If there is a better test, why not use it?
That is the question a group of nephrologists are asking directors of their laboratories about diagnosing metabolic acidosis. They are advocating that measuring a patient's urine ammonium level is more helpful than trying to estimate it from the urine anion gap (UAG).
More than 170 nephrologists signed a public letter making this request to “directors of clinical laboratories,” first published on Twitter as the introductory step in a campaign to make urine ammonium tests more available.
The letter notes that the test would be valuable “not only in the diagnosis of renal tubular acidosis…but also in managing acidosis in progressive [chronic kidney disease] CKD…and in evaluating and treating patients with kidney stones, where it will give us clues about the acid load the patients consume.”
Although the test is available at some reference labs, many institutions do not even offer physicians the option to request to send out the test.
Many nephrologists realize that “the urine anion gap is not a good test,” said David S. Goldfarb, MD, clinical chief of nephrology at New York University Langone Health and one of the leaders of the ammonium test campaign. “We are hoping that nephrologists will read this [letter] and say, ‘Yeah, why are we satisfied with a urine anion gap measurement which is clearly not satisfactory?’ If we can demonstrate that nephrologists are interested in this test, then perhaps it won't be such a big deal for [laboratories] to perform it.”
Uribarri J, Oh MS. The urine anion gap: Common misconceptions. J Am Soc Nephrol 2021; 32:1025–1028. doi: 10.1681/ASN.2020101509
Batlle DC, et al. The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis. N Engl J Med 1988; 318:594–599. doi: 10.1056/NEJM198803103181002
Goldstein MB, et al. The urine anion gap: A clinically useful index of ammonium excretion. Am J Med Sci 1986; 292:198–202. doi: 10.1097/00000441-198610000-00003