Improper blood specimen collection used for testing can cause false prolongation of PT or aPTT results. Table 4 covers several pre-examination variables affecting PT or aPTT test results.
Table 4. Pre-examination Variables Causing Falsely Elevated PT or aPTT Test Results.| Pre-examination 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. |
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