Effect of Pre-Analytic Variables on Prothrombin Time and Activated Partial Thromboplastin Time

Abstract

Introduction: Inadvertent inappropriate coagulation tests results are reported frequently inspite of using automated instruments and quality assurance measures, mostly due to pre analytical errors associated with sample collection and processing. Aim: This study was undertaken to evaluate pre-analytical variables in coagulation tests by inducing variables through simple experiments. The study also aimed to evolve rejection/ sample acceptability criteria for coagulation tests to minimise rejection of sample at the level of pre analytical phase. Materials and Methods: This cross sectional and observational study was carried out in the Pathology department of a tertiary care hospital from January 2014 to December 2014. Results obtained from reported blood samples with normal Prothrombin Time (PT) and Activated Partial Thromboplastin Time (APTT) values were used to induce processing variables. Coagulation tests results were collected by running samples in ideal conditions. Same samples were rerun with introducing variation in parameters and coagulation test measurements were taken. Differences between first and second coagulation measurements were noted and compared by applying t-test using IBM SPSS software version 25. For observational study, data were based on results of samples received and rejected in Clinical Pathology laboratory. Results: PT value was not significantly affected by haemolysis unlike aPTT which was altered due to haemolysis. By altering ratio of blood and anticoagulant, PT results were affected significantly when citrate concentration was increased by 0.4ml and aPTT results were affected more significantly when citrate concentration was increased by 0.1 ml. Centrifugation of samples for 10 minute at 1500xg or 3000xg did not alter the tests’ results significantly. PT values were not significantly affected upto 24 hours at room temperature. But aPTT values were significantly affected when sample was stored for more than 12 hours even on freezing temperature. In observational study, the major reason for rejection in the pre-analytical phase was clotted samples, followed by samples rejected due to underfilling. Conclusion: Samples should be properly identified and labeled. PT of haemolyzed samples giving results in normal range can be reported. A 67% of the optimum filled volume for PT and 86% for aPTT should be acceptable. A 10 minute centrifuge time at 1500xg might be suitable. Samples can be run for PT within 24 hours at room temperature and 48 hours in freezing condition and for aPTT within 12 hours.

Keywords

• coagulation tests
• sample collection
• sample processing