Comprehensive Metabolic Panel (CMP)

What it measures

Preparation

• Fasting for 8-12 hours is usually requested, because glucose is affected by meals. Water is fine.
• Tell the clinician about your medications and supplements. Biotin, diuretics, steroids, and some antibiotics can affect results.

Common abnormal patterns

• A high BUN and creatinine suggest reduced kidney function (AKI or CKD) or dehydration.
• A BUN:creatinine ratio above 20 suggests prerenal causes, such as volume depletion or GI bleeding.

• Low sodium: SIADH, heart failure, cirrhosis, diuretics, or excess water intake.
• High potassium: renal failure, ACE inhibitors, K-sparing diuretics, or hemolyzed sample. It can cause arrhythmias, so it needs prompt attention.
• Low bicarbonate: metabolic acidosis, such as DKA, lactic acidosis, or renal failure.
• High bicarbonate: vomiting, diuretics, or compensation for chronic respiratory acidosis.
• Anion gap = Na − (Cl + HCO3). A high gap points to causes such as DKA, lactic acidosis, uremia, or toxic ingestions.

• Fasting glucose of 126 mg/dL or higher on two occasions suggests diabetes.
• A fasting value of 100-125 mg/dL suggests prediabetes.

• High calcium is most often primary hyperparathyroidism or malignancy.
• Low calcium: hypoalbuminemia (correct for albumin), vitamin D deficiency, hypoparathyroidism, or CKD.

• A hepatocellular pattern (ALT and AST much higher than ALP) suggests viral hepatitis, fatty liver, alcohol or drug injury. AST:ALT above 2 suggests alcohol-related disease.
• A cholestatic pattern (ALP and bilirubin higher than ALT and AST) suggests bile duct obstruction, PBC or PSC, or infiltrative disease.
• Low albumin suggests chronic liver disease, nephrotic syndrome, malnutrition, or inflammation.

Limits

• It does not diagnose a specific disease. Abnormal results lead to targeted follow-up, such as an HbA1c, a urinalysis, a lipid panel, hepatitis serologies, or imaging.
• Mildly abnormal single values are often transient and are rechecked.
• It does not include magnesium, phosphate, or lipids.

Slide 1: Title

Slide 2: What is in the CMP?

• 14 tests, CPT 80053
• Glucose
• Kidney: BUN, creatinine
• Electrolytes and acid-base: sodium, potassium, chloride, bicarbonate (CO2), calcium
• Protein: albumin, total protein
• Liver: ALT, AST, ALP, total bilirubin
• BMP = first 8 tests only (glucose, BUN, creatinine, Na, K, Cl, CO2, Ca)
• Renal function panel adds phosphorus and drops the liver enzymes

Slide 3: A systematic approach

1. Check sample quality (hemolysis, fasting status, IV contamination)
2. Look at the trend, not a single value
3. Group results into organ systems
4. Find patterns across groups
5. Match against symptoms, medications, and history
6. Decide: repeat, add targeted tests, or act now

Slide 4: Reference ranges (adult, typical)

Slide 5: Glucose

• Fasting 100-125 mg/dL: prediabetes range
• Fasting 126 mg/dL or higher, confirmed: diabetes range
• Below 70 mg/dL: hypoglycemia (insulin or sulfonylureas, adrenal insufficiency, liver failure, sepsis, insulinoma)
• Always ask about fasting status
• Confirm with HbA1c or repeat testing

Slide 6: Kidney function (BUN and creatinine)

• Both high: AKI or CKD
• BUN:creatinine above 20: prerenal causes (dehydration, heart failure, GI bleed, high protein intake, steroids)
• Ratio 10-15: intrinsic renal disease
• Creatinine depends on muscle mass, so it can look falsely normal in frail or elderly patients
• Calculate eGFR and look at the trend over time
• Next steps: urinalysis, urine albumin:creatinine ratio, renal ultrasound

