Normal anion gap is influenced by which serum constituent according to the material?

• A. Serum Albumin
• B. Serum Calcium
• C. Serum Sodium
• D. Serum Phosphate

Answer: (A)

The normal anion gap is determined by unmeasured anions in the blood, and the largest contributor among these is albumin, a negatively charged protein. When albumin levels rise, there are more unmeasured negative charges, which can raise the gap; when albumin falls (hypoalbuminemia), these unmeasured charges drop and the gap decreases. This is why albumin heavily influences the baseline or “normal” value of the anion gap. In practice, changes in albumin can shift the gap even if the measured ions (sodium, chloride, bicarbonate) stay the same, and the common rule is that each 1 g/dL decrease in albumin lowers the gap by about 2.5 mEq/L. Calcium, sodium, and phosphate are not the primary determinants of the normal gap in the same way; sodium is part of the calculated value, while the others contribute less to the unmeasured anions that set the baseline.