Why is Anion Gap Low? Causes, Symptoms, and What to Do

A low anion gap causes no symptoms of its own. It is a calculated number, not a condition, and a low result is rare; when it happens, the most common reason is a low level of the blood protein albumin [1], and a laboratory or sampling error is another possible explanation [2]. Less often it reflects abnormal antibody proteins (as in multiple myeloma), lithium, or high calcium or magnesium. Any symptoms you have come from that underlying cause, so doctors usually repeat the test first and then look at your albumin and other results.

What the anion gap is

Your blood contains positively charged particles (such as sodium) and negatively charged ones (such as chloride and bicarbonate). The laboratory does not measure every charged particle, so it calculates the difference:

Anion gap = sodium − (chloride + bicarbonate) [3]

The "gap" stands for negatively charged substances the lab does not measure, including albumin. That is why the gap falls when albumin is low [2][3]. The main use of the number is to look for metabolic acidosis – too much acid in the blood – which typically raises the gap [1][3].

Reference range

Many labs write the unit as mmol/L. There is no universal normal value: each laboratory and analyzer sets its own range, so compare your result with the range printed on your report [2][4]. If albumin is low, the expected normal gap is lower too – Merck suggests lowering the normal range by about 2.5 mEq/L for every 1 g/dL fall in albumin [3][4].

Causes of a low anion gap

Most common

• Low albumin (hypoalbuminemia). Albumin is the main unmeasured negative charge, so less albumin means a smaller gap. Low albumin can come from liver disease, kidney disease, poor protein intake or absorption, infection or large burns [6].
• Laboratory or sampling error. Problems in collection, transport or processing of the blood sample can produce a falsely low result [2]. This is one reason a single low value is usually repeated.

Less common

• Lithium. Lithium treatment or lithium toxicity can reduce the gap [2][3].
• High calcium or high magnesium (hypercalcemia, hypermagnesemia) – extra positive charges [3].
• Excess antibody proteins (hypergammaglobulinemia), as in multiple myeloma. Some of these proteins carry a positive charge and narrow the gap [3].

Rare

• Bromide or iodide (halide) intoxication [3].
• Very high blood viscosity [3].
• Lab artefacts in severe hypernatremia (very high sodium), very high blood fats or bromide intoxication, which can even produce a negative gap [3].

Symptoms

A low anion gap itself does not cause symptoms. If low albumin is the reason, symptoms depend on why albumin is low: liver disease can cause swelling of the belly, ankles and legs and fatigue; kidney disease can cause swelling of the hands and feet, puffy eyelids and fatigue [6]. Multiple myeloma can cause bone pain (often in the back or ribs), fractures, weakness or fatigue, weight loss, frequent infections and fevers, and thirst [7]. Many people with a low result have no symptoms at all and the finding is incidental. MedlinePlus also notes that a low anion gap may go with alkalosis (blood that is less acidic than normal); if so, symptoms can include nausea and vomiting, hand tremor, muscle twitching, irritability, numbness or tingling in the face, hands or feet, and an irregular heartbeat [1].

High anion gap symptoms

Because the two are often confused: a high anion gap almost always means metabolic acidosis [3]. Its common causes are diabetic ketoacidosis, lactic acidosis (for example with shock or very heavy exercise), chronic kidney disease, and poisoning with methanol, antifreeze (ethylene glycol) or too much aspirin (salicylates) [1][4][5]. Symptoms come from the acidosis and its cause: shortness of breath or deep, rapid breathing (often without feeling breathless), nausea and vomiting, confusion, tiredness or lethargy, and a very fast heartbeat [1][5]. Symptoms appear at milder levels of acidosis when it develops quickly [5]. Mild rises with dehydration or hard exercise are also possible [1][2].

What usually happens next

• Repeat the test. A low result is often rechecked first to rule out a lab or sampling error [2].
• Look at albumin. Albumin, which may be included in a comprehensive metabolic panel [6], is the most common explanation for a low gap; the doctor may adjust the expected gap for it [1][3][4].
• Check calcium, magnesium and medicines. A history of lithium use and levels of calcium and magnesium are reviewed [3].
• Protein tests if needed. If albumin is normal and the gap stays low without another explanation, clinicians may look for abnormal antibody proteins – for example with protein electrophoresis, free light chains and a blood count – which are among the tests used to diagnose myeloma [7].

Frequently asked questions

What are the symptoms of a low anion gap?

There are none from the number itself. Symptoms, if any, belong to the cause – for example swelling and fatigue with low albumin, or bone pain and frequent infections with multiple myeloma [6][7].

What does a low anion gap mean?

Usually that your albumin is low or that the result is a lab artefact [1][2]. Unlike a high gap, a low gap does not point to acidosis: its recognised causes are low albumin, extra positively charged substances (calcium, magnesium, lithium, antibody proteins) or measurement problems [3].

Is a low anion gap dangerous?

The low value is not dangerous in itself. What matters is the reason, which is why a repeat test and an albumin level are the usual next steps [2].

Can dehydration cause a low anion gap?

Dehydration is more often listed as a cause of a high anion gap than a low one, although fluid status can affect results [2][4].

What are the symptoms of a high anion gap?

Those of metabolic acidosis: fast or deep breathing, shortness of breath, nausea and vomiting, tiredness, confusion and a fast heartbeat, plus symptoms of the cause such as uncontrolled diabetes or kidney disease [1][5].

How is the anion gap calculated?

Sodium minus the sum of chloride and bicarbonate. It is calculated from the results of an electrolyte panel, so no separate test is needed [1][3].

When to see a doctor

If a routine test shows a low anion gap and you feel well, it is worth discussing with your doctor at your next appointment or soon, so the result can be repeated and checked against your albumin level. Book sooner if you also have unexplained swelling, bone pain, repeated infections or ongoing tiredness. Seek urgent care for severe breathing difficulty, confusion, fainting or persistent vomiting, especially if you have diabetes – these can be signs of acidosis.

Sources

1. Anion Gap Blood Test — MedlinePlus, U.S. National Library of Medicine
2. Anion Gap Blood Test — Cleveland Clinic
3. Acid-Base Disorders — Merck Manual Professional Edition
4. Anion Gap (Blood) — University of Rochester Medical Center Health Encyclopedia
5. Metabolic Acidosis — Merck Manual Professional Edition
6. Albumin Blood Test — MedlinePlus, U.S. National Library of Medicine
7. Multiple Myeloma — MedlinePlus, U.S. National Library of Medicine

Decisions about your health should always be made in consultation with your healthcare provider.