Technical note: Evaluation of a colorimetric point-of-care test for measuring urine ammonium concentration in periparturient dairy cattle

Journal of Dairy Science
A A Megahed, Peter D Constable

Abstract

Urine ammonium concentration ([NH4+]) provides a clinically useful indicator of the magnitude of nutritionally induced systemic acidification in dairy cattle when urine pH < 6.1. The objective of this study was to evaluate the analytical performance of a low-cost point-of-care colorimetric test in measuring urine [NH4+] in dairy cattle consuming an acidogenic ration. A method comparison study was performed using 154 urine samples from 43 periparturient Holstein-Friesian cows. We compared urine [NH4+] measured by an indophenol blue colorimetric test (MColortest, Merck KGaA, Billerica, MA; test method) with levels measured by formaldehyde titration (reference method). Diagnostic performance was evaluated using Pearson correlation coefficient (r), Passing-Bablok regression, Bland-Altman plot, and binary logistic regression. Urine [NH4+] measured by the colorimetric test was strongly correlated (r = 0.98) with urine [NH4+] measured by formaldehyde titration. Method comparison studies indicated that the colorimetric test provided acceptable test performance when urine [NH4+] < 80 mmol/L. Logistic regression analysis indicated that the area under the receiver operating characteristic curve for the colorimetric ammonium test was high ...Continue Reading

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