Prothrombin time standardisation in canine samples with regard to inter-batch and inter-reagent variability

The objectives of this study were to evaluate the batch-to-batch and inter-reagent variability of prothrombin time (PT) measurements in canine plasma and to investigate the effect of different methods of standardisation. PT standard test (PTST) and PT modified test (PTMT; 1:20 sample dilution, fibrinogen supplementation) were measured using five different batches of each of two commercial reagents (the first a human placenta thromboplastin and the second a rabbit brain thromboplastin). In addition, inter-reagent variability was studied for PTMT in 86 canine samples (four standardised reagents) based on clotting times and after calculation of percentage activity, prothrombin ratio (PTR) or International Normalised Ratio (INR).PTST and PTMT measurements showed only a low variation between different batches of the two reagents (CV ⩽ 5%). Calculation of percentage activity or PTR reduced differences between different reagents when compared to results reported as coagulation times. Calculation of INR using the International Sensitivity Index values provided by manufacturers for use in humans showed less favourable results. In conclusion, standardisation to reduce inter-batch variability seems unnecessary for the studied reagents. Possibilities for PT standardisation in order to achieve a reagent-independent result are limited.