Mixing test

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The mixing test is a medical laboratory study used to clarify the differential diagnosis of blood clotting abnormalities. Other names for the test include mixing studies, PT mixing study, PTT mixing study, circulating anticoagulant screening test, or inhibitor screen.

The test is most often initiated when a patient has an abnormally long prothrombin time (PT) or more usually, a prolonged partial thromboplastin time (PTT), to distinguish whether the underlying problem is a clotting factor deficiency on the one hand, or a factor inhibitor on the other hand.

The test relies on the fact that the circulaing level of any coagulation factor must be reduced to a level significantly less than 50% in order to affect the PT or PTT. Thus when the patient's blood is mixed with an equal volume of blood donated by a person known to have normal clotting, the mixture (which will have at least 50% factor activity for all factors) will clot more quickly if the initial disorder was because of a deficiency, while it will not clot any faster if the initial disorder was because of a factor inhibitor.

If the PTT of the mixed blood is normal, factor deficiency is indicated, and further testing will be performed for specific factors. If the PTT of the mixed blood is still prolonged, an inhibitor is suggested, and further studies (typically starting with an assay for lupus anticoagulant) will be performed. This result, however, can also be seen if the patient is taking heparin or another anticoagulant. If the PTT of the mixed blood initially normalized, but becomes prolonged again after a two hour incubation, the test is suggestive of a factor VIII inhibitor (because their effect requires this amount of time to become effective). When a mixing test shows this result, a factor VIII inhibitor assay will be performed.