Chemical Pathology and Metabolic Disorders
Electrolytes and acid-base
Master the interpretation of sodium, potassium, chloride, and bicarbonate. Hyponatraemia is classified by volume status and osmolality; SIADH causes euvolaemic hypo-osmolar hyponatraemia with inappropriately concentrated urine. Hyperkalaemia has ECG consequences (peaked T waves, widened QRS) and is treated with calcium, insulin-dextrose, and beta-agonists. Acid-base interpretation uses the Henderson-Hasselbalch relationship, the anion gap, and compensation rules; high anion gap acidosis includes ketoacidosis, lactic acidosis, renal failure, and toxins (methanol, ethylene glycol, salicylates).
Glucose and lipids
Diabetes diagnosis uses fasting glucose, oral glucose tolerance, HbA1c, or random glucose with symptoms. Know diabetic ketoacidosis (ketones, anion gap, potassium shifts) versus hyperosmolar hyperglycaemic state. Lipid disorders: familial hypercholesterolaemia (LDL receptor defect) causes tendon xanthomas and premature atherosclerosis.
Liver, renal, and endocrine markers
Liver enzymes: ALT is more liver-specific than AST; alkaline phosphatase and GGT suggest cholestasis. Bilirubin metabolism distinguishes prehepatic (unconjugated), hepatic, and posthepatic (conjugated, dark urine) patterns. Renal markers include creatinine, urea, and eGFR; the urea-to-creatinine ratio rises in pre-renal states. Thyroid function tests: TSH is the most sensitive first-line test; primary hypothyroidism shows high TSH and low free T4.
Traps
Do not interpret an isolated abnormal result without considering pre-analytical error (haemolysis raises potassium and LDH). Correct sodium for hyperglycaemia. A normal anion gap does not exclude acidosis (hyperchloraemic acidosis).
Rule of thumb
For acid-base questions, calculate the anion gap first, then assess compensation; if the anion gap is elevated, check for a mixed disorder using the delta ratio.