What is the calculated anion gap for an elderly man with stage 2 chronic kidney disease, albumin 4.2 g/dL, normal calcium, chloride 106 mmol/L, bicarbonate 29 mEq/L (assuming a serum sodium around 140 mEq/L)?

Anion Gap Calculation

For this patient with albumin 4.2 g/dL, sodium ~140 mEq/L, chloride 106 mmol/L, and bicarbonate 29 mEq/L, the calculated anion gap is 5 mEq/L using the standard formula, which is at the lower end of the normal range (5-12 mmol/L). 1

Calculation Method

Using the standard anion gap formula: AG = Na⁺ - (Cl⁻ + HCO₃⁻) 2

• Sodium: 140 mEq/L (assumed)
• Chloride: 106 mmol/L
• Bicarbonate: 29 mEq/L

Anion gap = 140 - (106 + 29) = 5 mEq/L 2

Interpretation in Context

Normal Reference Range

• The modern reference range for anion gap using ion-selective electrode methodology is 5-12 mmol/L (not the outdated 8-16 mmol/L range). 1
• This patient's value of 5 mEq/L falls at the lower boundary of normal. 1

Albumin Correction Considerations

In this case, albumin correction is NOT necessary because the albumin is normal at 4.2 g/dL. 3, 4

• The correction formula (adjusted AG = observed AG + 0.25 × [normal albumin - observed albumin] in g/L, or 2.5 if using g/dL) is only applied when albumin deviates from the normal value of 4.0 g/dL. 4, 5
• Since this patient's albumin is 4.2 g/dL (essentially normal), the observed anion gap already accurately reflects the true gap anion concentration. 4
• Each g/L decrease in serum albumin causes the observed anion gap to underestimate total gap anions by 0.25 mEq/L, but this patient has normal albumin. 4

Clinical Significance in Stage 2 CKD

This low-normal anion gap (5 mEq/L) with normal albumin and mid-range calcium is reassuring and does not suggest metabolic acidosis or accumulation of unmeasured anions. 1, 6

• The elevated bicarbonate (29 mEq/L, upper limit of normal) combined with low-normal anion gap suggests no significant acid-base disturbance. 2, 6
• In Stage 2 CKD with preserved kidney function, metabolic acidosis is uncommon, and this patient shows no evidence of it. 2
• An anion gap <2 mmol/L would be concerning for laboratory error or specific conditions like multiple myeloma, but 5 mEq/L is within normal limits. 1

Important Clinical Caveats

• Do not apply the outdated reference range of 8-16 mmol/L, as this leads to misinterpretation when using modern ion-selective electrode methodology. 1
• Anion gaps exceeding 24 mmol/L suggest significant metabolic acidosis and warrant immediate investigation. 1
• In patients with hypoalbuminemia, failure to correct the anion gap can mask significant gap acidosis, but this does not apply to this patient with normal albumin. 4, 5
• The correction formula itself has limitations and should not be routinely applied when albumin is normal, as the law of electroneutrality already accounts for protein effects in the measured electrolytes. 7