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Lumbar Puncture (LP) and CSF Analysis

Sources: Pfenninger & Fowler's Procedures for Primary Care (3rd ed.), Harrison's Principles of Internal Medicine (22nd ed., 2025), Roberts & Hedges' Clinical Procedures in Emergency Medicine, Quick Compendium of Clinical Pathology (5th ed.), Textbook of Family Medicine (9th ed.), Tietz Textbook of Laboratory Medicine (7th ed.)

1. LUMBAR PUNCTURE

Definition

Lumbar puncture (LP) is a procedure in which a spinal needle is inserted into the subarachnoid space at the lumbar level under local anesthesia to withdraw CSF for diagnostic or therapeutic purposes. The spinal cord ends at approximately L1-L2 in adults, so the needle is inserted safely below this level - at L3-L4 or L4-L5 - where only the cauda equina floats freely and the risk of cord injury is minimal.

Tietz Textbook of Laboratory Medicine, 7th Ed.

Indications

Urgent/emergent:

Suspected CNS infection (bacterial/viral/fungal/TB meningitis, encephalitis)
Suspected subarachnoid hemorrhage (SAH) with negative CT

Non-urgent/elective:

Dementia workup (when normal-pressure hydrocephalus, syphilis, vasculitis suspected)
Unexplained neurologic disorders (altered consciousness, polyneuropathy, seizures)
Multiple sclerosis workup (oligoclonal bands)
Meningeal carcinomatosis / lymphomatous meningitis
Intrathecal drug administration (antibiotics, chemotherapy, anesthesia)
Imaging procedures (myelography, cisternography)
Pseudotumor cerebri - both diagnosis and therapeutic drainage
Pfenninger & Fowler's Procedures for Primary Care, p. 1438

Contraindications

Type	Condition
Absolute	Local skin infection at the puncture site
Absolute	Raised intracranial pressure with papilledema (risk of herniation)
Absolute	Supratentorial mass with midline shift or obliterated cisterns on CT
Relative	Severe coagulopathy or platelet count <20,000/mm³
Relative	Anticoagulated patient (INR >1.4)
Relative	Uncooperative patient / hemodynamically unstable

Key note: The absence of papilledema is NOT always reassuring - papilledema may take >48 hours to develop and is absent in up to 15% of adults and 50% of children with early raised ICP. Always check for clinical risk factors (Box 206-1 criteria include: age ≥60, altered level of consciousness, focal neurologic deficits, seizure within 1 week, immunocompromised state, history of CNS disease).

Pfenninger & Fowler's Procedures for Primary Care, pp. 1438-1439

Equipment

Figure: Lumbar puncture equipment tray (Pfenninger & Fowler)

Standard LP tray includes:

Povidone-iodine swabs and alcohol swabs
Fenestrated drape and sterile gloves
Manometer with three-way stopcock (to measure opening pressure)
1% lidocaine in a 3-mL syringe; 25-27 gauge skin needle for local anesthesia
20-22 gauge spinal needle (Quincke with beveled tip) - standard choice. A 24-27 gauge atraumatic needle reduces post-LP headache but cannot measure opening pressure reliably
Four numbered, capped sterile test tubes
Sterile dressing
Pfenninger & Fowler's Procedures for Primary Care, p. 1439

Patient Positioning

Lateral decubitus (fetal/embryo position): Patient lies on their side at the edge of the bed, knees drawn up to the abdomen, neck slightly flexed. Shoulders and pelvis must be exactly perpendicular to the bed (no forward or backward tilt). This is the preferred position when measuring opening pressure.

Seated upright position: Patient sits and leans forward. Easier to identify the midline in obese patients, but cannot measure accurate opening pressure. Most commonly used in adults.

Patient in lateral decubitus position showing L4 landmark

Figure: Lateral decubitus position with L4 anatomic landmark

Anatomic Landmark Identification

Iliac crest line crossing L3-L4 with sacrum labeled

Figure: A horizontal line connecting the superior aspects of the iliac crests (Tuffier's line) intersects the body of L4 - use this to identify the L3-L4 interspace

The L3-L4 interspace is the primary entry point. The L2-L3 or L4-L5 interspaces may also be used if needed.

Step-by-Step Technique (Midline Approach)

Step 1 - Pre-procedure safety check: Patients with focal neurologic deficits, altered mentation, immunocompromise, papilledema, or suspected SAH should have CT imaging first. However, if bacterial meningitis is suspected, do NOT delay antibiotics waiting for CT.

Step 2 - Positioning: Place the patient in lateral decubitus or seated position at the edge of the bed. The patient should "roll up into a ball" to open the interspinous spaces. Maintain strict alignment of shoulders, back, and pelvis.

Step 3 - Set up sterile field: Open the spinal tray aseptically. Put on sterile gloves. Pre-assemble the manometer by joining its two pieces and inserting it into the vertical port of the three-way stopcock. Open and upright the four numbered test tubes in order.

