engleski

Are differences in PT, aPTT and fibrinogen between the first and the second blood draw significant?

Background: Preanalytical phase in plasma-based haemostasis testing is source of numerous errors. Among others, sample collection represents one of the most important elements of preanalytical phase in coagulation testing. Tourniquet time, test tube filling and mixing as well as the order of draw are recognised as fundamental factors which have influence on the haemostasis results. CLSI H3-A6 and CLSI H21-A5 guidelines stated that is not necessary to discard the first tube if only coagulation tests are requested. However, this refers only for routine blood coagulation assays (PT, aPTT). Nevertheless, statistically, though not clinically, significant differences for routine blood coagulation tests results have been demonstrated in a large patient group. The aim of this work was to examine the differences in routine coagulation tests results between the first and the second blood tube. Materials and methods: For 171 subjects, blood was drawn in the first and the second evacuated tube containing sodium citrate (3.2%). Paired samples were analysed for PT, aPTT and fibrinogen at the same time on the coagulation analyser BCS (Siemens, Marburg, Germany). The tests were performed using the original reagents. Bias was calculated between the first and second draw and compared with desirable imprecision based on biological variation (0.5CVw). Analytical CVs were 4.5%, 3.6% and 4.5% for PT, aPTT and fibrinogen, respectively. Results: The difference between the first and the second draw was not acceptable for 96/171 (56%) of patients for PT, in 32/171 (19%) patients for aPTT and in 27/171 (16%) patients for fibrinogen. Differences for PT, aPTT and fibrinogen were outside the limits of analytical imprecision in our laboratory in 16%, 13% and 22% patients, respectively. Conclusion: Differences in PT, aPTT and fibrinogen level between the first and the second blood draw exceed the limits of desirable imprecision based on biological variation.

coagulation testing; biological variation; discard tube

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