Basic investigation course:- CBC , LFT, KFT , .what to see every parameter and how to interpret diagnosis according to investigation. Explain in detail
Basic investigation course:- CBC , LFT, KFT , ESR and CRP clinical significance, lipid profile urine report - physical, chemical and microscopic examination.what to see every parameter and how to interpret diagnosis according to investigation. Explain in detail. How to conclude diagnosis by seeing their test .
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Basic Investigation Course: Complete Guide to CBC, LFT, KFT, ESR, CRP, Lipid Profile & Urine Examination
PART 1 — COMPLETE BLOOD COUNT (CBC)
The CBC is the most ordered laboratory test. It evaluates all three cellular lines of blood.
1.1 Hemoglobin (Hb)
| Male | Female | Children | |
|---|---|---|---|
| Normal | 13.5–17.5 g/dL | 12.0–15.5 g/dL | 11–16 g/dL |
Low Hb = Anemia. Always look at three indices together to classify:
| Index | What it measures | Normal |
|---|---|---|
| MCV (Mean Corpuscular Volume) | Average RBC size | 80–100 fL |
| MCH (Mean Corpuscular Hemoglobin) | Hb per cell | 27–32 pg |
| MCHC (Mean Corpuscular Hemoglobin Concentration) | Hb concentration in cells | 32–36 g/dL |
Anemia Classification by MCV:
| MCV | Type | Key Causes |
|---|---|---|
| Low (<80 fL) — Microcytic | Iron deficiency, thalassemia, sideroblastic anemia, chronic disease (late) | Commonest = Iron deficiency |
| Normal (80–100 fL) — Normocytic | Acute blood loss, hemolytic anemia, aplastic anemia, chronic kidney disease, chronic disease (early) | |
| High (>100 fL) — Macrocytic | Vitamin B12 deficiency, Folate deficiency, hypothyroidism, liver disease, alcohol, drugs (MTX, hydroxyurea) |
How to narrow it down further:
- Iron deficiency anemia: Low MCV, low MCH, low serum ferritin, high TIBC, low serum iron. Peripheral smear shows pencil cells, target cells.
- B12/Folate deficiency: High MCV, hypersegmented neutrophils on smear, low B12/folate levels, elevated homocysteine.
- Thalassemia trait: Low MCV but the cell count (RBC) is actually high or normal — RBC count >5 million (Mentzer index <13 = thalassemia; >13 = iron deficiency).
- Hemolytic anemia: Normocytic, raised reticulocyte count, raised bilirubin (indirect), raised LDH, reduced haptoglobin.
High Hb = Polycythemia:
- Primary (Polycythemia Vera): JAK2 mutation, splenomegaly, raised WBC and platelets also.
- Secondary: Chronic hypoxia (COPD, high altitude), EPO-secreting tumors, smoking.
1.2 White Blood Cells (WBC / Leukocytes)
Normal total WBC: 4,000–11,000 cells/µL
Always look at the differential count, not just the total:
| Cell | Normal % | Normal Absolute | Significance |
|---|---|---|---|
| Neutrophils | 50–70% | 1,800–7,500/µL | Bacterial infection, inflammation |
| Lymphocytes | 20–40% | 1,000–4,800/µL | Viral infections, immune function |
| Monocytes | 2–8% | 200–900/µL | Chronic inflammation, TB, monocytic leukemia |
| Eosinophils | 1–4% | 40–500/µL | Allergies, parasites, Addison's disease |
| Basophils | 0–1% | 0–100/µL | Allergic reactions, CML |
Leukocytosis (WBC >11,000):
- Neutrophilia: Bacterial infection (commonest), stress, steroids, MI, burns. Left shift (band forms, metamyelocytes) = severe bacterial infection or sepsis.
- Lymphocytosis: Viral infections (EBV, CMV, hepatitis), CLL, whooping cough. Atypical lymphocytes = EBV (infectious mononucleosis).
- Eosinophilia: Allergic (asthma, urticaria), parasitic infections (roundworm, filaria), drugs (penicillin), Addison's disease, malignancy.