Step 4 - Skin preparation and draping: Clean the selected interspace plus one above and below with povidone-iodine in a circular motion (at least 10 cm diameter). Apply the fenestrated drape.

Step 5 - Local anesthesia: Draw 3 mL of 1% lidocaine. Raise a skin wheal at the L3-L4 midline interspace with a 25-27 gauge needle. Inject deeper into the posterior spinous region along the direction the spinal needle will travel.

Step 6 - Needle insertion: Insert the 20-22 gauge spinal needle through the skin in the midline, angled approximately 15 degrees cephalad toward the umbilicus. Keep the bevel of the needle parallel to the longitudinal axis of the spine (bevel turned up/down in lateral decubitus; turned sideways when seated) - this spreads rather than cuts the longitudinal dural fibers and reduces post-LP headache.

The needle passes through:

Skin and subcutaneous tissue
Supraspinous ligament
Interspinous ligament
Ligamentum flavum
Epidural space
Dura mater and arachnoid
Subarachnoid space (target - CSF flows freely)

Cross-sectional anatomy diagram showing needle passing through ligaments into subarachnoid space at L3

Figure: Lateral approach cross-section at L3 showing layers traversed, cauda equina in the subarachnoid space, and the 20-degree angle of entry (Pfenninger & Fowler)

A "give" or "pop" is felt as the needle penetrates the ligamentum flavum, and again as it pierces the dura-arachnoid. If bone is encountered, withdraw slightly and redirect more cephalad.

Step 7 - Measure opening pressure: Remove the stylet. Attach the manometer via the three-way stopcock as CSF flows in. Record the opening pressure in mm H₂O. Normal adult range: 70-180 mm H₂O.

Step 8 - CSF collection: Turn the stopcock to allow CSF to flow into the numbered test tubes. Collect 2-3 mL per tube (at least 4 tubes). Label tubes in the order collected.

If the fluid is bloody and does not clear by the 3rd tube: this suggests true SAH (traumatic tap clears progressively)
Replace the stylet before removing the needle (reduces post-LP headache risk)

Step 9 - Needle removal and dressing: Once sufficient CSF is obtained, replace the stylet, withdraw the needle, and apply a sterile dressing.

Step 10 - Post-procedure: Have the patient lie flat for 1-4 hours (reduces post-LP headache, though evidence is mixed).

Lateral Approach (Alternative)

Used when the midline approach fails or in elderly patients with calcified interspinous ligaments. Insert the needle 1.5-2 cm lateral to the midline, directed 20 degrees toward midline and 15 degrees cephalad. This bypasses the calcified midline ligaments.

Pfenninger & Fowler's Procedures for Primary Care, pp. 1439-1441

Tube Order and Tests Sent

Tube	Test
Tube 1	Bacteriology: Gram stain, acid-fast stain, culture (bacteria, fungal, TB, viral)
Tube 2	Biochemistry: glucose, protein, protein electrophoresis*
Tube 3	Hematology: cell count, differential
Tube 4 (optional)	VDRL, India ink, cryptococcal antigen, cytology, oligoclonal bands, myelin basic protein, PCR studies

If clinically indicated.

Practical tip: Tube 1 is most likely contaminated with blood from needle insertion. Use Tube 3 for the definitive cell count. For traumatic tap assessment, compare RBC count in Tube 1 vs. Tube 4 - a decreasing count favors traumatic tap. Correction factor: for every 700 RBCs, CSF leukocytes increase by 1 and CSF protein rises by 1 mg/dL.

Pfenninger & Fowler's Procedures for Primary Care, p. 1441

2. CSF ANALYSIS

Normal CSF Values

Parameter	Neonate	Infant	Child	Adult
Opening pressure (mm H₂O)	-	50-80	50-80	70-180
WBC count (/mm³)	8 (0-22)	-	<7	<5
Neutrophils	61%	-	None	None
Glucose (mg/dL)	34-119 mg/dL	-	40-80	50-80
CSF:serum glucose ratio	44-128%	-	~50%	60-70%
Protein (mg/dL)	20-170 (mean 90)	-	5-40	15-45

Pfenninger & Fowler's Procedures for Primary Care, p. 1440

Opening Pressure

Normal: 70-180 mm H₂O in adults (lateral decubitus; higher in seated position)
Elevated (>250 mm H₂O): suggests meningitis, SAH, intracranial mass, pseudotumor cerebri, or venous sinus thrombosis
Reduced: suggests CSF leak, dehydration, or spinal block below the tap level

Appearance / Color

Appearance	Interpretation
Clear and colorless	Normal
Turbid / cloudy	Cell count >200 WBC/mm³ or >400 RBC/mm³ - suggests infection
Bloody (tube 1 → clears by tube 3-4)	Traumatic tap
Uniformly bloody (all tubes)	True SAH
Pink xanthochromia	Free hemoglobin - suggests SAH
Yellow xanthochromia	Bilirubin from hemoglobin metabolism - begins ~12 hrs after hemorrhage, peaks at 72 hrs, disappears in 2-4 weeks
Artifactual xanthochromia	Hyperbilirubinemia, CSF protein >150 mg/dL, carotenoids, rifampin, delay >1 hour before analysis

Xanthochromia and erythrophagocytosis (or hemosiderin-laden macrophages) confirm true subarachnoid blood vs. traumatic tap.

