Nephrology and Fluid-Electrolyte Disorders
This section tests systematic acid-base and electrolyte reasoning, AKI staging, and recognition of when dialysis can no longer wait.
Acute kidney injury
KDIGO: creatinine rise ≥0.3 mg/dL within 48 h, ≥1.5× baseline within 7 days, or urine output <0.5 mL/kg/h for 6 h. Stage 3 is ≥3× baseline, creatinine ≥4.0 mg/dL, or initiation of renal replacement therapy. Urgent indications for dialysis: refractory hyperkalaemia, acidosis, pulmonary oedema, uraemic encephalopathy/pericarditis, and toxin removal.
Hyponatraemia
First check serum osmolality. If hypo-osmolar, check urine osmolality: <100 mOsm/kg suggests primary polydipsia; >100 means measure urine sodium. Urine Na <30 mmol/L suggests hypovolaemia; >30 in a euvolemic patient suggests SIADH. Correct by ≤8-10 mmol/L per 24 h to prevent osmotic demyelination; use 3% saline with a DDAVP clamp when seizures occur.
Hyperkalaemia
Calcium gluconate for ECG changes (membrane stabilisation, no change in potassium), then insulin with glucose and nebulised salbutamol to shift potassium intracellularly, then removal with diuretics, binders, or dialysis.
Acid-base
Winter's formula: expected pCO2 = 1.5 × HCO3 + 8 ± 2. Correct the anion gap for albumin (add 2.5 per 1 g/dL below 4).
Worked example
pH 7.25, HCO3 12, pCO2 26, Na 138, Cl 100, albumin 2.0. Anion gap = 138 − (100 + 12) = 26; albumin-corrected = 31, so this is a high-anion-gap acidosis. Expected pCO2 = 1.5(12) + 8 = 26 ± 2, which matches - compensation is appropriate and there is no second respiratory disorder.
Traps that cost marks
- Correcting sodium faster than 8-10 mmol/L/day.
- Using calcium as if it lowered potassium.
- Accepting a simple acid-base label without checking Winter's formula.