Pre-analytical storage and reagent-dependent sensitivity as sources of variability in prothrombin time and activated partial thromboplastin time measurements.

BACKGROUND

Prothrombin time (PT) and activated partial thromboplastin time (aPTT) are widely used coagulation assays whose results can vary due to pre-analytical and analytical factors. Understanding these sources of variability is essential for reliable laboratory practice and external quality assessment (EQA).

OBJECTIVES

This study evaluates the impact of pre-analytical storage and method-dependent sensitivity on PT and aPTT measurements and provides guidance for EQA programmes and clinical interpretation.

METHODS

Plasma samples from 42 healthy volunteers were analysed within 4 h of collection or after storage at -20 °C, -80 °C, or snap-frozen in liquid nitrogen (LN₂) and subsequent storage at -80 °C. Additionally, pooled plasma was spiked with direct oral anticoagulants, heparins, and lupus anticoagulant antibodies to assess reagent-specific sensitivity. PT and aPTT were measured on multiple routine platforms using commonly used reagents from 4 different manufacturers.

RESULTS

Storage at -80 °C or LN₂ preserved PT and aPTT values, whereas storage at -20 °C caused variable prolongation. In healthy volunteers, PT and aPTT showed marked inter-method variability. Sensitivity to anticoagulants and lupus anticoagulant also differed considerably between reagents, with the largest variation observed for edoxaban. This study provides an overview of inter-method variability, which may affect clinical decision-making and inter-laboratory comparability.

CONCLUSIONS

PT and aPTT are screening tests whose accurate interpretation depends on optimal sample handling and awareness of reagent- and method-specific sensitivities. These differences can affect clinical decision-making in anticoagulated patients. EQA programmes could increase their value by using frozen plasma, including clinically relevant anticoagulants, and implementing case-based scenarios rather than reporting only clotting times.