Discard tube for coagulation testing: the debate continues

International Council for Standardisation in Haematology (ICSH) recommendations for collection of blood samples for coagulation testing

This guidance document has been prepared on behalf of the International Council for Standardisation in Haematology (ICSH). The aim of the document is to provide guidance and recommendations for collection of blood samples for coagulation tests in clinical laboratories throughout the world. The following processes will be covered: ordering tests, sample collection tube and anticoagulant, patient preparation, sample collection device, venous stasis before sample collection, order of draw when different sample types need to be collected, sample labelling, blood-to-anticoagulant ratio (tube filling) and influence of haematocrit. The following areas are excluded from this document, but are included in an associated ICSH document addressing processing of samples for coagulation tests in clinical laboratories: sample transport and primary tube sample stability; centrifugation; interfering substances including haemolysis, icterus and lipaemia; secondary aliquots-transport and storage; and preanalytical variables for platelet function testing. The recommendations are based on published data in peer-reviewed literature and expert opinion.

Routine coagulation testing in Vacutainer® Citrate Plus tubes filled at minimum or optimal volume

Background Filling of citrate tubes with appropriate amount of blood is essential for obtaining reliable results of coagulation testing. This study aimed to verify whether results of routine coagulation tests are comparable when the new Becton Dickinson Vacutainer® Citrate Plus tubes are filled at minimum or optimal volume. Methods The study population consisted of 133 patients (40 on oral anticoagulant therapy), who had blood collected for routine coagulation testing. Two sequential Vacutainer® Citrate Plus tubes of the same type and lot were drawn. The first tube was collected after a butterfly needle was inserted into the vein, so that the air in the tubing was aspirated into the tube before blood (minimum fill volume), whilst the second was drawn at optimal fill volume. Experiments were repeated using 2.7-mL (n = 86) and 1.8-mL (n = 47) tubes. Results Prothrombin time (PT) and fibrinogen values were slightly but significantly decreased in tubes with minimum than in those with optimal fill volume. The activated partial thromboplastin time (APTT) was slightly prolonged in tubes with minimum than in those with optimal fill volume, but the difference was not statistically significant. An identical trend was noted in separate analyses for the 2.7-mL and 1.8-mL tubes. Spearman's correlations between the two fill volumes were always >0.94 and bias was always within the quality specifications. Conclusions Blood drawing into Vacutainer® Citrate Plus tubes at minimum fill volume does not clinically bias routine coagulation testing.

Routine coagulation testing: do we need a discard tube?

When coagulation tests are performed, the recommended guideline is that a discard tube be used and the coagulation testing should be done only on the second tube. This guideline is however inconsistently enforced and most laboratories follow a single tube draw for routine coagulation testing. Few studies have however, challenged this guideline and have shown that comparable results can be obtained in both tubes when a two tube draw is used. This prospective study was done over a 3 months period in the hematology laboratory under the Clinical Hematology unit of a tertiary care teaching institution in North India. Fifty-six paired specimens were drawn from healthy volunteers following the prescribed "two tube draw" method. Prothrombin time (PT) and activated partial thromboplastin time (APTT) were performed within 1 h of sample collection on a fully automated photo-optical coagulation instrument (Ceveron-Alpha). Paired results for PT and APTT were compared using Bland-Altman plots for method comparison. There was good correlation between the PT, INR and APTT of the first and second tubes with bias of 0.09, -0.05 and 0.3 respectively). Bland-Altman plots showed acceptable agreement between the two values with 95 % confidence interval ranging from -0.62 to 0.79 for PT, -0.05 to 0.06 for INR and -3.9 to 4.6 for APTT. Our study has shown no significant difference between PT and APTT values for the first and second tubes. Hence the use of a discard tube is not required.