Kidney Week

Abstract: SA-PO1101

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

Session Information

• Pathology and Lab Medicine: Clinical
  October 27, 2018 | Location: Exhibit Hall, San Diego Convention Center
  Abstract Time: 10:00 AM - 12:00 PM

Category: Pathology and Lab Medicine

• 1502 Pathology and Lab Medicine: Clinical

Authors

• Gruzdys, Valentinas, University of Utah, Salt Lake City, Utah, United States

Valentinas Gruzdys,

• Cahoon, Kenneth, ARUP Laboratories, Salt Lake City, Utah, United States

Kenneth Cahoon,

• Pearson, Lauren N., University of Utah & ARUP Laboratories, Salt Lake City, Utah, United States

Lauren N. Pearson,

• Lehman, Christopher, University of Utah, Salt Lake City, Utah, United States

Christopher Lehman,

• Raphael, Kalani L., VA Salt Lake City Health Care System, Salt Lake City, Utah, United States

Kalani L. Raphael,

Background

Low urine (u) NH_4⁺ 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