Version May 2024
| Cause | Comments |
| Low serum anion gap | |
| Laboratory error | Most frequent cause of low anion gap |
| Underestimation of serum sodium | Most frequent with severe hypernatremia or hypertriglyceridemia |
| Overestimation of serum chloride | Rare with ion-selective electrodes |
| Overestimation of serum bicarbonate | Spurious elevation in serum HCO3– if cells not separated from sera |
| Hypoalbuminemia | Second-most common cause of low serum anion gap |
| Monoclonal IgG gammopathy | Level of anion gap correlates with serum concentration of paraprotein |
| Polyclonal gammopathy | Might be more common cause of low anion gap than monoclonal gammopathy |
| Bromide intoxication | Anion gap depends on serum bromide concentration |
| Lithium intoxication | Low anion gap with lithium >4 mEq/L |
| Hypercalcemia | Inconsistent finding more likely with hypercalcemia associated with primary hyperparathyroidism |
| Hypermagnesemia | Theoretical cause but not documented in literature |
| Polymyxin B | Anion gap depends on serum level; occurs with preparation with chloride |
| Iodide intoxication | Rare cause |
| Negative serum anion gap | |
| Laboratory error | May be most common cause |
| Bromide intoxication | Second-most common cause; values as low as –60 mEq/L reported |
| Multiple myeloma | Rare cause, more likely to have low anion gap |
| Iodide intoxication | Rare cause of negative anion gap |
| Lithium[1] | Negative anion gap with extreme elevations of serum lithium |
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