Use of the activated partial thromboplastin time for the diagnosis of congenital coagulation disorders: problems and possible solutions.

Abstract

The activated partial thromboplastin time (APTT) is a commonly performed laboratory procedure which is used for multiple purposes including monitoring of heparin therapy, detection of coagulation factor deficiency, and detection of lupus anticoagulants. Among the hereditary coagulation deficiencies, factor VIII and factor IX are the most common. APTT reagents differ widely in both their sensitivity to factor VIII and factor IX deficiencies as well as their responsiveness. Sensitivity may be defined as the ability to identify a deficiency state while responsiveness is indicated by the degree of prolongation of the APTT result as compared to the upper limit of normal. Reagents may be both sensitive and responsive or alternatively sensitive and relatively nonresponsive. Consequently, it is extremely important for each laboratory to carefully identify the upper limit of the normal range. A variety of preanalytical variables will also effect the sensitivity of the APTT to factor deficiency states. These variables include specimen handling and the preparation of platelet poor plasma. The instrument effect is also of importance. Selection of the reagent tends to have the most impact on sensitivity and responsiveness while instrumentation affects the precision of a given APTT. The composition and concentration of phospholipid in APTT reagents does have an effect on reagent responsiveness and sensitivity. Sensitivity to factor deficiencies does not necessarily parallel sensitivity to lupus anticoagulants.