Исследование смешивания плазмы: дифференциальная диагностика нарушений коагуляции.

Mixing studies are tests performed on blood plasma of patients or test subjects to distinguish factor deficiencies from factor inhibitors, such as lupus anticoagulant, or specific factor inhibitors, such as antibodies directed against factor VIII. Mixing studies are screening tests widely performed in coagulation laboratories. The basic purpose of these tests is to determine the cause of prolongation of Prothrombin Time (PT), Partial Thromboplastin Time, or sometimes of thrombin time (TT). Mixing studies take advantage of the fact that factor levels that are 50 percent of normal should give a normal Prothrombin time (PT) or Partial thromboplastin time (PTT) result. Factor deficient plasmas (Adsorbed Plasma and Aged Plasma, etc.) are used in mixing studies. Plasma with known factor deficiencies are commercially available but are very expensive, so they are often prepared in the laboratory and can then be used for mixing tests.

Fresh normal plasma has all the blood coagulation factors with normal levels. Plasma from patients on oral anticoagulants (Warfarin etc.) for 48–72 hours is deficient in Factor VII. Adsorbed plasma or plasma from patients on oral anticoagulants (Warfarin etc.) for a week or more is deficient in Factor II, Factor VII, Factor IX, and Factor X. Aged plasma is deficient in Factor V & Factor VIIIC. Serum is deficient in factors I, V & VIIIC. If the problem is a simple factor deficiency, mixing the patient plasma 1:1 with plasma that contains 100% of the normal factor level results in a level ≥50% in the mixture (say the patient has an activity of 0%; the average of 100% + 0% = 50%). The PT or PTT will be normal (the mixing study shows correction). Correction with mixing indicates factor deficiency; failure to correct indicates an inhibitor. Performing a thrombin time on the test plasma can provide useful additional information for the interpretation of mixing tests.

A common problem is an unexplained increase in the PT and/or PTT. If this is observed, the test should be repeated with a fresh sample. Another consideration is heparin. It is possible that the blood sample was mistakenly drawn though a running line. Interference by heparin can be detected by absorbing the heparin with a resin (“Heparsorb”) or by using an enzyme to digest the heparin (“Hepzyme”). Also, the patient's history should be checked, especially with regard to anticoagulant use or liver disease. Provided that the abnormal result is reproduced on a fresh specimen and there is no obvious explanation from the history, a mixing study should be performed. If the mixing study shows correction and no prolongation with incubation, factor deficiency should be looked for, starting with VIII and IX. Vitamin K dependent and nonvitamin K–dependent factors should be considered to rule out accidental or surreptitious warfarin ingestion.