Quick Compendium of Clinical Pathology, 5th ed.; Tintinalli's Emergency Medicine

Glucose

Normal CSF glucose = ~60-70% of serum glucose (always check a simultaneous serum glucose)
Hypoglycorrhachia (CSF glucose <40 mg/dL, or CSF:serum ratio <30%): bacterial meningitis, TB meningitis, fungal meningitis, carcinomatous meningitis, hypoglycemia
Normal glucose: viral meningitis, early bacterial meningitis, brain abscess

Protein

Normal adult: 15-45 mg/dL
Elevated protein: infection (bacterial > viral), MS, Guillain-Barré syndrome, brain tumor, SAH, hypothyroidism, diabetes, spinal cord compression
Very high protein (Froin's syndrome - >500 mg/dL, yellowish, clots spontaneously): spinal block
CSF:serum albumin ratio <1:230 when blood-brain barrier is intact
CSF glutamine is elevated in hepatic encephalopathy

Cell Count and Differential (Pleocytosis)

Cell type predominance	Causes
PMN (neutrophils)	Bacterial meningitis (early), early TB/fungal meningitis, chemical meningitis
Lymphocytes/monocytes	Viral meningitis, TB meningitis, fungal meningitis (chronic), MS, sarcoidosis, syphilis
Eosinophils	Parasitic infections (angiostrongyliasis, gnathostomiasis, cysticercosis), fungal meningitis (coccidioidal - 6-20% eos), lymphoma, sarcoidosis

Harrison's Principles of Internal Medicine, 22nd Ed.

Comparison of CSF Findings in Meningitis

Parameter	Bacterial Meningitis	Viral Meningitis	TB/Fungal
Opening pressure	>180 mm H₂O	Often normal or elevated	Elevated
WBC count (/mm³)	1,000-10,000 (median 1,195)	<300 (median 100)	100-500
Cell type	Neutrophils >80%	Lymphocytes >80%	Lymphocytes
Glucose (mg/dL)	<40	>40 (normal)	Low
Protein (mg/dL)	100-500	Normal to mildly elevated	Elevated
Gram stain	Positive 60-90%	Negative	Negative
Culture	Positive 70-85%	Positive ~50% (viral PCR better)	Low yield

Textbook of Family Medicine, 9th Ed., p. 1233

Special CSF Tests

Test	Indication/Significance
India ink preparation	Cryptococcal meningitis (visualizes capsule)
Cryptococcal antigen	More sensitive than India ink for Cryptococcus
VDRL	Neurosyphilis (highly specific, ~70% sensitive; negative does not exclude)
Oligoclonal bands	Multiple sclerosis (intrathecal IgG synthesis); also seen in CNS infections
Myelin basic protein	Active myelin destruction (MS, CNS injury)
Cytology / flow cytometry	Malignant cells in leptomeningeal carcinomatosis or lymphoma
PCR (16S rRNA)	Broad bacterial detection; useful in partially treated meningitis
18S/28S rRNA PCR	Broad fungal detection
Beta-glucan assay	Fungal meningitis when other tests negative
Metagenomic NGS	Challenging infectious cases; unbiased pathogen detection
CSF beta-2 transferrin	CSF leak detection (rhinorrhea/otorrhea)
CSF glutamine	Hepatic encephalopathy

Harrison's Principles of Internal Medicine, 22nd Ed.

3. COMPLICATIONS

Complication	Details
Post-LP headache	Most common (10-25%); positional (worse sitting, better lying); 90% within 48 hrs, may last up to 2 weeks. Treat with oral caffeine 300 mg or theophylline 200 mg; IV caffeine benzoate 500 mg for refractory cases; epidural blood patch (15 mL autologous blood) if no relief at 24 hrs - 85% immediate relief, 98% after 2nd patch
Traumatic/bloody tap	Inadvertent puncture of spinal venous plexus; self-limiting in most; risk of spinal hematoma in coagulopathy
Spinal epidural hematoma (SEH)	Rare but catastrophic; risk increased with anticoagulants, traumatic LP, concurrent aspirin + heparin. Diagnose with emergency MRI; treat with urgent laminectomy
Brain herniation	Occurs if LP done with unrecognized raised ICP and mass lesion; always check fundi for papilledema; obtain CT if any risk factors present
Intracranial subdural hematoma	Due to downward brain displacement from persistent CSF leak; tearing of bridging veins
Epidermoid tumor	Late complication when LP done without stylet (implanted dermal cells)
Infection	<0.01% risk; meningitis or epidural abscess
Seizures	Rare; reported with post-LP headache

Reducing post-LP headache:

