A Protac-based screening test for activated protein C-resistant factor Va and other defects of the protein C anticoagulant pathway

Haemostasis patterns in patients with acute-on-chronic liver failure and acute decompensation of cirrhosis including thromboelastometric tests with and without the addition of Protac: a pilot study

BACKGROUND

Thromboelastometry is considered the best method to assesses hemostasis in liver disease. Diagnostic performance could be improved by adding protein C activators such as thrombomodulin or Protac®. We assessed changes in ROTEM parameters after the addition of Protac® in patients with acute-on-chronic liver failure (ACLF), acute decompensation (AD), and healthy individuals (HI) to define different hemostasis patterns, considering standard and velocity ROTEM parameters, and assess whether Protac® can improve the definition of the pattern.

METHODS

Pre-test, we investigated whether diluted EXTEM reagent improved the effect of Protac® on the clotting time (CT)-ratio with and without Protac®. Ten ACLF and 20 AD patients and 21 HI were included in the main study.

RESULTS

Standard EXTEM was used in the main study. INTEM CFT, INTEM A5 (inverse), and INTEM TPI (inverse) were the parameters that best differentiated liver disease from HI (ROC AUC, 0.921, 0.906, and 0.928, respectively; all P-values < 0.001). Combining INTEM CFT with EXTEM LI60-ratio only slightly improved the diagnostic performance (ROC AUC, 0.948; P < 0.001). EXTEM LI60 and INTEM maxV-t were the parameters that best differentiated between ACLF and AD patients (ROC AUC, 0.743, P = 0.033; and 0.723, P = 0.050; respectively). Combining EXTEM LI60 + INTEM maxV-t moderately improved the diagnostic performance (ROC AUC, 0.81, P < 0.001).

CONCLUSIONS

ROTEM velocity, fibrinolysis parameters and the indices calculated improve the diagnosis in combination with standard parameters (e.g., CFT and A5). Ratios calculated with and without Protac® (e.g., EXTEM LI60-ratio) only slightly increased the diagnostic performance in discriminating hemostasis patterns.

Detection of the imbalance of procoagulant versus anticoagulant factors in cirrhosis by a simple laboratory method

Patients with cirrhosis possess an imbalance in procoagulant versus anticoagulant activity due to increased factor VIII and decreased protein C. This imbalance can be detected by thrombin-generation assays performed in the presence/absence of thrombomodulin (predicate assay) that are not readily available in clinical laboratories. We sought to assess this hypercoagulability with a simpler thrombin-generation assay performed in the presence/absence of Protac, a snake venom that activates protein C in a manner similar to thrombomodulin (new assay). We analyzed blood from 105 patients with cirrhosis and 105 healthy subjects (controls). Results for the predicate-assay or the new-assay were expressed as ratio (with:without thrombomodulin) or as Protac-induced coagulation inhibition (PICI%). By definition, high ratios or low PICI% translate into hypercoagulability. The median(range) PICI% was lower in patients (74% [31%-97%]) than controls (93% [72%-99%]; P < 0.001), indicating that patients with cirrhosis are resistant to the action of Protac. This resistance resulted in greater plasma hypercoagulability in patients who were Child class C than those who were A or B. The hypercoagulability of Child C cirrhosis (63% [31%-92%]) was similar to that observed for patients with factor V Leiden (69% [15%-80%]; P = 0.59). The PICI% values were correlated with the levels of protein C (rho = 0.728, P < 0.001) or factor VIII (rho = -0.517, P < 0.001). Finally, the PICI% values were correlated with the predicate assay (rho = -0.580, P < 0.001).

CONCLUSION

The hypercoagulability of plasma from patients with cirrhosis can be detected with the new assay, which compares favorably with the other markers of hypercoagulability (i.e., high factor VIII and low protein C) and with the predicate-assay based on thrombin-generation with/without thrombomodulin. Advantages of the new assay over the predicate assay are easy performance and standardized results. Prospective trials are needed to ascertain whether it is useful to predict thrombosis in patients with cirrhosis.