- Monocytosis: TB, infective endocarditis, chronic infections, monocytic leukemia.
- Basophilia: CML (hallmark), hypothyroidism, allergies.
Leukopenia (WBC <4,000):
- Viral infections (dengue, HIV), drug-induced (chemotherapy, clozapine, carbimazole), SLE, bone marrow failure, B12/folate deficiency.
- Neutropenia <1,500: Risk of bacterial infections. <500 = severe risk (febrile neutropenia).
Blast cells on differential: Acute Leukemia (AML/ALL) — requires urgent bone marrow biopsy.
Leukemoid reaction vs. CML: WBC >50,000 with left shift. Leukemoid reaction (LAP score high), CML (LAP score low, Philadelphia chromosome positive).
1.3 Platelets (Thrombocytes)
Normal: 1,50,000–4,00,000/µL (1.5–4 lakh)
| Finding | Clinical Significance |
|---|---|
| Thrombocytopenia (<1,50,000) | Dengue, ITP, DIC, hypersplenism, aplastic anemia, drug-induced, HIV |
| <50,000 | Risk of spontaneous bleeding |
| <20,000 | Risk of intracranial bleeding — consider transfusion |
| Thrombocytosis (>4,00,000) | Reactive (iron deficiency, infection, post-splenectomy) vs. Primary (ET, PV, CML) |
Dengue pattern: Rapid fall in platelets + rising hematocrit (hemoconcentration) = impending dengue shock.
1.4 Peripheral Smear — Always Read With CBC
| Finding | Diagnosis |
|---|---|
| Hypersegmented neutrophils (>5 lobes) | B12/Folate deficiency |
| Target cells | Thalassemia, liver disease, HbC disease |
| Sickle cells | Sickle cell anemia |
| Schistocytes (fragmented RBCs) | TTP, HUS, DIC, mechanical heart valve |
| Spherocytes | Hereditary spherocytosis, autoimmune hemolytic anemia |
| Teardrop cells (dacrocytes) | Myelofibrosis |
| Blast cells | Acute leukemia |
| Reed-Sternberg cells | Hodgkin's lymphoma (in tissue, not blood) |
| Smear cells (smudge cells) | CLL |
| Rouleaux formation | Multiple myeloma, infections |
PART 2 — LIVER FUNCTION TESTS (LFT)
LFTs test multiple liver functions. Interpret them as a panel, not individually.
2.1 Bilirubin
| Parameter | Normal |
|---|---|
| Total Bilirubin | 0.2–1.2 mg/dL |
| Direct (Conjugated) | <0.3 mg/dL |
| Indirect (Unconjugated) | <0.8 mg/dL |
Jaundice appears when total bilirubin >2.5–3 mg/dL
| Type | Indirect ↑ | Direct ↑ | Causes |
|---|---|---|---|
| Pre-hepatic (Hemolytic) | ↑↑ | Normal | Hemolytic anemia, G6PD deficiency, sickle cell |
| Hepatic | Both ↑ | Both ↑ | Hepatitis (viral, alcoholic, autoimmune), cirrhosis |
| Post-hepatic (Obstructive) | Normal | ↑↑ | Choledocholithiasis, carcinoma head of pancreas, cholangitis |
2.2 Liver Enzymes (Transaminases)
| Parameter | Normal | Significance |
|---|---|---|
| AST (SGOT) | 10–40 U/L | Liver, heart, muscle, kidney |
| ALT (SGPT) | 7–56 U/L | More liver-specific |
| ALP (Alkaline Phosphatase) | 44–147 U/L | Liver (cholestasis), bone, gut |
| GGT (Gamma-GT) | M: 8–61, F: 5–36 U/L | Alcohol, cholestasis, drugs |
Interpretation patterns:
| Pattern | Likely Cause |
|---|---|
| ALT/AST >10× normal | Acute viral hepatitis, drug/toxin hepatitis (paracetamol overdose), ischemic hepatitis |
| ALT/AST 2–10× normal | Chronic hepatitis, NAFLD, alcoholic hepatitis |
| ALP >>AST/ALT (cholestatic pattern) | Obstructive jaundice (stones, tumor), primary biliary cholangitis, drugs |
| AST:ALT ratio >2:1 | Alcoholic liver disease (classic) |
| AST:ALT ratio <1 | Viral hepatitis, NAFLD |
| Isolated GGT rise | Alcohol use, enzyme-inducing drugs, fatty liver |
| All ↑ with synthetic function failure | Fulminant hepatic failure |
2.3 Synthetic Function Tests (Most Important for Severity)
| Parameter | Normal | Significance |
|---|---|---|
| Serum Albumin | 3.5–5.0 g/dL | Low = chronic liver disease, malnutrition, nephrotic syndrome |
| Prothrombin Time (PT/INR) | INR 0.8–1.2 | Prolonged = liver failure (Factor V, VII) — can't be corrected by Vit K in severe disease |
| Total Protein | 6–8.3 g/dL |
Low albumin + prolonged PT = severe liver dysfunction (poor prognosis)
Albumin is a chronic marker (half-life ~21 days). PT/INR is an acute marker (reflects current synthetic function).