Use a 24-27 gauge atraumatic (pencil-point/Whitacre) needle
Use the smallest gauge that allows adequate flow
Orient the bevel parallel to the longitudinal axis of the spine
Replace the stylet before removing the needle
Use a 22-gauge in practice (balance between headache risk and technical success)
Pfenninger & Fowler's Procedures for Primary Care, pp. 1441-1442; Roberts & Hedges' Clinical Procedures in Emergency Medicine

4. ANTICOAGULATION AND LP

Drug	Recommendation
Aspirin / NSAIDs alone	Generally safe without stopping (unless combined with heparin or another antiplatelet)
Clopidogrel / ticlopidine	Stop 7-10 days before (ASRA/AHA guideline)
Abciximab	Stop 24-48 hours before
Tirofiban / eptifibatide	Stop 8 hours before
IV heparin	Stop; 2% risk of SEH if LP done while on heparin; wait 4 hours after stopping
LMWH	Follow ASRA guidelines
Warfarin	INR must be ≤1.4 before LP

If LP must be performed on anticoagulated patients: most experienced operator, smallest needle available, and monitor closely for SEH.

Roberts & Hedges' Clinical Procedures in Emergency Medicine; Pfenninger & Fowler's Procedures for Primary Care

Clinical pearl: In suspected bacterial meningitis, never delay empiric antibiotics while waiting for CT or LP. Blood cultures + immediate antibiotics, then LP as soon as safely possible. In suspected SAH with negative CT, LP is essential - xanthochromia (yellow tint of centrifuged CSF) confirms the diagnosis and takes at least 12 hours to develop after hemorrhage, so LP should ideally be done ≥12 hours after headache onset.

CSF analysis and lumber puncture procedure explain from standard medicine book

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Lumbar Puncture (LP) and CSF Analysis

Sources: Pfenninger & Fowler's Procedures for Primary Care (3rd ed.) | Henry's Clinical Diagnosis and Management by Laboratory Methods (23rd ed.) | Tietz Textbook of Laboratory Medicine (7th ed.) | Harrison's Principles of Internal Medicine (22nd ed., 2025) | Quick Compendium of Clinical Pathology (5th ed.)

PART 1 - LUMBAR PUNCTURE

Anatomy and Rationale

The spinal cord ends at approximately L1-L2 in adults. Below this level, only the cauda equina (a bundle of nerve roots) floats freely in the subarachnoid space, making the lumbar region safe for needle insertion. The needle enters the subarachnoid space between L3-L4 (most common), L2-L3, or L4-L5.

CSF physiology (Henry's Clinical Diagnosis, 23rd ed.):

~500 mL produced per day (0.3-0.4 mL/min); total volume 90-150 mL in adults
~70% derived from choroid plexuses; remainder from ventricular ependymal lining
Total CSF volume replaced every 5-7 hours
Reabsorbed at arachnoid villi along the superior sagittal sinus
Functions: buoyancy (1500g brain weighs ~50g in CSF), shock absorption, CNS waste excretion, ionic homeostasis, transport of hypothalamic factors

Indications

Urgent indications:

Suspected CNS infection: bacterial/viral/fungal/TB meningitis, encephalitis
Suspected subarachnoid hemorrhage (SAH) after negative CT

Non-urgent indications:

Dementia workup (normal-pressure hydrocephalus, neurosyphilis, vasculitis)
Unexplained neurologic disorders: altered consciousness, polyneuropathy, seizures
Multiple sclerosis workup (oligoclonal bands, IgG index)
Meningeal carcinomatosis / lymphomatous meningitis
Intrathecal drug delivery (chemotherapy, antibiotics, anesthesia)
Myelography, cisternography (contrast instillation)
Pseudotumor cerebri (diagnosis and therapeutic drainage)

- Pfenninger & Fowler's Procedures for Primary Care, p. 1437

Contraindications

Type	Condition
Absolute	Local skin infection at the puncture site
Absolute	Raised ICP with papilledema - risk of transtentorial herniation
Absolute	Supratentorial mass with midline shift, obliterated cisterns, or posterior fossa mass on CT
Relative	Platelet count <20,000/mm³ or rapidly dropping; INR >1.4
Relative	Hemodynamically unstable patient (shock, status asthmaticus)
Relative	Uncooperative patient

Important note: Absence of papilledema does NOT exclude raised ICP - papilledema may take >48 hours to develop and is absent in up to 15% of adults and 50% of children with early raised ICP. Always assess clinical risk factors:

Box 206-1 - Clinical Risk Factors for Intracranial Pathology (do CT first):

Age ≥60 years
Altered level of consciousness
History of CNS disease
Immunocompromised state
Seizure within 1 week of presentation
Focal neurologic deficits (arm/leg drift, facial palsy, gaze palsy, visual field abnormality)
Abnormal language; inability to answer 2 questions or follow 2 commands

Critical pearl: In suspected bacterial meningitis, never delay antibiotics while awaiting CT. Give empiric antibiotics after blood cultures, then perform LP as soon as safely possible.

