A patient generally needs a level greater than 40% of each factor detected by the test procedure to achieve a normal (non-prolonged) PT or aPTT test result. Therefore, a patient with an inadequate level, meaning less than 40%, of one or more coagulation factors will have a prolonged PT or aPTT test.
In the mixing study, an aliquot of abnormal patient plasma is mixed with an equal amount of pooled normal plasma (PNP), which contains approximately 100% of all coagulation factors. The newly mixed plasma sample includes at least 40% of each factor after the mix, including the factors that may have been present at deficient levels in the original patient sample.
For example, a prolonged PT test would occur if a patient's sample contained 10% of the average amount of factor VII. After adding an equal amount of the pooled normal plasma, with approximately 100% of factor VII, the resulting quantity of factor VII in the sample would be 55%, enough to "correct" the PT test result.
When this new mixture is retested for PT or aPTT, the results that were initially prolonged due to low coagulation factor concentrations will be "corrected." If an inhibitor is present that can counteract and nullify the newly added factors, the assay will not be corrected, and the PT or aPTT will remain prolonged.
