Anion gap calculation for differential diagnosis of metabolic acidosis.
The anion gap is the difference between measured cations and anions in plasma. It reflects unmeasured anions (phosphates, sulfates, proteins, organic acids). Formula: AG = Na – (Cl + HCO3). Normal range – 8-12 mmol/L. With hypoalbuminemia an albumin-corrected value is used: AG corrected = AG + 2.5 × (4.4 – albumin g/dL). Clinical application. Differential diagnosis of metabolic acidosis. Elevated AG (>12) – accumulation of unmeasured anions: lactic acidosis (sepsis, shock, metformin), ketoacidosis (diabetic, alcoholic, starvation), ethylene glycol, methanol, salicylate poisoning, uremia, rhabdomyolysis. Normal AG – bicarbonate loss: diarrhea, renal tubular acidosis, carbonic anhydrase inhibitors. Delta gap. For diagnosis of mixed acid-base disorders use delta-gap: (AG – 12) / (24 – HCO3). Around 1 – pure high AG acidosis. >2 – concurrent metabolic alkalosis. <0.4 – concurrent normal AG acidosis. Limitations. Lithium, bromide, iodide can falsely lower AG. Hypoalbuminemia drops baseline by about 2.5 mmol/L per 1 g/dL albumin deficit – without correction, true AG can be missed in septic and oncology patients.
Emmett M, Narins RG. Clinical use of the anion gap. Medicine (Baltimore). 1977;56(1):38-54.
Formula version: 2024-v1
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