What Do I Do When the Coagulation Sample Is Drawn Incorrectly?
Preanalytic variables represent important sources of error in patient testing and accuracy of results. In hemostasis testing, sample integrity is paramount. Areas in which sample integrity are most affected are in phlebotomy practices, transport and handling of specimens, choice of coagulation tubes, and patient variables.
The sample must be provided from an atraumatic draw, on a properly identified patient, and the tube must be inverted three to four times for proper mixing of anticoagulant. The order of draw in coagulation testing is important to avoid contamination of the sample with tissue thromboplastin. Therefore, if multiple tubes are drawn, the coagulation tube should be last. If only a coagulation sample is requested and the sample is drawn through a butterfly, then a discard tube should be drawn first. If a syringe is needed for phlebotomy, a needle gauge of 12 to 19 is optimal. Additionally, the tubes must be filled to 90% capacity to preserve a 1:9 anticoagulant-to-blood ratio.
There are several coagulation proteins that are labile, namely factors V and VIII. The activity of these factors will be lost if the sample is not tested in an appropriate time span. For maximum activity, testing should be performed within 4 hours for aPTT and up to 24 hours for PT. Plasma can be removed from the sample and stored at -20oC for up to 2 weeks. Additionally, samples must be centrifuged for a period of time that enables them to become platelet poor plasma. Platelet poor plasma is defined as having a platelet count of less than 10,000, which depends upon proper centrifugation. If samples are not platelet poor, falsely shortened results may occur as a result of activation of platelet factor 4. Activation of platelet factor 4 may also occur in heparinized samples that are allowed to sit on red cells for longer than 4 hours. In this case the platelet factor 4 may inactivate the heparin giving a falsely shortened PTT result.
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