An anion gap is a calculated value that helps medical professionals understand the balance of electrolytes in a person’s blood. It is a tool used to assess acid-base balance and can indicate if there are imbalances in the body’s charged particles.
What Anion Gap Means
The anion gap is a measurement derived from standard blood tests, specifically an electrolyte panel. It represents the difference between the primary measured positively charged ions (cations) like sodium, and the primary measured negatively charged ions (anions) like chloride and bicarbonate. The “gap” reflects the concentration of unmeasured ions, predominantly negatively charged proteins, as not all ions are routinely measured.
A normal anion gap typically falls within a range of 4 to 12 milliequivalents per liter (mEq/L). A low anion gap means the calculated difference between measured cations and anions is smaller than expected. This can occur due to a decrease in unmeasured negatively charged particles or an increase in unmeasured positively charged particles in the blood.
Reasons for a Low Anion Gap
A low anion gap is a relatively uncommon finding, and its presence often prompts further investigation. The most frequent cause is hypoalbuminemia, which refers to lower-than-normal levels of albumin in the blood. Albumin is the most abundant negatively charged protein in the body, accounting for a significant portion of the unmeasured anions. When albumin levels decrease, the anion gap also decreases. Hypoalbuminemia can result from various conditions, including malnutrition, liver disease, kidney disease, severe burns, or chronic inflammation.
Beyond hypoalbuminemia, certain intoxications can also lead to a low anion gap. Lithium, a medication sometimes prescribed for mood disorders, is a positively charged ion not typically measured in electrolyte panels. Elevated levels of lithium, often due to toxicity, can increase unmeasured cations, reducing the calculated anion gap. Another less common cause is bromide intoxication, which can occur from exposure to certain sedatives or other substances. Bromide can interfere with chloride measurements, leading to a falsely elevated chloride result and a low or even negative anion gap.
Sometimes, a low anion gap result may simply be due to a laboratory error. Errors can occur during the collection, transportation, or processing of the blood sample, or during the analytical measurement of the electrolytes. Laboratory error is often cited as the most common reason for an initial low anion gap result. Other less common reasons for a low anion gap include an increase in positively charged proteins, such as those seen in certain blood disorders like multiple myeloma.
Next Steps After a Low Anion Gap Result
If a low anion gap is detected in a blood test, it is important to consult with a healthcare professional for a thorough evaluation. An abnormal anion gap is rarely an isolated finding and is typically interpreted within the broader context of a patient’s overall health, medical history, and other laboratory results. Given that laboratory error is a frequent cause, a doctor may recommend repeating the blood test to confirm the finding.
It is important to avoid self-diagnosis or undue worry based solely on a single low anion gap result. The medical team will conduct further tests and assessments to determine the underlying reason for the low value. This comprehensive approach allows for an accurate diagnosis and the development of an appropriate treatment plan, if necessary. Open communication with your doctor about any concerns or symptoms is encouraged.