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Abstract: SA-PO1101

Validation of a Modified Serum Ammonia Assay to Quantify Urine Ammonium in CKD

Session Information

Category: Pathology and Lab Medicine

  • 1502 Pathology and Lab Medicine: Clinical

Authors

  • Gruzdys, Valentinas, University of Utah, Salt Lake City, Utah, United States
  • Cahoon, Kenneth, ARUP Laboratories, Salt Lake City, Utah, United States
  • Pearson, Lauren N., University of Utah & ARUP Laboratories, Salt Lake City, Utah, United States
  • Lehman, Christopher, University of Utah, Salt Lake City, Utah, United States
  • Raphael, Kalani L., VA Salt Lake City Health Care System, Salt Lake City, Utah, United States
Background

Low urine (u) NH4+ excretion is associated with higher risk of GFR decline in CKD. Few clinical laboratories measure uNH4+ limiting clinical application, and urine anion and osmolar gaps poorly estimate uNH4+ in CKD. We determined whether a clinically-approved serum NH3 assay reliably quantifies uNH4+ in CKD.

Methods

uNH4+ was measured using the Randox (Ireland; glutamate dehydrogenase method) serum NH3 assay on the Architect ci8200 chemistry analyzer. Preliminary studies found that 1:40 dilution of urine with Architect on-board diluent quantified uNH4+ within limits of the assay (20–1180 μmol/L). Precision studies were performed using synthetic urines with known [NH4+] (2.5, 15.6, and 29.5 mmol/L) over 20 days (n=80 measurements per synthetic urine). We then compared [uNH4+] obtained by the Randox assay and by formalin-titration in 22 CKD patients (n=58 samples) using Deming regression.

Results

Randox serum NH3 assay total imprecision (%CV) with 1:40 dilution was 17.7%, 5.1%, and 2.2% in the 2.5, 15.6 and 29.5 mmol/L uNH4+ synthetic urines, respectively. In all 58 patient samples, 1:40 dilution quantified uNH4+ within the range of the Randox NH3 assay. After accounting for the pH of formalin, the correlation between uNH4+ by formalin-titration and the modified Randox assay was 0.9778 with a regression slope of 1.01 and intercept of 0.70 mmol/L (Figure).

Conclusion

After 1:40 urine dilution, precision of the Randox serum NH3 assay to quantify uNH4+ was better with higher uNH4+. Nevertheless, agreement between uNH4+ by this approach and by formalin-titration was excellent across a wide range of values in CKD. uNH4+ can be reliably quantified in CKD after 1:40 dilution using a standard plasma NH3 assay and an automated chemistry analyzer.

Comparison of uNH4+ by the modified Randox serum NH3 assay and formalin-titration

Funding

  • Veterans Affairs Support