Further Evidence that Activated Protein C Resistance Affects Protein C Coagulant Activity Assays

Laboratory Limitations of Excluding Hereditary Protein C Deficiency by Chromogenic Assay: Discrepancies of Phenotype and Genotype

Protein C (PC) deficiency is associated with an increased risk for venous thromboembolism (VTE). In daily practice, exclusion of a hereditary PC deficiency is often based on a single determination of PC activity, by either clotting time–based or mostly chromogenic assay. However, diagnosis of hereditary PC deficiency is challenging due to several laboratory and clinical limitations. We compared the potential of PC activity values measured by either chromogenic or clotting time–based assay to predict a variation in the PROC gene. One hundred one (35%) of 287 patients carried variations within the PROC gene, including 2 previously not published variations. In 20 (20%) patients with identified variation, PC activity, determined by chromogenic assay, was within the reference range. For prediction of an underlying genetic defect determined by chromogenic and clotting time–based assay, sensitivity was 80% versus 99%, specificity 75% versus 18%, positive predictive value 64% versus 39%, and negative predictive value (NPV) 88% versus 97%. The lower NPV of chromogenic versus clotting time–based PC assay can be mainly explained by the presence of PC deficiency type IIb. Following our proposed diagnostic algorithm, additional measurement of PC activity by clotting time–based assay in case of a positive VTE history improves detection of this subtype of PC deficiency. Considering potential therapeutic consequences for primary and especially for secondary VTE prophylaxis, genetic analysis is required not only for confirmation but also for clarification of PC deficiency.

Recommendations for clinical laboratory testing for protein C deficiency, for the subcommittee on plasma coagulation inhibitors of the ISTH

Inherited protein C (PC) deficiency increases risk of venous thromboembolism (VTE) by 5 to 10-fold in thrombosis-prone families; however, heterozygous PC deficiency alone does not determine that a subject has thrombophilia. Protein C deficient subjects, who lack additional inherited risk factors such as factor V Leiden or have no major acquired risk factors, may not suffer from VTE. In addition, PC deficiency may be acquired, often due to vitamin K antagonist treatment or liver disease. In contrast, homozygous or compound heterozygous PC deficiencies are rare and serious disorders, and affected infants are often in families with no history of PC deficiency or thrombosis. Laboratories commonly use the chromogenic PC assay to diagnose deficiency. Chromogenic assay is recommended due to its good specificity, but this assay fails to detect the rare type 2b deficiency where the defect is due to poor interaction with calcium ions, phospholipid, protein S, and factor Va and factor VIIIa. The clotting-based assay of PC is capable of detecting type 2b deficiency but it has reduced specificity. Importantly, PC level varies with age, adult reference ranges cannot be applied to babies or children and levels may not reach those of adults even in adolescence. Pre-analytical variables in the specimen affect measurement of PC, and can be assay-dependent; for example, a partially clotted sample will have falsely raised PC level by chromogenic assay but reduced level by clotting-based assay. Direct oral anticoagulants falsely raise PC level in the clotting-based assay but the standard chromogenic assay is unaffected.

Quality Assurance Issues and Interpretation of Assays

The consequences of an erroneous thrombophilia diagnosis may be serious if it is used to determine clinical management. Therefore careful selection, assessment, and control of laboratory tests for thrombophilia are essential. As for other coagulation tests, the pre-analytical phase must be carefully controlled with attention to the specific problems associated with each type of assay. The investigator must then recognize that for most laboratory tests of thrombophilia, there are a number of assay types available, often based on different principles of analysis. This creates the potential for different users to obtain varying results depending on the technique employed. Such problems can occur in assays of antithrombin activity, depending on whether the assay employs factor Xa, human thrombin, or bovine thrombin. In clot-based assays of protein C and protein S, there can be specificity problems related to interference by factor V Leiden (FVL), antiphospholipid antibodies, and other substances. Even genetic tests can give erroneous results and should not automatically be seen as absolute without supporting evidence and careful quality-control measures. Whatever technique is selected, it is mandatory to incorporate sufficient concurrent quality-control samples to validate the results of thrombophilia tests. These should include assessment of the parameter at normal and abnormal levels to give confidence in results across the measurement range that would normally be encountered in routine practice. This should be used in conjunction with regular participation in external quality assessment (EQA) (which has been linked to improved laboratory performance in thrombophilia testing). Larger EQA programs can provide information concerning the relative performance of analytical procedures, including the method principle, reagents, and instruments. Herein, we describe many of the methodologic effects in detail. We use specific examples to illustrate the general principle that, in performing laboratory testing for thrombophilia, one must always consider the performance characteristics and limitations of the assay in use.