Improper collection of the blood specimen used for testing can cause false prolongation of PT or aPTT results. Table 1 covers several preanalytical variables that may affect PT or aPTT test results.
Table 1. Preanalytical Variables Causing Falsely Elevated PT or APTT Test Results.Preanalytical Variable | Cause of False Elevation of PT and or aPTT Test Result | Corrective Action |
Blood collection tube is inadequately filled. | Improper ratio of blood to anticoagulant. Excess anticoagulant causes prolonged PT or aPTT result. | Recollect specimen ensuring proper fill to achieve a blood-to-anticoagulant ratio of 9:1. |
Patient has a hematocrit level above 55%. | Improper ratio of blood to anticoagulant. Excess anticoagulant causes prolonged PT or aPTT result. | Prepare a specimen collection tube that contains less anticoagulant. Refer to your laboratory's procedure for the proper amount of anticoagulant. |
Specimen is clotted. | Coagulation factors have been activated; insufficient levels left in the plasma. PT and aPTT results will be affected. | Recollect the specimen. |
Specimen collected from an arm with a heparin lock or from a heparinized vascular access device (VAD). | Heparin contamination will prolong the aPTT. | Collect the blood from a vein rather than a VAD. If blood must be drawn from the VAD, flush it first with 5 mL of saline, and discard the first 5 mL of blood before collecting the specimen. |
Patient is receiving heparin therapy. | Heparin will prolong the aPTT. | If the patient is being evaluated for possible factor deficiencies or coagulation inhibitors, use a heparin digesting enzyme as a pretreatment before testing the PT or aPTT. |
.