- Pfenninger & Fowler's Procedures for Primary Care, pp. 1438-1439

Equipment

Standard LP tray contains:

Povidone-iodine swabs + alcohol swabs
Fenestrated sterile drape + sterile gloves
Manometer + three-way stopcock (to measure opening pressure)
3 mL syringe with 1% lidocaine; 25-27 gauge skin needle for local anesthesia; 20-23 gauge needle to draw up anesthetic
20-22 gauge spinal needle (Quincke) - standard. 24-27 gauge atraumatic (Whitacre/pencil-point) reduces post-LP headache but cannot reliably measure opening pressure
Four numbered sterile capped test tubes
Sterile dressing
Optional: EMLA cream (apply 30-60 min before), 1 mL syringe for gentle suction if CSF not flowing, ultrasound for obese patients

- Pfenninger & Fowler's Procedures for Primary Care, p. 1439

Patient Positioning

Lateral decubitus (preferred for opening pressure measurement):

Patient in lateral decubitus position with L4 landmark marked showing fetal position

Fig: Lateral decubitus ("fetal") position - patient curled up, knees to abdomen, neck slightly flexed; dashed line marks L4 level (Pfenninger & Fowler)

Patient lies on their side at the edge of the bed, knees drawn to the abdomen, neck slightly flexed. Shoulders and pelvis must be strictly perpendicular to the bed - any forward/backward tilt rotates the spine and makes needle insertion difficult.

Seated upright position: Patient sits and leans slightly forward. Easiest for landmark identification in obese patients. Cannot give accurate opening pressure. Most commonly used in practice.

Anatomic Landmark Identification

Iliac crest line crossing L3-L4 level with sacrum and iliac crests labeled

Fig: Tuffier's line - horizontal line connecting the superior iliac crests crosses the body of L4; the L3-L4 interspace lies just above this line (Pfenninger & Fowler)

Key surface landmark (Tuffier's line): A line connecting the tops of both iliac crests crosses the body of L4. The L3-L4 interspace is just above this line. Mark with a pen or gentle skin indentation.

Step-by-Step Procedure (Midline Approach)

Step 1 - Pre-procedure assessment: Consider CT imaging first if any clinical risk factors for raised ICP are present (Box 206-1 above).

Step 2 - Consent and positioning: Explain procedure, risks, and alternatives. Obtain informed consent. Position the patient (lateral decubitus preferred). A nurse or assistant should help maintain the patient's position.

Step 3 - Sterile setup: Open the LP tray aseptically. Don sterile gloves. Preassemble the manometer (two pieces slide together); attach to the three-way stopcock. Stand the four numbered test tubes upright in their tray slots.

Step 4 - Skin preparation: Swab the selected interspace plus one above and below with povidone-iodine in a widening circular motion (minimum 10 cm diameter). Apply fenestrated drape over the area.

Step 5 - Local anesthesia: Draw up 3 mL of 1% lidocaine. Raise a skin wheal at the L3-L4 midline with a 25-27 gauge needle. Infiltrate deeper along the path the spinal needle will follow (into the interspinous ligament region).

Step 6 - Needle insertion: Insert the spinal needle through the skin in the midline, angled ~15 degrees cephalad (toward the umbilicus), maintaining the needle in the sagittal midplane.

Critical technique point: Orient the bevel parallel to the longitudinal axis of the spine (bevel pointing up or down in lateral decubitus; pointing sideways when seated). This spreads - rather than cuts - the longitudinal dural fibers, significantly reducing post-LP headache.

The needle passes through these layers in sequence:

Skin and subcutaneous fat
Supraspinous ligament
Interspinous ligament
Ligamentum flavum (a definite "give" or resistance change is felt here)
Epidural space (fat, veins)
Dura mater + arachnoid (second "pop" may be felt)
Subarachnoid space - target

Sagittal view of needle entering subarachnoid space at L3-L4, showing dura, cauda equina

Fig: Sagittal view showing needle passing through dura into subarachnoid space between L3-L4, with cauda equina visible (Pfenninger & Fowler)

After advancing 3-4 cm, stop and withdraw the stylet to check for CSF flow. If no flow, replace stylet and advance a few mm more. If bone is hit, withdraw slightly and redirect more cephalad. Rotating the needle 90-180 degrees can help if flow is minimal. Dry tap is more often due to malposition than obliterated space - reposition the patient or try a different interspace.

Cross-sectional anatomy at L3 showing ligaments, epidural space, dura/arachnoid, and subarachnoid space containing cauda equina, with needle angle of 20 degrees

Fig: Cross-section at L3 showing all layers the needle traverses; the needle enters at 20° angle; the subarachnoid space (yellow) contains the cauda equina (Pfenninger & Fowler)

Step 7 - Measure opening pressure: Once CSF flows, anchor the needle hub firmly against the patient's back. Attach the stopcock-manometer assembly. Ask the patient to relax and extend the legs slightly (flexed neck/legs artificially elevate pressure). CSF rises in the manometer - read the opening pressure (in mm H₂O). Normal adult range: 70-180 mm H₂O (Henry's: 90-180 mm H₂O in lateral decubitus).