Endogenous Thrombin Potential for Predicting Risk of Venous Thromboembolism in Carriers of Factor V Leiden

Measurement of endogenous thrombin potential (ETP) detects hypercoagulability and can be used to identify activated protein C resistance due to factor V Leiden (FVL). However, not all carriers of FVL suffer thrombosis and therefore we sought to determine if the test for ETP could be modified in such a way as to enable detection of FVL patients who were at increased risk of venous thromboembolism. Protac, an activator of both protein C and factor V, was incorporated into the traditional thrombin generation reaction and ratios (reaction with Protac:reaction without Protac) were calculated. Plasma samples from 42 FVL heterozygotes (12 with a history of thrombosis and 30 with no prior thrombosis) and 38 controls (non-FVL with no history of thrombosis) were analysed. The mean ETP ratio was significantly higher in FVL heterozygotes (0.90 +/- 0.06) compared to normal controls (0.41 +/- 0.10; p = 0.00004). Multivariate analysis indicated that the average ETP ratio was significantly and inversely correlated with factor V levels in FVL heterozygotes (p = 0.002) but not controls. Within the FVL group, patients with a history of thrombosis had higher ETP ratios (0.92 +/- 0.06) compared to those without (0.89 +/- 0.05), however, this did not reach statistical significance (p = 0.09). Further investigation into the use of ETP for detecting risk of thrombosis in people who are genetically predisposed is warranted. The recent introduction of diagnostic ETP measurements in the form of the calibrated automated thrombin generation from Thrombinoscope and the TechnoThrombin from Baxter should facilitate such studies.

The reduced sensitivity of the ProC Global test in protein S deficient subjects reflects a reduction in the associated thrombotic risk

To investigate simultaneously a defect affecting the protein C/protein S (PC/PS) anticoagulant pathway is possible thanks to a methodological approach (ProC(R) Global; Dade Behring) based on the activation of endogenous plasma PC by a snake venom extract. Factor V (FV) Leiden, the most frequent cause of hereditary thrombosis, is well detected by the test with sensitivity of 100% irrespective of the presence/absence of thrombosis in the subjects investigated. The test is also suited to detect PC or PS defect, but in this case the in vitro impairment of the PC/PS pathway is less pronounced particularly for PS defects (sensitivity for PC and PS defect, 85-100 and 30-90%, respectively). In this study, we hypothesized that the lower sensitivity described for PS defect, compared with those of PC and FV Leiden defects, could also be related to the clinical condition of the subject investigated (symptomatic/asymptomatic) rather than solely to the PS plasma activity/level. Therefore, we analyzed 126 subjects with single congenital defects in the PC/PS pathway: 46 subjects with PS deficiency (26 thrombotic cases and 20 asymptomatic relatives), 40 subjects with PC deficiency (25 thrombotic cases and 15 asymptomatic relatives), and 40 heterozygous FV Leiden subjects (25 thrombotic cases and 15 asymptomatic relatives). By a cut-off of normalized Agkistrodon contortix snake venom ratio of 0.84, the sensitivity in the whole group of cases (sensitivity a) was 76.1, 95.0 and 100%, respectively, for PS, PC and FV Leiden defects. The test failed to detect 11 (23.9%) among the 46 PS-deficient subjects, and all these cases except two belonged to the asymptomatic subgroup (9/20; 45%). Excluding the 20 asymptomatic relatives, the new sensitivity (sensitivity b) for the PS defect was 92.3%. The comparison of the sensitivity in the symptomatic PS cases and in the asymptomatic ones was significantly different (P = 0.010). Among the 40 PC-deficient subjects, only two (5.0%) were not detected by the test and they belonged indifferently to the two subgroups. Finally, none of the 40 FV Leiden heterozygotes were misdiagnosed by the test. These results suggest that in symptomatic PS-deficient cases the test could reflect a post-thrombotic effect and/or reveal potential unidentified prothrombotic influences assessing a prothrombotic risk condition.

