Variable interference of activated protein C resistance in the measurement of protein S activity by commercial assays

Advances in laboratory testing for thrombophilia

Testing for hereditary thrombophilia typically includes tests for activated protein C resistance (APC-R) and/or factor V Leiden, protein C, protein S, antithrombin, and prothrombin G20210A. New options for these assays have become available in recent years, with different advantages and disadvantages among the currently available methods. Potential interferences for each assay type are discussed, including lupus anticoagulants, heparin, warfarin, direct thrombin inhibitors (such as argatroban, dabigatran, hirudin, or bivalirudin), rivaroxaban, factor deficiencies or elevations, factor V Leiden, and specific mutations that the assay(s) might not be able to detect. Causes of acquired deficiencies are also described, as these must be carefully excluded before diagnosing a hereditary deficiency of protein C, protein S, or antithrombin.

Low molecular weight heparin: a possible cause for higher protein S activity than free protein S concentration

Different assays for the assessment of protein S (PS) functional activity are commercially available. We were able to show that, considering the influence of factors known in respect of PS, good agreement can be reached between the results of the determination of free PS as obtained using an immunoassay with monoclonal antibodies and the determination of PS activity as obtained using a test based on activated factor X (factor Xa). However, values of PS activity higher than free PS concentration were obtained in plasma samples taken from patients undergoing therapy with low molecular weight (LMW) heparin. An in vitro incubation of plasma samples with LMW heparin in varying concentrations led, in every case, to an increase of clotting times and thus to an increase of PS activity. In all investigations, the ratios of clotting time with heparin to that without heparin were higher in plasma samples containing PS than in PS-deficient plasma. This result was independent of the use of commercially deficient plasma or the blocking of PS in reference plasma by addition of polyclonal PS antibodies. Obviously, heparin blockers in commercially available assays only neutralize the effect of conventional heparin, and the prolongation of the clotting time is mainly caused by the inhibition of factor Xa by LMW heparin. The reason for the stronger effect in plasma containing PS than in the same plasma after the blocking of PS with polyclonal antibodies as well as in PS-deficient plasma is unclear. Due to the unrecognizable influence of LMW heparin on global clotting assays, the assessment of PS activity values without clear documentation of the application of LMW heparin can lead to improper diagnoses.

Interaction of the protein C/protein S anticoagulant system, the endothelium and pregnancy

Normal pregnancy is associated with significant changes in haemostasis, lipid metabolism and endothelial function. This suggests that maternal adaptation in these systems is required for successful pregnancy outcome. A number of acquired and heritable prothrombotic abnormalities are associated with complications in pregnancy. A common feature of these abnormalities is their ability to alter endothelial function or the protein C/protein S system and increase thrombin generation. In this review the normal function of the endothelium and the protein C/protein S system is detailed. The changes which characterize normal and complicated pregnancies are outlined and the evidence for the impact of heritable and acquired disorders of the protein C/protein S system on pre-eclampsia and fetal loss are discussed.

Laboratory evaluation of hypercoagulable states

The number of well-characterized hereditary and acquired hypercoagulable conditions is increasing, such that in many thrombophilic patients, the laboratory can now identify a hypercoagulable condition. This review describes the currently known hypercoagulable states that predispose patients to venous, and in some instances, arterial thrombosis. For each condition, the discussion includes the incidence, magnitude of the thrombotic risk in the general population in comparison with symptomatic families, synergistic interactions among the various hypercoagulable conditions, molecular pathogenesis, and interpretation of laboratory test results. In addition, recommendations for laboratory testing are summarized.