2.4 Putting It Together — LFT Diagnostic Approach
| Condition | Key LFT Pattern |
|---|---|
| Acute viral hepatitis (A, B, E) | ALT/AST very high (>10×), bili ↑, albumin normal initially |
| Chronic hepatitis B/C | Moderate transaminase elevation, may progress to ↓albumin, ↑PT |
| Alcoholic hepatitis | AST:ALT >2:1, GGT very high, ALP moderately raised |
| Cirrhosis | ↓Albumin, ↑PT, ↑Bilirubin, transaminases moderately ↑ or even normal |
| Obstructive jaundice | ALP/GGT markedly ↑, direct bilirubin ↑↑, transaminases mildly ↑ |
| NAFLD | Mild ALT>AST rise, metabolic risk factors |
| Hemolytic jaundice | Indirect bili ↑, enzymes normal, LDH ↑ |
PART 3 — KIDNEY FUNCTION TESTS (KFT / RFT)
3.1 Serum Urea
Normal: 7–20 mg/dL (BUN) or 15–40 mg/dL (urea)
- Raised urea: Renal failure, dehydration, high protein diet, GI bleed (blood digested = protein load), catabolic states.
- Low urea: Liver failure (urea synthesis impaired), malnutrition, overhydration.
- BUN:Creatinine ratio: Normal ~10–20:1
-
20:1 = Pre-renal AKI (dehydration, GI bleed, poor perfusion)
- <10:1 = Liver disease or low protein intake
-
3.2 Serum Creatinine
Normal: Male 0.7–1.3 mg/dL | Female 0.6–1.1 mg/dL
- More specific than urea for kidney function.
- Rises only when >50% of nephron mass is lost (not sensitive for early damage).
- Affected by muscle mass — thin/elderly patients may have CKD with "normal" creatinine.
3.3 eGFR (Estimated GFR)
Gold standard for staging CKD (using CKD-EPI or MDRD equation):
| Stage | GFR (mL/min/1.73m²) | Description |
|---|---|---|
| 1 | ≥90 | Normal/high — with kidney damage markers |
| 2 | 60–89 | Mildly decreased |
| 3a | 45–59 | Mild–Moderate |
| 3b | 30–44 | Moderate–Severe |
| 4 | 15–29 | Severely decreased |
| 5 | <15 | Kidney failure (dialysis needed) |
3.4 Serum Electrolytes (Part of KFT Panel)
| Electrolyte | Normal | Low (causes) | High (causes) |
|---|---|---|---|
| Sodium (Na⁺) | 135–145 mEq/L | Hyponatremia: SIADH, heart failure, cirrhosis, hypothyroidism, Addison's | Hypernatremia: Dehydration, DI, excess Na intake |
| Potassium (K⁺) | 3.5–5.0 mEq/L | Hypokalemia: Diuretics, vomiting, Conn's syndrome, alkalosis | Hyperkalemia: AKI/CKD, Addison's, acidosis, ACE inhibitors |
| Bicarbonate | 22–29 mEq/L | Metabolic acidosis (DKA, renal failure, diarrhea) | Metabolic alkalosis (vomiting, diuretics) |
3.5 Uric Acid
Normal: Male 3.5–7.2 mg/dL | Female 2.6–6.0 mg/dL
- High (Hyperuricemia): Gout, renal failure, leukemia/lymphoma (high cell turnover), diuretics, alcohol.
