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

Abstract

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.

Guidelines on the laboratory aspects of assays used in haemostasis and thrombosis

“…Although PC CAs are more specific than clotting assays, type IIb PC deficiency can only be detected in the clotting assay. The consequences of failing to detect this rare functional defect by CA must be balanced against the poorer specificity and generally poorer precision of clotting assays 121,122 …”

Comprehensive review of the impact of direct oral anticoagulants on thrombophilia diagnostic tests: Practical recommendations for the laboratory

“…Protein C, a natural anticoagulant, is cleaved and activated by the complex formed by thrombin, thrombomodulin and endothelial PC receptor and converted into aPC which, in presence of P S, inhibit FVa and FVIIIa toning down the amplification phase of the coagulation (Figure 1). 21,27 The measurement of PC activity is required to detect PC deficiency, an risk factor of venous thrombosis with a prevalence of 0.2% that could increase risk of thromboembolism by 5 to 10‐fold 33,34 . Except immunoassays (eg ELISA or LIA) that measure antigen of PC, clot‐based (PT or aPTT‐based) or chromogenic assays are commonly used to assess the PC activity.…”

“…As recommended by current ISTH guidelines, clotting‐based PC assay should be avoided in patients taking DOAC in favour of unimpacted chromogenic assays. Although PT, aPTT and PS and AT measurement are recommended to help for the interpretation of results, these tests could be biased by the presence of DOACs 33,34 . Results and literature about assessment of PC activity suggest that the use of immunological assays or chromogenic assays not involving FX or FII are not impacted by presence of DOACs 1,4,6,7,9,12,19 …”

Genotype–Phenotype Relationships in a Large French Cohort of Subjects with Inherited Protein C Deficiency

“…For this reason, PC AMI assays have been recommended for screening PCD by the International Society for Thrombosis and Haemostasis. 31 However, our findings concerning type IIb mutations (frequency and influence on VTE risk) demonstrate the importance of measuring PC CLOT.…”