Critical differences of clinical chemical parameters in blood from dogs.

The study concerned the critical difference which may help to judge whether the difference between two consecutive analytical results may be safely ascribed to natural variation or not. To calculate the critical difference of nine canine clinical chemical parameters, blood samples from 20 apparently clinically healthy dogs were collected once weekly for five consecutive weeks. For each of the nine clinical chemical parameters, the total variance of the analytical results was divided into the component of variance between dogs (S2Inter), the component of variance for weeks within dogs (S2Intra) and the component of variance for measurements (S2Anal) using nested analysis of variance. The critical difference was then calculated from S2Intra and S2Anal as 0.22 mu kat litre-1 for alanine aminotransferase, 0.20 mu kat litre-1 for aspartate aminotransferase, 0.34 mu kat litre-1 for alkaline phosphatase, 2.36 mmol litre-1 for urea, 35 mumol litre-1 for creatinine, 2.8 g litre-1 for albumin, 6.3 g litre-2 for serum proteinTotal, 1.49 mmol litre-1 for glucose and 0.84 mmol litre-1 for cholesterolTotal. These critical differences may be used as guidelines to evaluate the difference between two consecutive analytical results of the above parameters. However, the analytical results should not be assessed by the critical differences alone, but should also be compared to the corresponding reference intervals.