Laboratory Testing for the Evaluation of Phenotypic Activated Protein C Resistance

Abstract

Activated protein C (APC) resistance (APCR) is considered a risk factor of venous thromboembolism (VTE). The most common genetic disorder conferring APCR is a factor (F) V Leiden mutation, but many other factors are also implicated, such as other F5 mutations (e.g., FV Hong-Kong and FV Cambridge), protein S deficiency, elevated factor VIII, exogenous hormone use, pregnancy and postpartum, depending on how APCR is defined. Considering the large population affected, the detection of this phenotype is crucial. Two types of tests are currently available: clotting time-based assays (with several versions) and thrombin generation-based assays with the endogenous thrombin potential (ETP)-based assay. The purpose of this review is therefore to discuss the performances of these tests and the cases in which it would be appropriate to use one over the other. Initially, as APCR was thought to be solely related to the FV Leiden mutation, the objective was to obtain a 100% specific assay. Clotting-time based assays were thus specifically designed to detect this inherited condition. Later on, an APCR condition without a FV Leiden mutation was identified and highlighted as an independent risk factor of VTE. Therefore, the development of a less specific assay was needed and a global coagulation test was proposed, known as the ETP-based APCR assay. In light of the above, these tests should not be used for the same purpose. Clotting time-based assays should only be recommended as a screening test for the detection of FV mutations prior to confirmation by genetic testing. On the other hand, the ETP-based APC resistance assay, in addition to being able to detect any type of APCR, could be proposed as a global screening test as it assesses the entire coagulation process.

The Use of Bypassing Treatment Strategies in Hemophilia and Their Effect on Laboratory Testing

“…Subsequent modifications to the assay included preanalytical mixing of patient plasma with factor Vdeficient plasma to reduce such interferences, and development of Russell viper venom-based assays which are more specific for factor V Leiden. 22 Protein C (PC) and protein S (PS) are vitamin K-dependent proteins. PC is activated to APC when it complexes with thrombin and endothelial thrombomodulin; APC then complexes with its cofactor PS to rapidly inactivate factors V and VIII, thus regulating thrombin generation and clot formation.…”

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“…Close attention must be paid to the preanalytical (specimen collection and handling), 1 analytical (the testing process), 2 and postanalytical (test interpretation contribution to diagnosis and treatment) 3 variables associated with coagulation testing to assure accurate reporting of assay result(s) and corresponding result interpretation(s). The manuscripts in this edition of the journal represent state of the art summaries of testing aspects for antiphospholipid antibodies (including lupus anticoagulant [LA] 4 and solid phase testing 5 ), D-dimer, 6 activated protein C (APC) resistance, 7 and von Willebrand factor (VWF), [8][9][10][11][12][13] highlighting information essential for accurate diagnosis and monitoring of common disorders that significantly impact health care systems and patient lives.…”

“…20 In the fourth manuscript of the issue, Morimont et al summarize laboratory testing to evaluate phenotypic APC resistance including a review of different mechanisms resulting in this phenotype and descriptions of the functional assays that can be used for detection. 7 Particularly valuable is the description of endogenous thrombin potential (ETP) assays that provide a more global view of APC resistance in contrast to clot-based assays that are optimized for detection of APC resistance due to the factor V Leiden mutation. For instance, the paper highlights ETP-based detection of an APCresistant phenotype due to hormonal contraceptive use.…”