If opening pressure is >200 mm H₂O in a relaxed patient, withdraw no more than 2.0 mL.

Step 8 - Collect CSF: Turn the stopcock to allow CSF to flow into the numbered test tubes (2-3 mL per tube). Collect at least 3-4 tubes. Note: if the fluid is bloody and does not clear from Tube 1 to Tube 3, this suggests true SAH rather than traumatic tap.

Tube allocation:

Tube	Contents
Tube 1	Bacteriology: Gram stain, acid-fast stain, culture (bacteria, fungal, TB, viral)
Tube 2	Biochemistry: glucose, protein, protein electrophoresis
Tube 3	Hematology: cell count, differential (use this for definitive cell count, not Tube 1)
Tube 4	Optional: VDRL, India ink, cryptococcal antigen, cytology, oligoclonal bands, myelin basic protein, PCR

Note from Henry's: Tube 1 should never be used for microbiology (skin contamination risk), and if Tube 1 is hemorrhagic from a traumatic tap, it should not be used for protein studies (e.g., suspected MS). Tube 3 should be examined for the primary purpose of CSF collection. Process specimens immediately - cellular degradation begins within 1 hour. Do NOT refrigerate culture specimens (Haemophilus, Neisseria are fastidious).

Step 9 - Therapeutic LP (if indicated): For pseudotumor cerebri: remove CSF until closing pressure reaches ≤100 mm H₂O (typically 25-35 mL). For suspected normal-pressure hydrocephalus: removal of 35-50 mL may cause transient improvement in gait or cognition (diagnostic).

Step 10 - Needle removal: Replace the stylet before withdrawing the needle (this reduces risk of post-LP headache). Apply sterile dressing.

Lateral Approach (Alternative)

Used when midline approach fails or calcified ligaments prevent entry (elderly patients). Insert needle 1.5-2 cm lateral to midline, directed 20 degrees toward midline and 15 degrees cephalad. This bypasses the ossified supraspinous and intraspinous ligaments.

Complications

Complication	Details
Post-LP headache	Most common (10-25%); positional - worse upright, better supine; 90% onset within 48 hrs (may appear up to 14 days later); may be debilitating
Traumatic/bloody tap	~20% of LPs; inadvertent puncture of epidural venous plexus; self-limiting
Spinal epidural hematoma (SEH)	Rare but catastrophic; highest risk with anticoagulants + traumatic LP; presents as back pain then paraplegia; diagnose with emergency MRI; treat with urgent laminectomy
Brain herniation	Occurs when LP is done with unrecognized mass lesion or raised ICP; check fundi for papilledema before every LP
Intracranial subdural hematoma	Rare; from downward brain displacement due to persistent CSF leak - tearing of bridging veins
Epidermoid tumor	Late risk when LP done without a stylet (implanted dermal cells) - especially in neonates
Infection (meningitis/abscess)	<0.01% risk

Treatment of post-LP headache:

First-line: bed rest, oral hydration, analgesics
Oral caffeine 300 mg or theophylline 200 mg
IV caffeine benzoate 500 mg over a few minutes (repeat in 1 hour for 85% relief rate)
Epidural blood patch (if no relief at 24 hours): inject 15 mL autologous blood into the epidural space - provides immediate relief in 85%; after 2nd patch, 98% relief

Minimizing post-LP headache:

Use a 24-27 gauge atraumatic (Whitacre/pencil-point) needle when opening pressure measurement is not critical
In practice, 22-gauge Quincke is used for balance of success rate and headache risk
Keep bevel parallel to the long axis of the spine
Replace the stylet before needle withdrawal

- Pfenninger & Fowler's Procedures for Primary Care, pp. 1441-1442

PART 2 - CSF ANALYSIS

Opening Pressure

Age Group	Normal Range
Infants	10-100 mm H₂O
Children	50-80 mm H₂O
Adults	70-180 mm H₂O (some sources 90-180)
Obese adults	May be up to 250 mm H₂O (normal for them)

>250 mm H₂O = intracranial hypertension (meningitis, hemorrhage, tumor, pseudotumor, venous sinus thrombosis)
Cryptococcal meningitis and pseudotumor cerebri may show elevated pressure as the only CSF abnormality
Low pressure: spinal-subarachnoid block, dehydration, CSF leak, circulatory collapse

- Henry's Clinical Diagnosis & Management, 23rd Ed.; Pfenninger & Fowler's

Gross Appearance

Appearance	Meaning
Clear and colorless	Normal
Turbid / cloudy	WBC >200/μL or RBC >400/μL; suggests infection
Frankly purulent	Severe bacterial meningitis
Uniformly bloody (all tubes)	True subarachnoid hemorrhage
Bloody, clears tube 1 → 3	Traumatic tap
Pink xanthochromia	Oxyhemoglobin from RBC lysis; SAH (detectable 2-4 hrs after bleed)
Yellow xanthochromia	Bilirubin from hemoglobin breakdown; SAH (appears ~12 hrs after bleed)
Orange	RBC lysis OR dietary hypervitaminosis A (carotenoids)
Yellow-green	Hyperbilirubinemia (biliverdin)
Brown	Meningeal metastatic melanoma
Viscous	Cryptococcal meningitis (capsular polysaccharide); mucin-producing carcinoma
Clot formation	Traumatic tap, complete spinal block (Froin syndrome), suppurative/TB meningitis

