Metabolic acidosis and its evaluation

Definition and the ageing kidney. Metabolic acidosis is a primary fall in serum bicarbonate (<22 mEq/L) with a low arterial pH (<7.35). The ageing kidney handles an acute acid load poorly — older people acidify the urine more slowly and take longer to recover, so acidosis is both commoner and more prolonged in old age.

First step — the anion gap. AG = Na⁺ − (Cl⁻ + HCO₃⁻), normal 8–12 mEq/L. Always correct for albumin: add 2.5 mEq/L to the measured gap for every 1 g/dL fall in serum albumin below 4 g/dL — otherwise a high-gap acidosis is missed in the malnourished older patient.

High anion gap causes (GOLD MARK): Glycols (ethylene glycol, propylene glycol), Oxoproline (chronic paracetamol), L-lactate (sepsis, shock, metformin), D-lactate, Methanol, Aspirin, Renal failure (uraemia), Ketoacidosis (diabetic, alcoholic, starvation).

Normal anion gap (hyperchloraemic) causes: diarrhoea, renal tubular acidosis — of which type 4 (hyporeninaemic hypoaldosteronism) is the classic one in the older diabetic with CKD — ureteric diversion, excessive saline, and drugs (acetazolamide, spironolactone, trimethoprim, amiloride).

Stepwise evaluation. ABG/VBG with simultaneous electrolytes → confirm the acidosis → calculate albumin-corrected anion gap → if high, calculate the delta ratio (ΔAG/ΔHCO₃) to detect a mixed disorder → if normal, calculate the urine anion gap (U + U − U ): a positive gap means RTA, a negative gap means GI Na K Cl bicarbonate loss → finally check respiratory compensation with Winter's formula (expected pCO₂ = 1.5 × HCO₃ + 8 ± 2).

Management. Treat the underlying cause — fluids and antibiotics for sepsis, insulin and fluids for ketoacidosis, dialysis for severe uraemia or toxic alcohols. Sodium bicarbonate is reserved for severe acidaemia (pH <7.1) or chronic CKD acidosis. In older adults, remember that bicarbonate carries a sodium load that can precipitate volume overload and heart failure, and rapid correction can cause hypokalaemia and hypocalcaemia. Low pH + low HCO₃ = metabolic acidosis ↓ Calculate albumin-corrected anion gap ↓ High AG Normal AG Lactate · Ketones · Urea · Toxins → urine anion gap → check delta ratio Positive = RTA · Negative = GI loss LATEST UPDATE (2023–PRESENT) — 1 POINT

Treating chronic metabolic acidosis of CKD is now standard renoprotective care. Current KDIGO-aligned practice is to give oral sodium bicarbonate to maintain serum bicarbonate ≥22 mEq/L in CKD, because chronic acidosis accelerates GFR decline and drives muscle protein catabolism, sarcopenia and bone demineralisation — all of which are especially damaging in the frail older patient. The hydrochloric-acid–binding polymer veverimer, developed as a sodium-free alternative, did not meet its primary endpoint in the VALOR-CKD trial, so sodium bicarbonate remains the treatment of choice, with careful attention to volume status and blood pressure in older adults.