Low Anion Gap
A low anion gap — conventionally under 8 mEq/L — is rare, encountered in well under 1% of routine chemistry panels (Kraut & Madias, StatPearls 'Serum Anion Gap'). Unlike a high gap, it is seldom an emergency, but it can be the first clue to serious underlying disease such as multiple myeloma or toxin exposure. Here is what a decreased anion gap means, its causes, and a practical workup.
What a low anion gap means
The anion gap is the difference between measured cations and measured anions: AG = Na⁺ − (Cl⁻ + HCO₃⁻). It exists because the serum also contains unmeasured ions — mostly anions such as albumin and phosphate — that fill the gap. A low anion gap therefore means one of three things: the pool of unmeasured anions has shrunk (most often because albumin has fallen), the pool of unmeasured cations has expanded (paraproteins in myeloma, cations such as lithium or bromide), or one of the measured values is wrong (laboratory error). Because the gap is normally small to begin with, any of these forces can push it below 8 or even make it negative.
The first practical step whenever the anion gap reads low is to confirm it. A single abnormal value may reflect a sampling or assay artifact, so repeat the chemistry panel before launching an extensive workup. If the low gap persists, the causes below come into focus, with hypoalbuminemia overwhelmingly the most common.
Causes of a low anion gap
| Cause | Mechanism | Notes |
|---|---|---|
| Hypoalbuminemia | Fewer albumin anions shrink the unmeasured-anion pool. | By far the most common cause; each 1 g/dL albumin drop lowers the gap ~2.5 mEq/L (Figge et al., Critical Care Medicine). |
| Paraproteinemia (multiple myeloma, MGUS) | Monoclonal immunoglobulins carry a net positive charge, adding unmeasured cations. | A persistently low or negative AG can be the presenting clue to myeloma. |
| Bromide toxicity | Bromide is measured as chloride by the assay, inflating Cl⁻ and collapsing the gap. | History of pyridostigmine bromide, sedatives, or fumigant exposure. |
| Hypercalcemia / hypermagnesemia | Extra unmeasured divalent cations narrow the gap. | Usually only matters at marked elevations. |
| Lithium therapy | Lithium cation accumulates as an unmeasured cation. | Seen at therapeutic-to-toxic lithium levels. |
| Laboratory error | Falsely high Cl⁻ or falsely low Na⁺ from assay interference. | Always repeat before pursuing rare diagnoses. |
For a deeper differential and the clinical scenarios behind each entry, the dedicated causes of low anion gap page expands each category with workup specifics. You can also feed the patient's actual electrolytes into the anion gap calculator to confirm the arithmetic and apply the albumin correction, which often "normalizes" a low gap that is simply albumin-driven.
Workup approach
- Repeat the panel. Confirm the low gap is reproducible before investigating further — lab error is common enough to rule out first.
- Check the albumin. If albumin is low, the explanation is usually found — apply the Figge correction and the apparent gap will often rise into the normal band.
- Consider paraproteinemia. A persistently low or negative anion gap in an older adult, especially with anemia, renal dysfunction, or bone pain, warrants serum protein electrophoresis (SPEP), urine protein electrophoresis (UPEP), and serum free light chains to evaluate for multiple myeloma or MGUS.
- Take a toxin and medication history. Bromide (pyridostigmine, certain sedatives, fumigants) and chronic lithium can each depress the gap; pair the history with serum drug levels where available.
- Review calcium and magnesium. Marked hypercalcemia or hypermagnesemia is a rare but real contributor.
A low gap is rarely dangerous in itself, but a negative anion gap (below zero) is highly unusual and should never be dismissed. In an adult over 50 — particularly with fatigue, bone pain, recurrent infections, or renal impairment — a negative or persistently low gap is a red flag for paraproteinemia and warrants prompt hematology evaluation with SPEP, UPEP, and serum free light chain assay. A negative gap with a compatible exposure history should also prompt bromide and lithium level testing.
In short: a low anion gap is uncommon, usually explained by hypoalbuminemia or lab error, and only occasionally the sign of something serious — but when it is, multiple myeloma is the diagnosis most worth excluding. Confirm the value, correct for albumin, and pursue paraproteinemia and toxins in the right clinical context.
When to worry about a low anion gap
Most of the time, a low anion gap is not a reason to worry. The single most common explanation is laboratory error — a spuriously high chloride or low sodium from assay interference — so the correct first move is to repeat the chemistry panel before pursuing anything else. If the gap comes back persistently low (< 6 mEq/L on a repeat draw), then a focused workup is warranted. Hypoalbuminemia is the most frequent real cause, because albumin is the dominant unmeasured anion and each 1 g/dL drop lowers the expected gap by ~2.5 mEq/L. After albumin, the diagnoses worth excluding are paraproteinemia (multiple myeloma or MGUS, especially in adults over 50 with anemia, bone pain, or renal impairment) and medication effects. For a deeper look at the malignant causes, see our page on cancers that cause a low anion gap (Haber LA, Cleve Clin J Med 2023).
| Medication | Mechanism |
|---|---|
| Lithium | Direct cation effect |
| Bromide (historical) | Interferes with chloride assay |
| High-dose acetazolamide | Bicarbonate depletion |