Sensitivity of the ProC global assay for protein C pathway abnormalities. clinical experience in 899 unselected patients with venous thromboembolism

The ProC Global is a clotting assay designed to globally evaluate the functionality of the protein C (PC) pathway, which is found defective in up to 30% of the Caucasian patients with thrombophilia. It is based on the ability of endogenous activated protein C (APC), generated by activation of PC by a snake venom extract, to prolong an activated partial thromboplastin time (APTT). This retrospective study was carried out to evaluate the ability of this assay to distinguish patients with or without any PC pathway abnormalities in a cohort of 899 unselected patients with a history of thrombosis. The result of the ProC Global assay, expressed in PC activation time-normalized ratio (PCAT-NR), was significantly lower in patients in whom was previously demonstrated an abnormality of the PC pathway compared to those without. The cut-off level of PCAT-NR=0.75 was found to provide the best sensitivity-specificity ratio, since all the patients with the factor V (FV) Leiden mutation (n=71), APC resistance (n=3), PC deficiency (n=22), or combined defects (n=19) had a PCAT-NR below that value. The sensitivity of the ProC Global assay for a low protein S (PS) level (n=56) was only 66%, and was even weaker in the case of hereditary PS deficiency (46.6%, n=15). The assay did not perform well in samples from patients on oral anticoagulant treatment (OAT, n=64) or with liver failure (n=4), as the PCAT-NR was reduced in most cases, even in the absence of any abnormality. These results suggest that the ProC Global assay could be validly used as a first-step screening test for the FV Leiden-related APC resistance and PC deficiency in patients not on OAT. Given the moderate sensitivity of the assay for PS deficiency, this coagulation inhibitor must be determined in every case. However, the overall benefit of such a screening strategy is limited since more than 38% of the 659 patients without abnormality had a decreased PCAT-NR.

Method-dependent influence of certain polymorphisms in the factor V B-domain on the response to activated protein C

The factor V (FV) B-domain is extremely important to the cofactor function of native FV for activated protein C (APC) in the inactivation of factor VIII (FVIII). In a previous study, we found that in the B-domain coding portion of DNA, the polymorphisms at nucleotide positions 2391, 2663, 2684, and 2863 were associated. In the major allele, all bases are A (A allele) and those in the minor allele are G (G allele). This study concerns itself with the question of whether or not there are differences in the APC response between the A allele and the G allele in plasma samples from persons without the FV Leiden. The APC ratios of homozygous carriers of the major A allele and the minor G allele do not differentiate themselves in classical activated partial thromboplastin time-based assays. In contrast, a test based on the deactivation of FVIII in the tenase complex in homozygous carriers of the minor G allele showed significantly lower APC ratios (P = 0.001) in comparison with the major A allele. The results of the investigation after modification of the test indicate that mutative changes in the B-domain apparently influence the interaction among phospholipids, APC, FV, and protein S. An increase in FVIII through the introduction of the FVIII concentrate Kogenate to the plasma samples was associated with a drop in the APC ratios of both genotypes. After defining 59 age- and sex-based matched pairs without the FV Leiden, the observed frequency of the minor G allele was higher in the non-thrombotic group (33.0%) than in the thrombotic group (22.8%). However, the difference did not reach the level of significance (odds ratio, 0.53; 95% confidence interval, 0.26-1.12). It does, nevertheless, appear possible that a homozygous condition for the minor allele in combination with a defect known to be associated with thrombophilia represents an additional thrombogenetic risk factor.

Screening for abnormalities of the protein C anticoagulant pathway using the ProC Global assay. Results of a European multicenter evaluation

