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The page below is a sample from the LabCE course Detecting and Evaluating Coagulation Inhibitors and Factor Deficiencies. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Preanalytical Variables That Can Cause Falsely Elevated PT or aPTT Results

Improper collection of the blood specimen that is used for testing can cause false prolongation of PT or aPTT results. The following table covers several preanalytical variables that may affect 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.


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