Anion gap means

i. The sum of cations and anions in ECF is always equal, so as to maintain the electrical neutrality. Sodium and potassium together account for 95% of the cations whereas chloride and bicarbonate account for only 86% of the anions. Only these electrolytes are commonly measured.ii. Hence, there is always a difference between the measured cations and the anions. The unmeasured anions constitute the anion gap. This is due to the presence of protein anions, sulphate, phosphate and organic acids. iii. The anion gap is calculated as the difference between (Na+ + K+) and (HCO3- + Cl-). Normally this is about 12 mmol/L.Ref: D M Vasudevan 7th edition Page no: 399