Urine Anion Gap
The three main causes of normal anion gap acidosis are:
- Loss of HCO3- from Gastrointestinal tract (diarrhea)
- Loss of HCO3- from the Kidneys (RTAs)
- Administration of acid
Distinguishing between the above 3 groups of causes is usually clinically obvious, but occasionally it may be useful to have an extra aid to help in deciding between a loss of base via the kidneys or the bowel. Calculation of the urine anion gap may be helpful diagnostically in these cases.
The measured cations and anions in the urine are Na+, K+, and Cl- ; thus the urine anion gap is equal to:
Urine anion gap = [Na+] + [K+] - [Cl-]
Urine anion gap = unmeasured anions – unmeasured cations
In normal subjects, the urine anion gap is usually near zero or is positive. In metabolic acidosis, the excretion of the NH4+ (which is excreted with Cl- ) should increase markedly if renal acidification is intact. Because of the rise in urinary Cl- , the urine anion gap which is also called the urinary net charge, becomes negative, ranging from -20 to more than -50 meq/L. The negative value occurs because the Cl- concentration now exceeds the sum total of Na+ and K+.
In contrast, if there is an impairment in kidney function resulting in an inability to increase ammonium excretion (i.e. Renal Tubular Acidosis), then Cl- ions will not be increased in the urine and the urine anion gap will not be affected and will be positive or zero.
In a patient with a hyperchloremic metabolic acidosis: A negative UAG suggests GI loss of bicarbonate (eg diarrhea), a positive UAG suggests impaired renal acidification (ie renal tubular acidosis).
As a memory aid, remember ‘neGUTive’ - negative UAG in bowel causes.
Remember that in most cases the diagnosis may be clinically obvious (severe diarrhea is hard to miss) and consideration of the urinary anion gap is not necessary.
See case 5 for an example.
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