Nephrology and Fluid/Electrolyte Disorders: AKI, CKD, Acid-Base and Electrolytes
Core concepts
Nephrology questions cover acute kidney injury (AKI), chronic kidney disease (CKD), glomerulonephritis, nephrotic syndrome, acid-base disorders and electrolyte emergencies. Stage AKI using creatinine and urine output criteria. Differentiate prerenal azotaemia from acute tubular necrosis using history, urinalysis, urine sodium and fractional excretion of sodium (FENa), remembering diuretics can raise FENa. CKD management includes blood pressure control, ACE inhibitor or ARB where appropriate, SGLT2 inhibitors, avoiding nephrotoxins and preparing for renal replacement therapy. Understand hyponatraemia, hyperkalaemia, metabolic acidosis and the indications for urgent dialysis (AEIOU).
Traps
FENa is unreliable in CKD, diuretic use and contrast nephropathy; fractional excretion of urea may help. A normal anion gap acidosis suggests bicarbonate loss or renal tubular acidosis, whereas a raised anion gap suggests ketoacidosis, lactic acidosis, uraemia or toxins. Correct sodium for hyperglycaemia before interpreting hyponatraemia. Do not give potassium in DKA before checking potassium. Hyperkalaemia with ECG changes needs calcium gluconate first, then insulin-dextrose and removal.
Worked example / rule of thumb
A dehydrated patient has creatinine 180 micromol/L, urine sodium 10 mmol/L, FENa 0.5%, bland urine and improves after saline. This is prerenal AKI. Rule: FENa below 1% usually suggests prerenal physiology, but always interpret with diuretics, CKD and clinical volume status.