Anion Gap

A raised gap points to added unmeasured anions — think lactate, ketones, urate in renal failure, and toxic alcohols, salicylate or metformin in poisoning. A normal gap with acidosis suggests bicarbonate loss (diarrhoea, renal tubular acidosis) or chloride excess, including from large-volume saline. Hypoalbuminaemia masks a raised gap: every 10 g/L fall lowers the apparent gap by about 2.5 mmol/L, so an “normal” gap in a hypoalbuminaemic patient can conceal a significant acidosis — correct before concluding otherwise.