Xanthochromia timeline (Henry's, 23rd Ed.):

Pink (oxyhemoglobin): detectable 2-4 hours after SAH; peaks at 24-36 hrs; disappears over 4-8 days
Yellow (bilirubin): appears ~12 hours after SAH; peaks at 2-4 days; persists 2-4 weeks

Artifactual xanthochromia (false positive): hyperbilirubinemia, CSF protein >150 mg/dL, traumatic tap with >100,000 RBCs/μL, carotenoids, rifampin therapy, detergent contamination, delay >1 hour without processing

Distinguishing traumatic tap from true SAH:

Bloody CSF that progressively clears from Tube 1 to Tube 3/4 = traumatic tap
Xanthochromia after centrifugation = true SAH
Erythrophagocytosis or hemosiderin-laden macrophages = true SAH
Very high opening pressure (up to 500 mm H₂O) = more consistent with SAH

- Henry's Clinical Diagnosis & Management, 23rd Ed., p. 589; Quick Compendium of Clinical Pathology, 5th ed.

Normal CSF Reference Values

Parameter	Adult Normal	Neonates
Opening pressure (mm H₂O)	70-180	10-100
WBC count (/μL)	0-5	0-30
Lymphocytes	62 ± 34%	20 ± 18%
Monocytes	36 ± 20%	72 ± 22%
Neutrophils	2 ± 5% (≤7% acceptable)	3 ± 5%
Glucose (mg/dL)	50-80	Varies
CSF:serum glucose ratio	60-70% (range 40-80%)	44-128%
Total protein (mg/dL)	15-45	20-170 (mean 90)
RBC	None	None (minor traumatic bleeding common)

Normal CSF: lymphocytes and monocytes in a 70:30 ratio. Higher proportion of monocytes in young children (up to 80% monocytes normal).

- Pfenninger & Fowler's Procedures for Primary Care, p. 1440; Henry's Clinical Diagnosis & Management, 23rd Ed.

Glucose

Normal CSF glucose = 60-70% of plasma glucose
Always order a simultaneous serum glucose (ideally obtained 2-4 hours before LP for equilibration)
Hypoglycorrhachia (CSF glucose <40 mg/dL, or CSF:serum ratio <30%): bacterial meningitis, TB meningitis, fungal meningitis, carcinomatous meningitis, hypoglycemia
Normal glucose favors: viral meningitis, early bacterial meningitis, aseptic meningitis

Protein

Normal adult: 15-45 mg/dL
Protein concentration increases from ventricular CSF (lowest) → lumbar CSF (highest) - because proteins diffuse from plasma along a concentration gradient down to the lumbar space
Elevated CSF protein seen in: bacterial meningitis (100-500 mg/dL), TB/fungal meningitis, viral meningitis (mild), MS, Guillain-Barré, tumors, SAH, hypothyroidism, spinal cord compression
Froin syndrome: protein >500 mg/dL, yellow xanthochromic CSF that clots spontaneously - indicates complete spinal block
More than 80% of CSF protein originates from plasma ultrafiltration; remainder from intrathecal synthesis

Special protein markers:

Oligoclonal IgG bands: intrathecal IgG synthesis - hallmark of MS (also seen in CNS infections)
Myelin basic protein: active myelin destruction (MS, acute CNS injury)
CSF albumin index (CSF albumin/serum albumin): assesses blood-brain barrier integrity; ratio <1:230 = intact BBB
Beta-2 transferrin (asialated transferrin): specific for CSF - diagnoses CSF rhinorrhea/otorrhea
Tau protein and beta-amyloid isoforms: Alzheimer disease diagnosis/prognosis
14-3-3 protein: Creutzfeldt-Jakob disease
S-100B, NSE, GFAP: traumatic/ischemic brain injury markers
CSF glutamine: elevated in hepatic encephalopathy

- Tietz Textbook of Laboratory Medicine, 7th Ed.; Henry's Clinical Diagnosis & Management, 23rd Ed.

Cell Count and Differential

Normal: 0-5 WBCs/μL in adults (0-30 in neonates). No RBCs.

