Urine anion gap

Urine Anion Gap is a calculation used in medicine to aid in the differential diagnosis of metabolic acidosis, a condition characterized by a decrease in the blood's pH. The urine anion gap is particularly useful in determining whether the cause of the metabolic acidosis is due to a gastrointestinal loss of bicarbonate or a renal (kidney) problem.

Calculation

The urine anion gap is calculated using the concentrations of the major ions found in the urine: sodium (Na+), potassium (K+), and chloride (Cl-). The formula is:

Urine Anion Gap = (Na+ + K+) - Cl-

In this equation, the concentrations are measured in milliequivalents per liter (mEq/L). A positive urine anion gap suggests that the metabolic acidosis is due to a problem with the renal tubules' ability to excrete acid, as seen in renal tubular acidosis. A negative urine anion gap is typically seen in cases where the acidosis is due to a gastrointestinal loss of bicarbonate, such as in diarrhea.

Clinical Significance

The urine anion gap is a useful tool in the evaluation of metabolic acidosis. Metabolic acidosis can be caused by an increase in acid production, a loss of bicarbonate, or a decrease in acid excretion. The urine anion gap helps to differentiate between these causes, particularly distinguishing between renal and gastrointestinal sources of bicarbonate loss.

A positive urine anion gap indicates that the kidneys are not properly excreting acids, which is characteristic of renal tubular acidosis. In contrast, a negative urine anion gap suggests that the kidneys are functioning properly and that the metabolic acidosis is likely due to a loss of bicarbonate from the gastrointestinal tract.

Limitations

The urine anion gap has limitations and should not be used in isolation for the diagnosis of metabolic acidosis. It is most useful when applied in conjunction with other laboratory tests and clinical findings. Factors such as the presence of other ions in the urine that are not measured in this calculation can affect the accuracy of the urine anion gap. Additionally, the urine anion gap is less reliable in the setting of mixed acid-base disorders.

See Also

• Metabolic acidosis
• Renal tubular acidosis
• Acid-base homeostasis