Anion Gap Calculation Does NOT Use Corrected Sodium
When calculating the anion gap on a comprehensive metabolic panel, you should always use the measured (uncorrected) serum sodium value, even in the setting of hyperglycemia. 1
Why Use Measured Sodium for Anion Gap
The anion gap calculation is designed to detect unmeasured anions (lactate, ketoacids, uremic toxins, ingested toxins) and is calculated as: Na⁺ − (HCO₃⁻ + Cl⁻), with normal values of 10–12 mEq/L. 2
The American Diabetes Association explicitly recommends using measured (uncorrected) sodium for calculating effective serum osmolality and monitoring treatment in hyperglycemic crises, not the corrected sodium. 1
The anion gap reflects the actual concentration of unmeasured anions in the serum at the time of measurement, and using measured sodium preserves this relationship. 3
An anion gap >12 mEq/L indicates accumulation of unmeasured anions regardless of the glucose level. 2
When Corrected Sodium IS Used (Not for Anion Gap)
Corrected sodium serves an entirely different clinical purpose—it guides fluid therapy decisions in hyperglycemic crises. 1
The formula for corrected sodium is: Corrected [Na⁺] = Measured [Na⁺] + 1.6 × ([Glucose in mg/dL - 100]/100). 1, 4
After initial resuscitation in DKA or HHS, if corrected sodium is normal or elevated, switch to 0.45% NaCl at 4-14 ml/kg/h; if corrected sodium is low, continue 0.9% NaCl at the same rate. 1
The American Diabetes Association explicitly warns: "never use measured sodium alone to guide fluid choice"—but this refers to fluid selection, not anion gap calculation. 1
Clinical Algorithm for Hyperglycemic Acidosis
When evaluating a patient with hyperglycemia and suspected metabolic acidosis:
Calculate anion gap using measured sodium: AG = Na⁺ − (HCO₃⁻ + Cl⁻). 2
If AG >12 mEq/L, suspect high anion gap metabolic acidosis (HAGMA) from DKA, lactic acidosis, or other causes. 2, 3
Separately calculate corrected sodium to guide fluid therapy: add 1.6 mEq/L for each 100 mg/dL glucose above 100 mg/dL. 1, 4
Use effective osmolality for severity assessment: 2[measured Na (mEq/L)] + glucose (mg/dL)/18—again using measured sodium. 1
Common Pitfalls to Avoid
Do not "correct" the sodium before calculating the anion gap—this will artificially alter the AG and potentially mask a HAGMA. 1, 3
In severe hyperglycemia (e.g., glucose 900 mg/dL), the measured sodium may appear falsely low due to osmotic water shifts, but this measured value is still correct for anion gap calculation. 1, 5
The anion gap can be affected by hypoalbuminemia (decreases AG by ~2.5 mEq/L per 1 g/dL decrease in albumin) and should be corrected for albumin if present, but never for glucose. 3
Adjusting chloride and bicarbonate for water balance (reflected by sodium concentration) can improve their correlation, but this is a separate analytical exercise from calculating the anion gap for clinical diagnosis. 6
Monitoring During Treatment
Check serum electrolytes, blood glucose, calculated effective osmolality (using measured sodium), and venous pH every 2-4 hours during DKA/HHS treatment. 1, 2
Recalculate the anion gap with each set of labs using the measured sodium to track resolution of the acidosis. 2
The corrected sodium may change during treatment due to ongoing osmotic diuresis and should be recalculated to guide ongoing fluid therapy. 5