- Low uric acid: Xanthinuria, allopurinol therapy, SIADH.
3.6 Pre-renal vs. Renal vs. Post-renal AKI
| Feature | Pre-renal | Intrinsic Renal | Post-renal |
|---|---|---|---|
| BUN:Cr ratio | >20 | <20 | Variable |
| Urine Na | <20 mEq/L | >40 mEq/L | Variable |
| FENa | <1% | >2% | Variable |
| Urine osmolality | >500 | <350 | Variable |
| Urine sediment | Normal, hyaline casts | Granular/"muddy brown" casts, RTE cells | May be normal |
| Cause | Dehydration, blood loss, heart failure | ATN, GN, AIN | Stones, BPH, tumor |
PART 4 — ESR (Erythrocyte Sedimentation Rate)
Normal: Male <15 mm/hr | Female <20 mm/hr (Westergren method)
(Often given as age/2 for males, (age+10)/2 for females)
How it works:
RBCs settle faster when there are more acute phase proteins (fibrinogen, globulins) that cause rouleaux formation.
Causes of Raised ESR:
| Degree | Range | Causes |
|---|---|---|
| Mild | 20–50 | Pregnancy, age, mild infection, anemia |
| Moderate | 50–100 | TB, RA, SLE, lymphoma, chronic infections |
| Markedly elevated | >100 | Multiple myeloma, temporal arteritis, giant cell arteritis, nephrotic syndrome, visceral malignancy |
| "Extreme" | >150 | Multiple myeloma (most classic), polymyalgia rheumatica, SBE |
Clinical Applications:
- TB screening: Sensitive but non-specific. Very high ESR in active TB.
- RA monitoring: ESR correlates with disease activity.
- Multiple myeloma: ESR >100 is highly suggestive — check serum protein electrophoresis.
- Temporal arteritis: ESR >50 is a diagnostic criterion.
- Paradoxically low ESR: Sickle cell disease, polycythemia, hypofibrinogenemia, early DIC.
- ESR is SLOW to respond (days–weeks). Not useful for acute assessment.
PART 5 — CRP (C-Reactive Protein)
Normal: <10 mg/L (high-sensitivity CRP <1 mg/L for cardiac risk)
Key Differences: CRP vs. ESR
| Feature | CRP | ESR |
|---|---|---|
| Response time | Rises within 6 hours of insult | Rises over 24–48 hours |
| Normalizes | Within days of resolution | Takes weeks |
| Specificity | More specific for inflammation | More affected by other factors |
| Best use | Monitoring acute infection, post-op | Chronic disease monitoring |
CRP Interpretation:
| Level | Interpretation |
|---|---|
| <10 mg/L | Normal or mild inflammation |
| 10–100 mg/L | Moderate inflammation — bacterial infection, RA flare, tissue injury |
| >100 mg/L | Severe bacterial infection, sepsis, major trauma, severe vasculitis |
| >200 mg/L | Strongly suggests bacterial sepsis |
Viral infections: CRP typically <40 mg/L (key differentiator from bacterial)
Bacterial infections: CRP typically >100 mg/L
High-sensitivity CRP (hsCRP) for Cardiovascular Risk:
| hsCRP | Cardiovascular Risk |
|---|---|
| <1 mg/L | Low |
| 1–3 mg/L | Intermediate |
| >3 mg/L | High |
PART 6 — LIPID PROFILE
Fasting sample (12 hours). Key parameters:
6.1 Reference Values and Interpretation (ATP III / ACC/AHA guidelines)
| Parameter | Desirable | Borderline | High Risk |
|---|---|---|---|
| Total Cholesterol | <200 mg/dL | 200–239 mg/dL | ≥240 mg/dL |
| LDL Cholesterol | <100 mg/dL (optimal) | 130–159 | ≥190 mg/dL (very high) |
| HDL Cholesterol | ≥60 mg/dL (protective) | 40–60 | <40 mg/dL (risk factor) |
| Triglycerides | <150 mg/dL | 150–199 | ≥500 = very high (pancreatitis risk) |
