Validation of a spectrophotometric method for GGT measurement in canine urine and determination of the urine GGT-to-creatinine ratio reference interval and biological variation in 41 healthy dogs.

The urine gamma-glutamyl transferase (GGT)-to-creatinine ratio has been used to monitor patients at risk of acute renal injury. We validated the spectrophotometric quantification of GGT in urine in a commercial biochemistry analyzer. The assay was precise, accurate, and linear. Intra-assay precision was 3.59% in 4 samples, with GGT concentrations of 47-195 U/L. Inter-assay precision in 3 samples with activities of 11-51 U/L was 7.74%. Accuracy was 97.3%, with an absolute bias of 2.7 U/L. Urine GGT was unaffected by hematuria, hemoglobinuria, or bacteriuria. Urine GGT was stable at 20°C and 4°C for up to 3 d. Storage by freezing at -20°C resulted in a significant reduction in enzyme activity. A pH outside the range of 6.5-8 resulted in reduced GGT activity. The biological variation of urine GGT-to-creatinine ratio provided an index of individuality of 1.6, indicating that a population-based reference interval (RI) can be used. The reference change value was calculated, and an increase in consecutive measurements >43% is required to be regarded as significant. The urine GGT-to-creatinine ratio RI obtained in a population of 41 healthy dogs was 8.5-28.5 U/g.