ProC Global is a new global clotting assay designed to evaluate the functionality of the protein C anticoagulant pathway. It is based on the ability of endogenous activated protein C, generated by activation of protein C by Protac, to prolong an activated partial thromboplastin time, and the results are expressed in protein C activation time normalized ratio (PCAT-NR), after normalization. This multicenter trial involving five European laboratories was designed in order to determine the ability of the ProC Global assay to distinguish patients with and without abnormalities of the protein C pathway. The PCAT-NR was significantly lower in the patients with a thrombotic history not on oral anticoagulant treatment (n = 627) than in the healthy controls (n = 148), even after exclusion from both groups of the patients with abnormality of the protein C pathway. Using receiver operator characteristics analysis, the cut-off level of PCAT-NR = 0.80 was found to provide the best sensitivity-specificity ratio. All the carriers of the factor V Leiden mutation (n = 73), as well as all the patients with activated protein C resistance (n = 42), had a PCAT-NR below 0.80. The ProC Global assay performed well in patients with combined defects (97.0%, n = 33) or protein C deficiency (91.3%, n = 46), but it failed to detect all of them, and one patient with combined defects as well as four patients with a low protein C level had a PCAT-NR above the cut-off level. The sensitivity of the assay for protein S deficiency (n = 58) was weak (only 69.0%) and, surprisingly, more than 40% of the 375 patients without any of these abnormalities of the protein C pathway had a PCAT-NR below the cut-off level.

Screening for risk factors for thrombosis using a new generation of assays developed to evaluate the functionality of the protein C anticoagulant pathway

These results, obtained in a small series of patients, suggest that both the ProC Global assay and the PCP Test would be suitable, using well-defined cut-off levels, to identify all the carriers of the Factor V Leiden mutation and all the patients with a protein C deficiency or with combined defects of the protein C pathway. For both assays, however, the sensitivity for protein S deficiency was below 60%. These results in selected patients are congruent with those previously reported in the literature about the ProC Global assay, the PCP Test, and other assays evaluating the functionality of the protein C anticoagulant pathway. All demonstrated a weak sensitivity to protein S deficiency, suggesting that protein S plays only a minor role as an APC cofactor in such global assays. A major discrepancy between the two evaluated assays was obtained in the group of patients without abnormality of the protein C pathway. Actually, using the ProC Global assay, more than 40% of the patients had a decreased PCAT-NR while presenting with none of the three tested abnormalities, whereas none of the studied patients had a ratio below 1.80, and only 5 of 143 (3.5%) had a ratio below 2.00 when using the PCP Test. The observation that around 40% of the control patients had a decreased PCAT-NR could suggest the influence of currently unknown defects of the protein C/protein S pathway on the ProC Global assay. It could also be hypothesized that the higher factor VIII levels already reported in patients with a history of thrombosis than in controls had a significant role in the low responsiveness, but this parameter was not tested in the authors' series. In that connection, it is also well established that elevated factor VIII levels both shorten the APTT and reduce the anticoagulant effect of heparin when evaluated using APTT. Actually, some of the samples investigated in this study were obtained during the acute phase of thrombosis. It is not possible to draw out the hypothesis of an association of biologically undetectable minor changes in various factors involved in the protein C anticoagulant pathway; all the individual factors would remain within their normal ranges. Finally, because 40% of the patients without abnormalities of the protein C pathway had a decreased PCAT-NR, the question arises whether the ProC Global assay might in itself be a biologic marker of thrombophilia, independent of its sensitivity for abnormalities of the protein C anticoagulant pathway. In that connection, the correlation between the result of the ProC Global assay and the risk for thromboembolism was recently evaluated by two different groups. In both cases, the preliminary results suggested that a decreased response to the ProC Global assay might be an independent risk factor for venous thrombosis. The two global assays could therefore have distinctly different applications. If the global assays are used in the hemostasis laboratory to screen for abnormalities of the protein C pathway, and thus to rationalize the use of specific assays, the PCP Test should be chosen, because of its high specificity. Because only 3.5% of the control patients had a ratio below 2.00 (and none had a ratio below 1.80), the PCP Test could be accurately used as a first-step assay in the laboratory screening for these abnormalities of the protein C anticoagulant pathway. Using such a flow chart, the specific assays for APC resistance or the identification of the factor V Leiden mutation and protein C would be performed only in case of a ratio below a cut-off defined using receiver operating characteristic (ROC)-analysis in unselected patients. Because of the weak sensitivity of this assay to both constitutional and acquired protein S deficiencies (below 15% using 1.80 as the cut-off level, or 60% using 2.00), the measurement of this parameter had to be performed in all cases. If, on the other hand, the assay is used to screen for risk factors for thrombosis, the ProC Global assay could b