Measurement of Urinary Ammonium Using a Commercially Available Plasma Ammonium Assay

Area of Science:

• Clinical Chemistry
• Nephrology
• Biochemistry

Background:

• Urinary ammonium excretion is crucial for evaluating acid-base disorders, chronic kidney disease, and nephrolithiasis.
• Current measurement methods are limited to specialized laboratories in the United States, hindering widespread clinical application.
• This study aimed to adapt a plasma ammonium assay for quantifying urinary ammonium and assess its stability.

Purpose of the Study:

• To evaluate the performance of a modified enzymatic plasma ammonium assay for measuring urinary ammonium concentration.
• To determine the stability of urinary ammonium under various storage conditions (temperature, pH).
• To assess the suitability of the modified assay for routine clinical use.

Main Methods:

• An enzymatic plasma ammonium assay was modified for urine samples by employing a 40-fold dilution.
• Assay precision, limit of quantitation, and linearity were evaluated using an Abbott Architect ci8200 analyzer.
• Method comparison with the formalin titration method and stability studies at different temperatures and pH levels were conducted.

Main Results:

• The modified assay demonstrated good precision (2-18%) and linearity (0.7-45 mmol/L) with a limit of quantitation of 2.92 mmol/L.
• Comparison with the formalin method showed strong agreement (slope 0.98, intercept -0.37 mmol/L).
• Urinary ammonium was stable for 48 hours at room temperature, 9 days at 4°C/-20°C, and over 24 months at -80°C within a pH range of 5.6-6.3.

Conclusions:

• The modified enzymatic plasma ammonium assay reliably quantifies urinary ammonium at physiologic concentrations.
• The assay demonstrates excellent performance characteristics and stability, comparable to the established formalin titration method.
• This adapted assay is suitable for routine clinical use on automated chemistry analyzers, improving accessibility for patient diagnostics.