aPTT 1:1 Normal Plasma

Panels that include this biomarker

• Factor VIII Inhibitor Profile, Comprehensive 117157

  4 analytes · Plasma · Fasting not stated · Turnaround 2 - 3 days

  Method and collection

  Method

  The factor VIII inhibitor (Bethesda titer) assay is performed using an activated partial thromboplastin time (aPTT)-based test system.6 Serial dilutions are made of patient plasma with veronal buffered saline, then mixed with normal plasma containing close to 100% factor VIII activity and are then incubated for one hour. An aPTT-based factor VIII assay using factor VIII-depleted plasma substrate is then performed on these incubated mixtures. Results are compared to those of incubated normal plasma. One Bethesda unit is defined as the amount of factor VIII inhibitor that neutralized 0.5 IU of factor VIII in this system. The number of serial dilutions tested is based on the anticipated level of the inhibitor.

  Preparation

  The patient should not be anticoagulated. Do not draw from an arm with a heparin lock or heparinized catheter.

  Collection

  Citrated plasma samples should be collected by double centrifugation. Blood should be collected in a blue-top tube containing 3.2% buffered sodium citrate.1 Evacuated collection tubes must be filled to completion to ensure a proper blood to anticoagulant ratio.2,3 The sample should be mixed immediately by gentle inversion at least six times to ensure adequate mixing of the anticoagulant with the blood. A discard tube is not required prior to collection of coagulation samples, except when using a winged blood collection device (ie, "butterfly"), in which case a discard tube should be used.4,5 When noncitrate tubes are collected for other tests, collect sterile and nonadditive (red-top) tubes prior to citrate (blue-top) tubes. Any tube containing an alternate anticoagulant should be collected after the blue-top tube. Gel-barrier tubes and serum tubes with clot initiators should also be collected after the citrate tubes. Centrifuge for 10 minutes and carefully remove 2/3 of the plasma using a plastic transfer pipette, being careful not to disturb the cells. Deliver to a plastic transport tube, cap, and recentrifuge for 10 minutes. Use a second plastic pipette to remove the plasma, staying clear of the platelets at the bottom of the tube. Transfer the plasma into a Labcorp PP transpak frozen purple tube with screw cap (Labcorp No. 49482). Freeze immediately and maintain frozen until tested. Please print and use the Volume Guide for Coagulation Testing to ensure proper draw volume.

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