Monoclonal antibodies to crosslinked fibrin degradation products (XL-FDP). I. Characterization and preliminary evaluation in plasma

Evaluation of pre-analytical, demographic, behavioural and metabolic variables on fibrinolysis and haemostasis activation markers utilised to assess hypercoagulability

BACKGROUND

Measurement of some haemostatic factors and products formed during activation of haemostasis seems to be promising in the determination of hypercoagulability.

METHODS

The fibrinolytic variables euglobulin clot lysis time, tissue-type plasminogen activator, plasminogen activator inhibitor-1 and the haemostasis activation markers prothrombin fragment 1+2, thrombin-antithrombin complex and D-dimer were determined in 101 apparently healthy men and women aged 20-92 years (58+/-18 years, mean+/-SD) to establish variability due to several demographic, behavioural and metabolic factors.

RESULTS

None of the fibrinolytic variables were affected by smoking, while tissue-type plasminogen activator antigen was significantly lower in women compared to men. Multiple regression analysis revealed several independent associations between tissue-type plasminogen activator, plasminogen activator inhibitor, body mass index and lipid levels, describing up to 40% of the variance in fibrinolytic variables. For haemostasis activation markers, no gender difference or effect of smoking was observed. Only D-dimer was independently associated with age. The haemostasis activation markers determined proved to be extremely sensitive to blood sampling procedure and were significantly higher in samples obtained by an untrained nurse compared to a trained nurse.

CONCLUSIONS

Fibrinolytic variables are predominantly modulated by age, body mass index and blood lipids, while haemostasis activation markers are mainly un-influenced by these factors.

Decrease in sensitivity of D-dimer for acute venous thromboembolism after starting anticoagulant therapy

D-dimer testing is useful for the exclusion of acute venous thromboembolism (VTE). Anticoagulant therapy is expected to reduce D-dimer levels in patients with thrombosis and, consequently, it may not be safe to use D-dimer levels to exclude VTE after anticoagulant therapy has been started. The objectives of this study were to estimate the decrease in D-dimer levels after 24 h of heparin therapy and, applying this estimate to the results of a recent study, to calculate the expected reduction in sensitivity. Using pre-defined criteria, we first performed a literature review to determine whether, and by how much, D-dimer levels decrease within 24 h of starting heparin therapy in patients with acute VTE. Using D-dimer levels that were measured in a prospective study of patients with confirmed deep vein thrombosis and/or pulmonary embolism as baselines, we then determined the change in sensitivity (and specificity) that would result from the fall in D-dimer levels that the literature review suggested would have occurred after 24 h of heparin therapy. On the basis of the literature review, we calculated that mean D-dimer levels decrease by 25%, 24 h after starting heparin therapy in patients with acute VTE. This 25% decrease in D-dimer levels resulted in a decrease in sensitivity from 95.6% (95% confidence interval, 90.0-98.6) to 89.4% (95% confidence interval, 83.7-95.1). There is a decrease in D-dimer levels in patients with acute VTE 24 h after starting heparin therapy that is expected to result in a clinically important drop in sensitivity.

Fibrin degradation product (FnDP) assays: analysis of standardization issues and target antigens in plasma

The in vivo formation of fibrin and its subsequent secondary fibrinolytic digestion yields a variety of crosslinked fibrin degradation products (XL/FnDP). One of these is known as D-dimer and a variety of ELISA-type commercial kits using monoclonal antibodies to D-dimer have been developed. We have investigated the possibility of establishing a standard such that these various kits might indicate the same levels of D-dimer in the same samples. We have concluded that because it seems that each individual monoclonal antibody to D-dimer targets a unique and distinct epitope in the FnDP fraction the notion of introducing a standard D-dimer which will respond uniformly to each D-dimer monoclonal antibody is not feasible. Using various monoclonal and polyclonal antibodies in conjunction with gel exclusion chromatography we have examined human plasma derived from patients with disseminated intravascular coagulation (DIC) which contained high levels of fibrin degradation products (FnDP). The data suggested that the FnDP fraction in plasma contained mostly high molecular weight (> 2 x 10(6) daltons) crosslinked fragments which contain high levels of fibrinopeptide A. Many of these crosslinked fragments crossreact with antibodies to D-dimer because they each contain D-dimer as a structural component. On the basis of this data, a novel sequence of events is proposed which may occur during the aggregation of fibrin in vivo.

