Anion Gap Blood Test

The anion gap blood test is a medical laboratory test used to measure the difference between the concentrations of major cations (positively charged ions) and anions (negatively charged ions) in the blood.

What is measured?

The test measures the levels of three main ions in the blood:

• Sodium (Na+), a cation
• Chloride (Cl-), an anion
• Bicarbonate (HCO3-), an anion

Calculation of Anion Gap

The anion gap is calculated using the following formula:

Anion Gap = Sodium - (Chloride + Bicarbonate)

This calculation helps to estimate the concentration of unmeasured anions in the blood, such as phosphate, sulfate, and lactate.

Normal Values

The normal range for the anion gap is typically between 3-12 mmol/L, but this can vary slightly depending on the laboratory and the individual's condition.

Clinical Significance

An abnormal anion gap can indicate various conditions, including:

• Metabolic acidosis, a condition where the blood is too acidic
• Kidney disease or failure
• Lactic acidosis, a condition where the body produces too much lactic acid
• Toxicity from certain substances, such as salicylates or ethylene glycol

Interpretation of Results

A high anion gap may indicate:

• Increased production of acidic substances, such as lactic acid or ketones
• Decreased excretion of acidic substances by the kidneys
• Presence of unmeasured anions in the blood

A low anion gap may indicate:

• Dilutional effect, where the blood is diluted with fluids or other substances
• Laboratory error or interference

Limitations and Considerations

The anion gap test has some limitations and considerations, including:

• It is a calculated value and may not always reflect the true concentration of unmeasured anions
• It can be influenced by various factors, such as changes in sodium or chloride levels
• It should be interpreted in conjunction with other laboratory tests and clinical findings