Serum Anion Gap

Anion gap

The anion gap (AG) is an estimate of the anions in plasma that routine panels don't measure: AG = Na⁺ - (Cl⁻ + HCO₃⁻). Most labs treat 8–12 mEq/L as normal, though some that run newer ion-selective electrodes use 6–10. Plug in Na 140, Cl 104, HCO₃⁻ 24 and you get AG = 12, which sits at the upper edge of normal. Albumin is itself a major unmeasured anion, so for every 1 g/dL it drops, add about 2.5 to the measured value to get the corrected AG.

For high-AG metabolic acidosis the old mnemonic MUDPILES still does the job: Methanol, Uremia, Diabetic ketoacidosis, Paraldehyde, Iron/Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates. A normal-AG (hyperchloremic) acidosis points elsewhere, toward diarrhea, renal tubular acidosis, large saline infusions or ureteral diversions. Either way, the AG is what tells you which path you're on.

Clinical applications

It shows up in acute emergencies where lactic acidosis is in play (shock, sepsis, polytrauma), in poisonings from methanol, ethylene glycol or salicylates, in endocrinology with DKA or metformin-related lactic acidosis, in nephrology with uremia and renal tubular acidosis, and in day-to-day ICU monitoring. When a picture looks mixed, the delta gap (ΔAG/ΔHCO₃⁻) helps tease the disturbances apart.

FAQ

Should I correct AG for albumin? In critically ill or chronically malnourished patients, yes. Skip the correction and a hypoalbuminemic patient may read as "normal" when the true gap is actually high.

Low AG, what does it mean? Think hypoalbuminemia, lithium intoxication, multiple myeloma with its cationic paraproteins, severe hypercalcemia or hypermagnesemia, or simply a lab error.

AG with and without K⁺? Potassium isn't in the classic formula. A few authors do fold it in with AG = (Na + K) - (Cl + HCO₃), which pushes the normal range up to 12–16. Whichever version you choose, just be consistent about it.