The fibrinolytic response to ancrod therapy: characterization of fibrinogen and fibrin degradation products

Ancrod is a purified coagulant venom which renders blood incoagulable by cleaving fibrinopeptide A (FPA) from fibrinogen, but the mechanism involved in the clearance of fibrin from the circulation is unknown. To investigate the fibrinolytic response to ancrod, and to increase understanding of clearance mechanisms, six patients with peripheral vascular disease causing claudication were infused with ancrod at 2 u/kg over 6 h followed by 2 u/kg at 12 h intervals for 38 h. Venous blood samples were taken at time 0, 3, 6, 25 and 49 h for assay of fibrinogen (Fbg), fibrinopeptide A (FPA), total fibrin(ogen) degradation products (TDP), fibrin degradation products (FbDP), fibrinogen degradation products (FgDP), cross-linked fibrin degradation products (XL-FDP), tissue plasminogen activator (tPA), urinary type plasminogen activator (u-PA), plasminogen, alpha 2 antiplasmin (alpha 2 AP) and plasminogen activator inhibitor-1 (PAI-1). Fibrinogen (median and range) was 2.3 (1.4-3.90) g/l at time 0 and thereafter was undetectable. FPA rose from 2.5 (1.8-3.6) to 600 and 188 pmol/l at 3 h and 6 h and remained elevated. TDP, FbDP and FgDP increased greatly following ancrod while there was no evidence of XL-FDP. The surprising increase in FgDP during defibrination suggests either that fibrinogen is digested following its incorporation into circulating fibrin protofibrils or that some of the fibrin subunits in the photofibril retain one of the two fibrinopeptide A's. tPA and uPA remained unchanged. Plasminogen fell from 125 (100-155)% to 79 (40-118)% at 49 h and alpha 2 AP fell from 91 (75-107)% to 24 (10-35)% at 49 h. The level of PAI-1 was depressed during defibrination, with the exception of the 6 h data. The results demonstrate that ancrod removes FPA from fibrinogen to produce non-cross-linked (soluble) fibrin. This is cleared from the circulation without evidence of an increase in the circulating activities of the plasminogen activators, tPA or UK, but with evidence of plasminogen activation and consumption.

Fibrinolysis in cerebrospinal fluid after intraventricular haemorrhage

Concentrations of cross linked fibrin degradation products were measured in the cerebrospinal fluid from five 'normal' preterm infants (median 102 ng/ml), four preterm infants with intraventricular haemorrhage (median 315 ng/ml), and five infants with progressive post-haemorrhagic ventricular dilatation (median 1000 ng/ml). Serial samples of cerebrospinal fluid from one infant showed a peak concentration two weeks after the haemorrhage.

Measurement of D-dimer in plasma as diagnostic aid in suspected pulmonary embolism

The potential of plasma measurement of D-dimer (DD), a specific derivative of crosslinked fibrin, for diagnosis or exclusion of pulmonary embolism was investigated in a prospective series of 171 consecutive patients who attended an emergency department with suspected pulmonary embolism. The diagnosis was made or excluded by means of a clinical decision-making process which included clinical evaluation, ventilation-perfusion (VQ) lung scan, and, as indicated, pulmonary angiography, venography, or non-invasive examination of the leg veins. Pulmonary embolism was diagnosed by this process in 55 (32%) of 170 patients with sufficient data. All but 1 of these 55 patients had a DD concentration of 500 micrograms/l or above. The sensitivity and specificity of this cutoff concentration for the presence of pulmonary embolism were 98% and 39%, respectively, which give positive and negative predictive values of 44% and 98%. Among the 115 patients (68%) who had inconclusive VQ scans, 31 were diagnosed as having pulmonary embolism. 29 of the remaining 84 patients without pulmonary embolism had DD concentrations below 500 micrograms/l, which means that further diagnostic procedures could have been avoided in a quarter of the patients with inconclusive VQ scans. The sensitivity of the plasma measurement of DD remained high even 3 and 7 days after presentation (96% and 93%). Plasma measurement of DD therefore has a definite place in the diagnostic procedure for suspected acute pulmonary embolism in attenders at emergency departments: a concentration below 500 micrograms/l rules out the diagnosis.

