Step Two
The next step is retesting the PT and/or aPTT test with the newly created sample mixture. The results should be documented on a worksheet to compare to the original PT and/or aPTT test results.
Step Three
The sample made for the mixing study consisted of pooled normal plasma and the patient plasma should also be incubated to rule out any slow-reacting inhibitors. To do this, the "mixed" specimen is incubated at 37°C for 1–2 hours or as long as required by your laboratory's procedure. A set of control tubes should also be incubated at the same time as the mixed sample tube. A pure patient plasma sample and a pure pooled standard plasma sample will serve as the controls in this procedure. You may incubate all three tubes in a water bath or heat block (refer to the image on the right). The incubation of these controls will account for the heat-labile state of some coagulation factors.
After the incubation phase, the PT and/or aPTT tests should be repeated. If coagulation inhibitors were present in the patient sample, the incubation phase would have given the ideal temperature and time for the antibody-coagulation factor reaction. This is especially helpful in the case of anti-factor VIII inhibitors since they are often slow-acting or weak inhibitors.