| Non-HDL cholesterol | <130 mg/dL | — | — |
VLDL: Calculated as Triglycerides ÷ 5 (Normal: <30 mg/dL)
LDL: Calculated by Friedewald equation: LDL = Total Cholesterol − HDL − (TG/5) (Not valid if TG >400 mg/dL)
6.2 Clinical Patterns:
| Pattern | Likely Diagnosis |
|---|---|
| High LDL, normal TG | Familial hypercholesterolemia, diet-induced |
| High TG, low HDL | Metabolic syndrome, DM, alcohol, hypothyroidism |
| High TG >1000 mg/dL | Familial hypertriglyceridemia — risk of pancreatitis |
| High total cholesterol + high LDL | Hypothyroidism, nephrotic syndrome, liver disease |
| Low HDL alone | Smoking, sedentary lifestyle, central obesity |
| All lipids elevated | Hypothyroidism, nephrotic syndrome, familial combined hyperlipidemia |
6.3 LDL Targets (Risk-based):
| Patient | LDL Target |
|---|---|
| Low risk | <130 mg/dL |
| Moderate risk | <100 mg/dL |
| High risk (DM, multiple risk factors) | <70 mg/dL |
| Very high risk (post-MI, established CVD) | <55–70 mg/dL |
PART 7 — URINE EXAMINATION (Urinalysis)
A complete urine examination has three components: Physical, Chemical, Microscopic.
7.1 PHYSICAL EXAMINATION
| Parameter | Normal | Abnormal & Significance |
|---|---|---|
| Volume | 800–1800 mL/day | Oliguria (<400 mL/day): AKI, dehydration; Anuria (<100 mL): severe AKI; Polyuria (>3L): DM, DI |
| Color | Pale yellow to amber | ↓ Yellow (clear/pale): dilute urine, DI; Dark yellow/amber: dehydration, concentrated; Red/pink: hematuria, myoglobinuria, hemoglobinuria, beets, drugs; Brown/tea: hemoglobinuria, hepatitis, alkaptonuria; Orange: bile pigments (obstructive jaundice), rifampicin; Green/blue: Pseudomonas UTI, amitriptyline; Milky: chyluria, pyuria, phosphaturia |
| Appearance | Clear | Turbid/cloudy: pyuria (UTI), phosphates, urates; Frothy: proteinuria (nephrotic syndrome) |
| Specific Gravity | 1.003–1.030 | High (>1.030): dehydration, DM; Low (1.001–1.005): DI, overhydration; Fixed 1.010: renal failure (isosthenuria) |
| Smell | Slightly aromatic | Ammonia smell: bacterial decomposition (UTI, stale sample); Fruity/sweet: DKA (acetone); Fishy: trimethylaminuria; Foul: infected urine |
7.2 CHEMICAL EXAMINATION (Dipstick)
| Parameter | Normal | Positive/Abnormal — Clinical Significance |
|---|---|---|
| pH | 4.5–8.0 (usually ~6) | Alkaline urine (>7): UTI (urea-splitting organisms — Proteus), vegetarian diet, RTA type 1; Acidic urine (<5): DKA, uric acid stones, acidosis |
| Protein | Negative (trace only) | +1 = ~30 mg/dL; Persistent proteinuria: nephrotic syndrome (>3.5g/day), GN, DM nephropathy, preeclampsia; Transient: fever, exercise, UTI |
| Glucose (Glycosuria) | Negative | Positive: DM (when blood glucose >180 mg/dL, renal threshold), pregnancy (lower threshold), Fanconi syndrome (low renal threshold with normal blood glucose) |
| Ketones | Negative | Positive: DKA (most important), starvation, vomiting, high-fat diet, alcoholic ketoacidosis |
| Blood | Negative | Positive: Hematuria (RBCs), hemoglobinuria, myoglobinuria. If blood +ve on dipstick but NO RBCs on microscopy = hemoglobinuria or myoglobinuria |
| Bilirubin | Negative | Positive: Direct hyperbilirubinemia (hepatic/obstructive jaundice). Absent in hemolytic jaundice (indirect bilirubin not water soluble) |
