• 1.

    Uribarri J, Oh MS. The urine anion gap: Common misconceptions. J Am Soc Nephrol 2021; 32:10251028. doi: 10.1681/ASN.2020101509

  • 2.

    Raphael KL, et al. Urine ammonium predicts clinical outcomes in hypertensive kidney disease. J Am Soc Nephrol 2017; 28:24832490. doi: 10.1681/ASN.2016101151

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3.

    Vallet M, et al. Urinary ammonia and long-term outcomes in chronic kidney disease. Kidney Int 2015; 88:137145. doi: 10.1038/ki.2015.52

  • 4.

    Asplin JR. Evaluation of the kidney stone patient. Semin Nephrol 2008; 28:99110. doi: 10.1016/j.semnephrol.2008.01.001

Letter to Directors of Clinical Laboratories (September 2021)

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Calculation of the urine anion gap (UAG) was suggested in the 1980s as an easy way to indirectly estimate urine ammonium (NH4) in patients with hyperchloremic metabolic acidosis. This calculation was used by necessity because clinical laboratories were not measuring UNH4 at that time. Despite significant technological advances ever since, most clinical laboratories in this country still do not measure UNH4. The UAG has fallen short as a surrogate for UNH4 for many reasons, and its shortcomings have been recently reviewed in detail (1). The undersigned believe that direct measurement of UNH4 is a test [that] is long

Calculation of the urine anion gap (UAG) was suggested in the 1980s as an easy way to indirectly estimate urine ammonium (NH4) in patients with hyperchloremic metabolic acidosis. This calculation was used by necessity because clinical laboratories were not measuring UNH4 at that time. Despite significant technological advances ever since, most clinical laboratories in this country still do not measure UNH4. The UAG has fallen short as a surrogate for UNH4 for many reasons, and its shortcomings have been recently reviewed in detail (1). The undersigned believe that direct measurement of UNH4 is a test [that] is long overdue. It has value not only in the diagnosis of renal tubular acidosis, as mentioned above, but also in managing acidosis in progressive [chronic kidney disease] CKD (2, 3), and in evaluating and treating patients with kidney stones, where it will give us clues about the acid load the patients consume (4). One argument of the clinical labs is that the test may not be ordered in sufficient volume to justify them developing the required complex proficiency and validation tests. We believe the test is not being ordered, not because clinicians do not think it worthwhile, but because of its limited availability. If at least a number of clinical laboratories were available to perform the test as a “send-out,” we all would order UNH4 with greater frequency. We therefore petition you to make UNH4 a readily available test to which clinicians throughout the country have access. We, the undersigned ([174 of us]), are nephrologists who strongly support this initiative and appreciate your consideration of our request.

Sincerely,

Jaime Uribarri, MD, Professor of Medicine, Icahn School of Medicine at Mt. Sinai

David S. Goldfarb, MD, Professor of Medicine, NYU Grossman School of Medicine, NYU Langone Health

Kalani Raphael, MD, Professor of Medicine, Oregon Health & Science University

Anna Zisman, MD, Associate Professor of Medicine, University of Chicago

References

  • 1.

    Uribarri J, Oh MS. The urine anion gap: Common misconceptions. J Am Soc Nephrol 2021; 32:10251028. doi: 10.1681/ASN.2020101509

  • 2.

    Raphael KL, et al. Urine ammonium predicts clinical outcomes in hypertensive kidney disease. J Am Soc Nephrol 2017; 28:24832490. doi: 10.1681/ASN.2016101151

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3.

    Vallet M, et al. Urinary ammonia and long-term outcomes in chronic kidney disease. Kidney Int 2015; 88:137145. doi: 10.1038/ki.2015.52

  • 4.

    Asplin JR. Evaluation of the kidney stone patient. Semin Nephrol 2008; 28:99110. doi: 10.1016/j.semnephrol.2008.01.001

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