Dear George,
I am finding reference to the use of aged serum or adsorbed plasma in mixing study board review questions, yet no information in the current texts. Are those out of date procedures?
Thank you, Tania Puro
Hi, Ms. Puro. You are correct, the substitution assays are no longer used and are not likely to appear the American Society for Clinical Pathologists (ASCP) Board of Registry examination nor the National Accrediting Agency for Clinical Laboratory Personnel (NCAMLP) examination. A few substitution assay questions appear in the older examination review books, but not in those published since 2003.
Until about 1985, we used barium sulfate adsorbed normal plasma, deficient in factors VII, IX, X and prothrombin, and aged normal serum, deficient in factors V and VIII, prothrombin, and fibrinogen (Weiss AE. The Hemophilias. In Corriveau DM, Fritsma GA: Hemostasis and Thrombosis in the Clinical Laboratory. Lippincott, Philadelphia, 1988). You would treat a plasma that had a prolonged prothrombin time (PT) or activated partial thromboplastin time (PTT, APTT) with both reagents and observe patterns of correction to conclude which coagulation factor was deficient. This assumes the deficiency is congenital and there is only one factor that is deficient.
This approach may have made sense when factor assays were laboriously performed by hand, but was abandoned when automated assays became regularly available. Now, typically, if the PT is normal, PTT is prolonged and the patient is bleeding, we test immediately for factors VIII, IX, and XI. Likewise, in the rare event both are prolonged, we test for prothrombin, V, or X. These rules apply primarily when a congenital single factor deficiency is anticipated.
Several years ago I called Judy Gillison at George King Bio-Medical Inc., who said they shipped their last set of aged serum and adsorbed plasma in 1991 or 92.
The term mixing studies is now reserved for instances when we mix a specimen with a prolonged PT or PTT with reagent normal plasma and test for correction. This method is presented in our lectures, Lupus Anticoagulant 1 and Lupus Anticoagulant 2, found in Educational Modules. Thank you for your question. Geo.
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