The venous anion gap typically falls within which range?

• A. 0–3 mmol/L
• B. 5–14 mmol/L
• C. 15–25 mmol/L
• D. 25–35 mmol/L

Answer: (B)

The concept here is the anion gap, which estimates unmeasured anions in the blood. It’s calculated as the difference between the measured cation (usually sodium) and the sum of the measured anions (chloride plus bicarbonate): Na+ − (Cl− + HCO3−). In venous blood, this gap is normally around 5 to 14 mmol/L, allowing for small lab-to-lab variations in measurements and whether potassium is included in the sodium value. Values within this range indicate a normal balance of ions. A higher gap suggests accumulation of unmeasured anions (lactate, ketoacids, kidney failure toxins, etc.), pointing to metabolic acidosis with an increased anion gap. A lower gap is less common and can occur with artifacts or conditions like hypoalbuminemia. So the range 5–14 mmol/L best reflects the typical normal venous anion gap.