Biochemistry · Short notes

Clinical Biochemistry: Short Notes & Cheat Sheet

Reference ranges, acid–base disorders and the anion gap, liver, kidney and cardiac function tests, and other common laboratory markers.

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Short notes

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The four acid–base disorders

Metabolic acidosis↓ HCO₃⁻: DKA, lactic, diarrhoeaMetabolic alkalosis↑ HCO₃⁻: vomiting, diureticsRespiratory acidosis↑ CO₂: COPD, opioid overdoseRespiratory alkalosis↓ CO₂: anxiety, early salicylateAnion gapNa⁺ − (Cl⁻ + HCO₃⁻) ≈ 8–12CompensationLungs fast, kidneys slowAcid–base
Look at the pH first, then decide whether the primary change is in bicarbonate (metabolic) or CO₂ (respiratory).

Acid–base

  • Normal arterial pH is 7.35–7.45, PaCO₂ 35–45 mmHg and bicarbonate 22–26 mmol/L.
  • Metabolic acidosis (low bicarbonate) with a high anion gap: methanol, uraemia, DKA, lactic acidosis, ethylene glycol, salicylates, iron or isoniazid.
  • Normal-anion-gap metabolic acidosis: diarrhoea and renal tubular acidosis.
  • Metabolic alkalosis: vomiting and diuretics. Respiratory acidosis: hypoventilation (COPD, opioids). Respiratory alkalosis: hyperventilation (anxiety, early salicylate poisoning).
  • Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻), normally about 8–12 mmol/L.

Liver and kidney

  • Raised ALT and AST suggest hepatocellular damage; raised ALP and GGT suggest cholestasis. An AST:ALT ratio above 2 suggests alcoholic liver disease.
  • Unconjugated bilirubin rises in haemolysis and Gilbert's syndrome; conjugated bilirubin rises in obstruction (dark urine, pale stools).
  • Albumin and prothrombin time reflect the liver's synthetic function.
  • Kidney function: serum creatinine and eGFR (CKD-EPI). For drug dosing, creatinine clearance is often estimated by Cockcroft–Gault: (140 − age) × weight ÷ (72 × serum creatinine in mg/dL), × 0.85 for women.
  • A high urea-to-creatinine ratio suggests a prerenal cause (dehydration, GI bleeding).

Cardiac and other markers

  • Troponin I or T is the most specific marker of myocardial injury: it rises in 3–6 hours, peaks at 12–24 hours and stays raised for 7–10 days.
  • CK-MB returns to normal in 2–3 days, so it helps detect re-infarction.
  • BNP and NT-proBNP rise in heart failure.
  • Lipase is more specific than amylase for pancreatitis.
  • TSH is the first-line thyroid test: high TSH with low free T4 means primary hypothyroidism.
  • CRP and ESR are general inflammation markers; procalcitonin suggests bacterial infection.

Cheat sheet

High-yield facts in one place. Cover the right-hand column and test yourself.

Common reference ranges (adults)

TestApproximate range
Fasting glucose70–99 mg/dL
Sodium135–145 mmol/L
Potassium3.5–5.0 mmol/L
Bicarbonate22–26 mmol/L
Creatinine≈ 0.6–1.2 mg/dL
Calcium (total)8.5–10.5 mg/dL
Albumin3.5–5.0 g/dL
Total bilirubin0.3–1.2 mg/dL
Total cholesterol< 200 mg/dL
Triglycerides< 150 mg/dL
Arterial pH7.35–7.45

Pattern → meaning

PatternSuggests
↑ ALT, ↑ ASTHepatocellular injury
↑ ALP, ↑ GGTCholestasis
AST : ALT > 2Alcoholic liver disease
↑ Unconjugated bilirubinHaemolysis, Gilbert's
↑ TroponinMyocardial injury
↑ LipaseAcute pancreatitis
↑ TSH, ↓ free T4Primary hypothyroidism

Memory aids

MUDPILES
High anion gap acidosis: Methanol, Uraemia, DKA, Propylene glycol, Iron/Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates.

Exam traps

  • Salicylate poisoning causes respiratory alkalosis first and metabolic acidosis later.
  • Diarrhoea causes a normal-anion-gap acidosis; vomiting causes metabolic alkalosis.
  • Troponin stays raised for about a week, so CK-MB is better for detecting re-infarction.
  • Cockcroft–Gault estimates creatinine clearance for drug dosing; it is not the same as eGFR.

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