Critical difference of some bovine haematological parameters

The purpose of the present study was to calculate the critical difference between 2 analytical results for the red blood cell count (RBC), the white blood cell count (WBC), the haemoglobin concentration (Hb), and the haematocrit (PCV) in blood from Red Danish Dairy cows. The critical difference can help to judge whether the difference between 2 consecutive analytical results from the same animal may be safely ascribed to natural variation or not. To calculate the critical differences, blood samples from 20 clinically healthy lactating cows were collected once daily for 5 consecutive days. The total variance of the analytical results was divided into the component of variance between cows (S2Inter), the component of variance for days within cows (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 0.61 x 10(12)/l for RBC, 2.2 x 10(9)/l for WBC, 0.79 mmol/l for Hb, and 0.07 for PCV. The critical differences may be used as guidelines to indicate potentially important changes in the parameters. However, the analytical results should not be assessed by the critical differences alone, but should also be compared to the corresponding reference intervals.