Slide 7: Sodium and potassium

• Low: assess volume status and osmolality (SIADH, heart failure, cirrhosis, diuretics, polydipsia)
• High: free water deficit (dehydration, diabetes insipidus)
• Correct slowly to avoid osmotic demyelination

• High: renal failure, ACE inhibitors or ARBs, K-sparing diuretics, acidosis, or a hemolyzed sample
• Low: diuretics, vomiting, diarrhea, hypomagnesemia
• Severe changes cause arrhythmias, so check an ECG

Slide 8: Chloride, bicarbonate, and the anion gap

• Anion gap = Na − (Cl + HCO3)
• Normal 8-12 mmol/L
• High gap: unmeasured acid or toxin (DKA, lactic acidosis, uremia, ethylene glycol, methanol, salicylates)
• Normal gap with low HCO3: diarrhea, renal tubular acidosis, saline loading
• High HCO3: vomiting, diuretics, or compensation for chronic respiratory acidosis
• Worked example from the text: Na 144, Cl 108, HCO3 14 gives a gap of 22 (ethylene glycol ingestion)
• If the gap is high, consider an osmolal gap, lactate, and ketones

Slide 9: Calcium and albumin

• Correct calcium for albumin: corrected Ca = measured Ca + 0.8 × (4.0 − albumin)
• High calcium: primary hyperparathyroidism (outpatient) or malignancy (inpatient) most common
• Low calcium: low albumin, vitamin D deficiency, hypoparathyroidism, CKD, low magnesium
• Low albumin: liver disease, nephrotic syndrome, malnutrition, inflammation
• High total protein with low albumin suggests a gammopathy, so consider SPEP

Slide 10: Liver tests

• Hepatocellular pattern: ALT and AST markedly higher than ALP (viral hepatitis, fatty liver, drugs, ischemia)
• Cholestatic pattern: ALP and bilirubin higher than ALT and AST (bile duct obstruction, PBC, PSC, infiltrative disease)
• AST:ALT above 2: suggests alcohol-related injury
• AST and ALT in the thousands: ischemic injury, acetaminophen toxicity, acute viral hepatitis
• Isolated indirect hyperbilirubinemia: Gilbert syndrome or hemolysis
• ALT and AST are injury markers. Albumin and INR reflect synthetic function.

Slide 11: Patterns that matter

Slide 12: Pitfalls

• Hemolysis falsely raises K, AST, and LDH
• Hyperglycemia or hyperlipidemia lowers measured sodium (pseudohyponatremia)
• Drawing from an IV line contaminates results
• Reference ranges differ by lab, age, sex, and pregnancy
• Mild single abnormalities are often transient, so repeat before extensive workup
• Biotin supplements can interfere with some assays

Slide 13: When to act urgently

• K above 6.0 mmol/L or ECG changes
• Na below 120 mmol/L or symptoms
• Glucose below 54 mg/dL or above 400 mg/dL, or high anion gap acidosis
• Corrected Ca above 14 mg/dL, or symptomatic hypocalcemia
• Rapid creatinine rise (AKI)
• ALT or AST in the thousands, or jaundice with coagulopathy

Slide 14: Case example

• Anion gap = 128 − (96 + 18) = 14 (mildly raised)
• BUN:Cr = 20, suggesting a prerenal component
• Pattern: AKI with hyperkalemia and acidosis
• Next steps: ECG, volume assessment, stop nephrotoxins, urinalysis, and treat the hyperkalemia

Slide 15: Key takeaways

• Interpret by organ system and then by pattern
• Always calculate the anion gap and correct calcium for albumin
• Trends beat single values
• Match results to the clinical picture
• Use abnormal results to guide targeted follow-up testing

Slide 16: References

• Henry's Clinical Diagnosis and Management by Laboratory Methods (CPT 80053 panel composition)
• Roberts and Hedges' Clinical Procedures in Emergency Medicine (anion gap, osmolal gap)