The presence of a factor deficiency within the intrinsic and/or common pathway may be detected by performing an APTT assay on a mixture of patient plasma and normal pooled plasma without incubation. If the correction of elevated clotting time is observed, a factor deficiency is most likely present. Failure of the normal plasma to correct the elevated APTT would suggest the presence of an inhibitor.
The 9:1 blood/citrate ratio must be kept constant. Adjustments for the amount of anticoagulant should be made when the hematocrit is abnormal (above 55%). Mix immediately to prevent clotting.
Collection Instructions
Send whole blood sample(s) at room temperature as soon as possible.
If specimens cannot be shipped immediately, centrifuge and separate plasma within 1 hour of collection, send frozen plasma on dry ice
APTT mixing studies are not performed:
- if the APTT falls within the normal APTT Reference Interval. See Reporting.
- if the patient is on heparin or oral anticoagulant therapy. See Reporting.