| Urobilinogen | Trace (0.1–1 EU/dL) | High: Hemolytic jaundice, hepatocellular disease; Absent: Complete bile duct obstruction (no urobilinogen cycle) |
| Nitrites | Negative | Positive: Gram-negative bacteriuria (E. coli, Klebsiella, Proteus — reduce nitrate to nitrite). Gram-positive organisms do NOT reduce nitrite |
| Leukocyte Esterase | Negative | Positive: Pyuria — UTI, AIN, contamination. Combined with nitrites = strong UTI evidence |
| Specific Gravity | 1.003–1.030 | (see physical above) |
Urine Bilirubin & Urobilinogen Interpretation:
| Condition | Bilirubin in Urine | Urobilinogen in Urine |
|---|---|---|
| Hemolytic jaundice | Absent | ↑↑ |
| Hepatocellular jaundice | Present | ↑ (or normal) |
| Obstructive jaundice | Present ↑↑ | Absent (complete block) |
7.3 MICROSCOPIC EXAMINATION
Centrifuge urine, examine sediment under microscope.
Cells:
| Finding | Normal | Clinical Significance |
|---|---|---|
| RBCs | 0–2/HPF | >3/HPF = hematuria: GN (dysmorphic RBCs, RBC casts), UTI, stones, tumor, trauma, anticoagulants |
| WBCs (Pus cells) | 0–5/HPF | >5/HPF = pyuria: UTI (most common), AIN (sterile pyuria + eosinophils), TB (sterile pyuria — no bacteria grown) |
| Epithelial cells | Few | Renal tubular epithelial (RTE) cells in large numbers: ATN, nephrotoxins |
| Eosinophils | Absent | Eosinophiluria: Acute interstitial nephritis (AIN), atheroembolic renal disease |
Casts (Highly diagnostic):
Casts form in the renal tubular lumen — they have the shape of tubules.
| Cast Type | Significance |
|---|---|
| Hyaline casts | Normal in small numbers; increased in dehydration, fever, exercise, pre-renal AKI |
| RBC casts | Pathognomonic of GN (glomerulonephritis) — nephritic syndrome, lupus nephritis, IgA nephropathy |
| WBC casts | Pyelonephritis, acute interstitial nephritis |
| Granular casts ("muddy brown") | ATN (Acute Tubular Necrosis) — classic finding |
| Waxy casts | Advanced/chronic renal failure (CKD), amyloidosis |
| Broad casts | End-stage renal disease (very dilated tubules/collecting ducts) |
| Fatty casts / Oval fat bodies | Nephrotic syndrome (lipiduria) |
| Pigmented casts | Rhabdomyolysis (myoglobin), hemolysis |
Crystals:
| Crystal | Shape | Clinical Significance |
|---|---|---|
| Calcium oxalate | Envelope/dumbbell | Hyperoxaluria, ethylene glycol poisoning, idiopathic |
| Uric acid | Rhomboid/barrel | Gout, uric acid stones, tumor lysis syndrome |
| Struvite (triple phosphate) | Coffin lid | UTI with urea-splitting organisms (Proteus, Klebsiella), staghorn calculi |
| Cystine | Hexagonal | Cystinuria (pathological) |
| Calcium phosphate | Wedge/prism | Alkaline urine, hyperparathyroidism |
PART 8 — HOW TO CONCLUDE DIAGNOSIS: PUTTING IT ALL TOGETHER
Systematic Approach:
Step 1: Look for the CHIEF COMPLAINT / PRESENTING SYMPTOM
Step 2: Order relevant investigations
Step 3: Interpret each test within clinical context
Step 4: Correlate all abnormal values to arrive at diagnosis
Common Clinical Scenarios:
🔷 Patient with Fatigue + Pallor:
- CBC → Low Hb → Type of anemia (MCV)
- Microcytic → Serum ferritin, iron, TIBC
- Macrocytic → B12/Folate, peripheral smear (hypersegmented neutrophils)
- Add LFT (rule out liver disease), KFT (CKD anemia), ESR/CRP
