Study Of The Ratio Expression For Prothrombin Time And Activated Partial Thromboplastin Time Results

Prothrombin time (PT) and activated partial thromboplastin time (APTT) results are statistically different, not only in function of reagent used but also, with a same reagent, in function of the degree of automation of the measuring device. It has been claimed that using time ratio of pathological to normal samples would reduce these discrepancies. In a national voluntary quality control program using Verify Citrate Normal and Pathological I/II plasmas, pairs of results obtained in the same analytical batch were computed to compare mean values and reproducibility of ratios and seconds for both PT (n=1764) and APTT (n=1392). Results were screened simultaneously for origin of reagent and measuring technique. Concerning PT, the ratio expression did not improve the intralaboratory dispersion of results for both pathologies levels. Neither in group comparisons between couples reagent-device, nor for the whole set of pooled values, a significant improvement of the dispersion of the ratio results could be observed if compared to the dispersion expressed in seconds. In the 6 groups reagent-instrument, difference between average values was never greater than 1.0 sec for the Normal control plasma (mean: 11.7 sec), 2.4 sec for the level 1 plasma (mean: .18.9 sec) and 4.9 sec for plasma level 2 (mean: 29.3 sec). Concerning APTT, dispersion was strongly enhanced in semi-automated equipments. Chemical activators gave times longer than the particulate ones. Here again, no significant improvement was offered by the ratios. In conclusion, ratio expression of PT’s and APTT’s results does not abolish the influence of the couples ‘origin of reagent’ - ‘automation of measuring device’.