How is anion gap calculated?
The presence of new anions in plasma can be detected with the calculation of the P Anion gap .
Electroneutrality requires that Na + + unmeasured cations (UC) = Cl − + HCO 3 − + unmeasured anions (UA). Actually, almost everything is measured, but for simplicity’s sake we pretend that the potassium, calcium, phosphate, and others are not.
Then, rearranging, Na + – Cl − – HCO 3 − = UA – UC = anion gap. Assuming some usual normal values, 140 – 104 – 24 = 12 (normal range 6 to 12). The major UA is albumin—about 3 mEq negative charge for each g/dL. So with normal albumin concentration of 4.0 g/dL, it contributes 12 mEq/L, roughly equal to the anion gap. From that equation, it can be seen that an increase in the anion gap is due to an increase in the UA (e.g., lactate) and/or a decrease in the UC (e.g., globulins).
Accordingly, the baseline value of the P Anion gap must be adjusted for the P Albumin . A rough guide for correcting the baseline value of P Anion gap for P Albumin is that at P Albumin of 4.0 g/dL (40 g/L), the P Anion gap is 12 mEq/L; hence for every 1.0 g/dL (10 g/L) decrease in P Albumin , the P Anion gap will be lower by ∼3 mEq/L. The converse is true for a rise in the P Albumin