Method-dependent variation in PT and aPTT measurements
In external quality assessment (EQA) programmes for prothrombin time (PT) and activated partial thromboplastin time (aPTT) measurements, variations depending on the analytical method used are observed. The aim of this study is to map the differences between methods and to expand knowledge about the causes of these differences. This can contribute to the improvement of:
· EQA materials and conditions,
· interpretation of differences by EQA participants,
· harmonization of methods.
To study variation between methods, four analyzers and eight PT and seven aPTT reagents from Siemens Healthineers, Diagnostica Stago, Werfen, and Roche Diagnostics were compared. To evaluate pre-analytical conditions, citrated plasma was collected from 42 healthy volunteers, upon which PT and aPTT were measured in fresh plasma and in plasma stored for 4–12 weeks at −20°C, −80°C, or in liquid nitrogen. The influence of anticoagulant drugs (heparin, dalteparin, nadroparin, apixaban, rivaroxaban, edoxaban, dabigatran) and lupus antibodies was assessed by comparing PT and aPTT measurements in concentration series generated by spiking anticoagulants and lupus antibodies into Omniplasma. To assess commutability of EQA materials, PT and aPTT measurements on 10 recent EQA samples, 14 patient pools and 14 healthy individuals were compared between the different methods.
Storage of plasma at −20°C resulted in longer PT and aPTT compared to fresh plasma, whereas storage at −80°C or in liquid nitrogen yielded comparable results. The median PT of healthy volunteers (stored at −80°C) ranged from 9.0–13.2 s, and the median aPTT from 24.1–32.5 s, with Roche and Siemens methods giving the shortest clotting times and Stago the longest. A similar trend was observed in the anticoagulant sensitivity tests (concentration series stored at −80°C), in which STA-NeoPTimal (Stago) showed a greater PT prolongation with increasing concentrations of apixaban/edoxaban/dabigatran than PT Rec (Roche). Pathrombin SL (Siemens) exhibited the steepest increase in aPTT with increasing concentrations of heparin, dalteparin and nadroparin. In the lupus antibody concentration series, the PTT/Autom.Sta APTT reagent (Stago) showed the greatest aPTT prolongation. Remarkably, recent EQA schemes did not include any samples containing direct oral anticoagulants (DOACs).
Storage of plasma at −20°C results in prolonged clotting times, while plasma samples stored at −80°C could be suitable for use in EQA programmes. Different PT and aPTT reagents yield variable results and differ in sensitivity to anticoagulant drugs including DOACs, and lupus antibodies. This should be taken into account in clinical practice, e.g. when determining reference intervals and monitoring patients on anticoagulant therapy. The design and interpretation of EQA programmes should also keep up with innovation in anticoagulant use by dispatching clinically representative samples and considering the effects of novel anticoagulants on test results.