Prevalence of factor V Leiden in children with thrombo-embolism

Hereditary resistance to the anticoagulatory action of activated protein C (APC resistance, APCR) was identified as a possible new thrombophilic factor in a high percentage (17%-60%) of young adults with thrombotic events. A single missense mutation (R506Q) due to a G/A transition (G1691A) in exon 10 of the factor V gene is regarded as the causative molecular defect, resulting in factor V Leiden which is correlated with APCR. Identification of this mutation by polymerase chain reaction-based methods is easy to perform and prevents pre-analytical and analytical errors in the coagulometric assay for APCR. Since the impact of this mutation in children with thrombo-embolic disease has not been determined to date, we initiated a multi centre prevalence study in two paediatric populations, with and without thrombo-embolic events. We compared 125 paediatric patients with thrombosis, divided into three different age groups (0 to < 0.5 years; > 0.5 to < 10 years; > 10 to < 18 years) with a normal population of 159 children. Although the mutation G1691A was found with an unexpectedly high prevalence of 12% in our normal controls, the prevalence was significantly higher in the age groups; 0 to < 0.5 years (26%) and > 10 to < 18 years (30%). In patients between > 0.5 and < 10 years the overall prevalence was similar to that of the control group (13%). However, in patients of this age with spontaneous thrombosis, G1691A was also a significant risk factor (5/17 approximately equal to 29%). Homozygosity for G1691A was detected in three patients but not in the control group. Including deficiencies of protein C, protein S, antithrombin, and the presence of anti-phospholipid antibodies, thrombosis was correlated with endogenous thrombophilic factors in 38/125 patients (30.4%).

CONCLUSION

Our results emphasize the impact of factor V Leiden on thrombogenesis in children. However, the significance is age-dependent and may reflect the different physiology of haemostasis in the three age groups. The diagnostic workup of children with thrombosis should include tests for factor V Leiden. The correlation of factor V Leiden with the clinical course of thrombo-embolism in children is essential to establish rational guidelines for therapy and prophylaxis of APCR-related thrombosis which are not yet available.

Inherited defects of the protein C anticoagulant system in childhood thrombo-embolism

Childhood thrombo-embolism is mostly the result of inherited thrombophilia or vascular insults combined with risk factors such as peripartal asphyxia, fetopathia diabetica, exsiccosis, septicaemia, central lines, congenital heart disease, cancer, trauma, surgery or elevated antiphospholipid antibodies. Inherited thrombophilia includes mainly defects of the protein C pathway, resistance to activated protein C, protein C or protein S deficiency. Resistance to activated protein C, in the majority of cases caused by the point mutation Arg 506 Gln of the factor V gene, has emerged as the most important hereditary cause of thrombo-embolism in adults and children. However, since an acquired risk of thrombo-embolic complications frequently masks the inherited deficiency in affected children, children with thrombo-embolism should have adequate laboratory evaluation for inherited coagulation disorders, especially the protein C pathway. Until more data on childhood thrombo-embolism are available, treatment recommendations will continue to be extrapolated from guidelines for adults.

APC resistance in neonates and infants: adjustment of the APTT-based method

Resistance to activated protein C (APCR) has emerged as the most important hereditary cause of venous thromboembolism. Using an aPTT-based method together with DNA technique we investigated 120 healthy neonates and infants < 12 months of age and 24 infants with septicaemia for the presence of this mutation. In addition, data of 11 neonates with vascular occlusion, heterozygous (+/-) for the Arg 506 Gln mutation were included. Results of an aPTT-based method (clotting time using the APC/CaCl2 solution obtained in an undiluted, 1:5 and 1:11 dilution with factor V deficient plasma divided by clotting time with CaCl2 in the same plasma dilution) are shown: Whereas 7 (5.5%) out of 120 healthy neonates were (+/-) carriers for the factor V Arg 506 Gln mutation, concordance with the aPTT-based method (cut-off defined as ratio < 2) was found only when using the 1:11 plasma dilution. Six (four) out of 24 infants with sepsis, not carrying the factor V mutation, would have been classified as APC resistant when using the 1:1 (1:5) plasma dilution. Four (two) out of 18 patients, (+/-) for the Arg 506 Gln mutation showed APC ratios > 2 in the 1:1(1:5) plasma dilution.

Central retinal vein and branch artery occlusion associated with inherited plasminogen deficiency and high lipoprotein(a) levels: a case report

We describe a case of central retinal vein and branch artery occlusion associated with inherited type I plasminogen deficiency (68%) and permanent elevation of Lp(a) (460 mg/l, S2 phenotype) in a 45 year old white woman with no associated local or systemic risk factors. Pedigree analysis revealed inheritance of plasminogen deficiency from the deceased father and of high Lp(a) levels from the mother. Both the patient's sons had plasminogen deficiency, but they had normal Lp(a) levels. In a series of 40 consecutive patients with central retinal vein occlusion we previously reported the observation of high Lp(a) levels--consistently associated with the S2 phenotype--in 30% of the patients as compared to a 10% incidence in controls. This case emphasizes the importance of screening patients with occlusion of the retinal vessels and no associated risk factors for coagulation abnormalities predisposing to thrombosis.