Pleocytosis patterns and clinical significance:

Neutrophilic (PMN) pleocytosis - Causes (Box 30.3, Henry's):

Bacterial meningitis (>60% PMNs; PMN count >1,180/μL has 99% predictive value for bacterial meningitis)
Early viral meningoencephalitis (PMNs may predominate first 24-48 hrs, then shift to lymphocytes)
Early TB and fungal meningitis
Amebic encephalomyelitis
CNS abscess, subdural empyema
Following seizures, CNS hemorrhage, or CNS infarct
Metastatic tumor in contact with CSF
Injection of foreign material (methotrexate, contrast media)

Lymphocytic pleocytosis - Causes:

Viral meningitis (aseptic meningitis)
Tuberculous meningitis
Fungal meningitis (cryptococcal, coccidioidal)
Syphilitic meningoencephalitis
MS, Guillain-Barré syndrome
Sarcoidosis, subacute sclerosing panencephalitis
Parasitic CNS infestations (cysticercosis, toxoplasmosis)
Early bacterial meningitis with low leukocyte counts

Eosinophilic pleocytosis (rare):

Parasitic infections: Angiostrongylus cantonensis, Gnathostoma, Baylisascaris, cysticercosis, schistosomiasis
Fungal meningitis (coccidioidal: 6-20% eosinophils with lymphocytic pleocytosis)
Lymphoma, leukemia, metastatic carcinoma
Sarcoidosis, hypereosinophilic syndrome

- Henry's Clinical Diagnosis & Management, 23rd Ed., pp. 590-591

CSF Lactate

Normal: ~1.1-2.4 mmol/L
Increased in: bacterial meningitis (marked), fungal/TB meningitis (mild-moderate), viral meningitis (normal to mildly elevated)
Can help differentiate bacterial from viral meningitis when Gram stain is negative

Typical CSF Findings in Meningitis (Comparison Table)

Parameter	Bacterial	Viral	Fungal	Tuberculous
Opening pressure	Elevated (>180 mm H₂O)	Usually normal	Variable	Variable (often elevated)
WBC count	≥1,000/μL	<100/μL	Variable	Variable
Cell differential	Mainly neutrophils*	Mainly lymphocytes†	Mainly lymphocytes	Mainly lymphocytes
Protein	Mild to marked increase (100-500 mg/dL)	Normal to mild increase	Increased	Increased
Glucose	Usually ≤40 mg/dL	Normal	Decreased	Decreased (<45 mg/dL)
CSF:serum glucose	Normal to marked decrease	Usually normal	Low	Low
Lactic acid	Mild to marked increase	Normal to mild increase	Mild-moderate increase	Mild-moderate increase
Gram stain	Positive 60-90%	Negative	Negative	Negative
Culture	Positive 70-85%	~50% (PCR better)	Low-moderate	Low yield

* Lymphocytosis present in ~10% of bacterial meningitis cases † Neutrophils may predominate early in viral disease (shift to lymphocytes in 2-3 days)

- Henry's Clinical Diagnosis & Management, 23rd Ed., p. 598

Special/Advanced CSF Tests

Test	Use
India ink	Cryptococcus neoformans - visualizes large capsule
Cryptococcal antigen (latex agglutination)	More sensitive than India ink for cryptococcal meningitis
VDRL	Neurosyphilis - highly specific but only ~70% sensitive
Oligoclonal bands + IgG index	Multiple sclerosis (intrathecal IgG synthesis)
Myelin basic protein	Active demyelination
Beta-2 transferrin	CSF leak diagnosis (rhinorrhea, otorrhea)
Cytology / flow cytometry	Leptomeningeal malignancy, lymphoma, leukemia
PCR (16S rRNA)	Broad bacterial detection; useful in partially treated meningitis
Multiplex PCR panels	Bacteria, viruses, fungi simultaneously
Beta-glucan assay	Fungal meningitis when other tests negative
Metagenomic next-generation sequencing	Unbiased pathogen identification in challenging cases
Tau / beta-amyloid	Alzheimer disease workup
14-3-3 protein	Creutzfeldt-Jakob disease
PLAP (placental alkaline phosphatase)	Pineal germ cell tumors (radio-sensitive)
CSF lactate + adenosine deaminase (ADA)	TB meningitis workup
Ferritin	Late-presenting SAH (elevated)

Diseases Detected by CSF Laboratory Examination (Henry's Clinical Diagnosis, 23rd Ed.)

Diagnostic Accuracy	Conditions
High sensitivity + High specificity	Bacterial meningitis, TB meningitis, viral meningitis, fungal meningitis, viral encephalitis
High sensitivity + Moderate specificity	Subarachnoid hemorrhage, multiple sclerosis, neurosyphilis, infectious polyneuritis, paraspinal abscess
Moderate sensitivity + High specificity	Meningeal malignancy
Moderate sensitivity + Moderate specificity	Intracranial hemorrhage, subdural hematoma

Practical summary for traumatic tap correction (Henry's): For every 700 RBCs in CSF (traumatic tap), CSF WBC count increases by ~1 cell/μL and CSF protein rises by ~8 mg/dL per 10,000 RBCs/μL. Formally:

WBC corrected = WBC observed - (WBC blood × RBC csf / RBC blood)

These corrections are valid only if all measurements are made from the same tube and peripheral counts are not extremely abnormal.

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