A patient would generally need a level > 40% of each factor that is being detected by the test procedure to achieve a normal (non-prolonged) aPTT or PT test result. Therefore, a patient with an inadequate level, meaning less than 40%, of one or more coagulation factor 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 new mixed plasma sample contains at least a 40% level of each factor after the mix, including the factors that may have been present in very low levels in the original patient sample.
For example, if a patient's sample contained 10% of the normal amount of factor VII, a prolonged PT test would occur. After adding an equal amount of the pooled normal plasma, with an approximate 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 originally prolonged due to low coagulation factor concentrations will "correct". If an inhibitor is present that can counteract and nullify the newly added factors, there will be no correction in the assay, and the PT or aPTT will remain prolonged.