🔷 Patient with Jaundice:
- LFT → Bilirubin (direct vs indirect) → Type of jaundice
- Urine: bilirubin + urobilinogen pattern (see table above)
- AST/ALT → Hepatocellular vs. cholestatic
- CBC → Hb low + high reticulocytes = hemolytic
- If obstructive → ALP/GGT very high, ultrasound abdomen
🔷 Patient with Swollen Legs + Proteinuria:
- Urine: protein 3+ or 4+, fatty casts, oval fat bodies
- LFT: low albumin (<2.5)
- KFT: creatinine (may be normal initially)
- Lipid profile: high cholesterol (compensatory synthesis)
- Diagnosis: Nephrotic Syndrome
🔷 Patient with Fever + Dysuria:
- Urine: WBC >5/HPF, positive nitrites, leukocyte esterase
- Urine culture and sensitivity
- CBC: leukocytosis with neutrophilia
- CRP: >100 mg/L (bacterial)
- Diagnosis: UTI / Pyelonephritis (if WBC casts present = upper tract)
🔷 Patient with Polyuria + Glycosuria:
- Urine: glucose positive, may have ketones (if DKA)
- KFT: creatinine (nephropathy screening)
- Blood glucose: fasting + 2hr PP or HbA1c
- Lipid profile (metabolic syndrome)
- Diagnosis: Diabetes Mellitus
🔷 Suspected Sepsis:
- CBC: high WBC with neutrophilia + left shift (or low WBC if overwhelming)
- CRP: >100 mg/L (often >200)
- ESR: raised (less useful acutely)
- KFT: creatinine may rise (sepsis-related AKI)
- LFT: transaminases may rise
- Urine: rule out source of infection
🔷 Chest Pain + Cardiac Risk Assessment:
- Lipid profile: LDL, HDL, total cholesterol, TG
- hsCRP: cardiovascular risk
- CBC: rule out anemia as cause of angina
- ESR/CRP: rule out pericarditis, myocarditis
- KFT: before statin/contrast use
🔷 Suspected Multiple Myeloma:
- CBC: normocytic normochromic anemia, rouleaux on smear
- ESR: markedly elevated (>100 mm/hr — often >150)
- KFT: raised creatinine (myeloma kidney), hypercalcemia
- Urine: Bence Jones protein (dipstick may be negative — requires sulfosalicylic acid test or specific immunoelectrophoresis)
Key Diagnostic Pearls:
| Rule | Clinical Implication |
|---|---|
| Never interpret one test in isolation | Always correlate with history and examination |
| CRP rises fast, falls fast | Use for acute infection monitoring |
| ESR rises slowly, falls slowly | Use for chronic disease activity |
| LFT patterns > single values | AST:ALT ratio is more informative than either alone |
| Creatinine is insensitive | GFR may be halved before creatinine rises |
| RBC casts = GN until proven otherwise | Act immediately — nephrology referral |
| Urine glucose with normal blood glucose | Think Fanconi syndrome or proximal RTA |
| Sterile pyuria | Think TB, AIN, inadequately treated UTI |
| ESR >100 | Always think myeloma, TB, temporal arteritis |
| BUN:Creatinine >20 | Pre-renal cause, look for dehydration/GI bleed |
Sources: Henry's Clinical Diagnosis and Management by Laboratory Methods; Comprehensive Clinical Nephrology 7th Ed.; Rosen's Emergency Medicine; Fitzpatrick's Dermatology; Textbook of Family Medicine 9e.
This is a comprehensive framework for interpreting basic investigations in clinical practice. For every patient, always combine test results with the clinical history and physical examination — no investigation result should ever be interpreted in isolation.
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