Coagulation disturbances following ascites recirculation

To investigate the effects of extra-corporeal ascites recirculation on coagulation, several coagulation variables were measured in ascitic return fluid and plasma before, during and after this procedure in 16 patients with diuretic resistant ascites. Small but significant reductions in plasma fibrinogen levels and platelet counts and increases in plasma X-oligomer were observed during and after the procedure compared with before. These findings are consistent with the view that ascites recirculation induces disseminated intravascular coagulation although this would appear to be only mild and of no clinical significance in the majority of patients. Although increased levels of activated factor VII were observed in ascitic fluid indicating activation of the extrinsic pathway of coagulation, a significant increase in plasma activated factor VII during the procedure was not demonstrated. Increased fibrinolytic activity was observed in ascitic fluid due to the presence of tissue plasminogen activator. Increased X-oligomer levels were observed in ascitic fluid indicating that lysis of cross-linked fibrin is also an active process within ascites.

Imaging of human thrombi in the rabbit jugular vein: I: Comparison of two fibrin-specific monoclonal antibodies

The development of monoclonal antibodies with a specificity for cross-linked fibrin may have a potential role in the detection and of thrombi and thrombolytic therapy. In this study, two monoclonal antibodies with a specificity for fibrin have been examined. In vitro studies have shown NIBn 123 (which has a high affinity for X-oligomer) and DD-3B6 to bind to immobilised fibrin on PVC plates as well as plasma clots which were incubated in the presence of plasma. The Km values for NIBn 123 and DD-3B6 wre 1.0 x 10(10)/7.7 x 10(8) M and 2.6 x 10(8) M respectively. No significant binding to fibrinogen either immobilised or in solution was found. The binding of these antibodies to a human thrombus in the jugular vein of the rabbit was monitored over a 24 hour period. Preferential binding of each antibody reached a ratio of approximately 1.0 (jugular/heart) at 24 hours and an image was detected.

Acute changes in blood glucose concentration do not promote thrombin generation or fibrin breakdown in type 1 diabetes

To investigate the effect of blood glucose concentration on thrombin generation and fibrinolytic activity, six Type 1 patients had the blood glucose concentration maintained for 1 h at 5, 15, and 25 mmol l-1, and 8 patients underwent hypoglycaemia of 20 min duration after the blood glucose had been kept at 8 mmol l-1 for 1 h. During hyperglycaemia plasminogen activator activity rose from 214 (11-625) (median, range) to 478 (18-772) units (p less than 0.05) at a blood glucose of 5 mmol l-1 and to 511 (89-816) (p less than 0.05) and 535 (33-976) (p less than 0.05) units at a blood glucose of 15 and 25 mmol l-1, respectively. Cross-linked fibrin degradation products (FDP) were 45 and 53 micrograms l-1 at a blood glucose of 5 mmol l-1 and remained unchanged at higher glucose levels. Fibrinopeptide A was 1.3 (0.6-2.8) nmol l-1 at a blood glucose of 5 mmol l-1, and remained unchanged with hyperglycaemia, being 1.3 (0.9-1.3) nmol l-1 after 1h at 25 mmol l-1. During hypoglycaemia, plasminogen activator activity rose from 155 to 745 units (p less than 0.05) while both fibrinopeptide A and cross-linked FDP remained unchanged. The results indicate that acute fluctuations in blood glucose concentration do not lead to thrombin generation. Additionally, increased fibrinolytic activity measured in vitro is not associated with an increase in cross-linked FDP. This suggests that short-term hyper- and hypoglycaemia do not affect the end-products of the coagulation and fibrinolytic pathways.

The relationship between plasma vasopressin and changes in coagulation and fibrinolysis during hip surgery

To investigate whether vasopressin (aVP) could have a role in the regulation of coagulation and fibrinolysis during hip surgery, venous blood samples were taken for assay of FVIII:C, FVIII R:Co, vWF:Ag, fibrinopeptide A (FPA), euglobulin clot lysis time (ECLT), high molecular weight fibrin breakdown products (XL-FDP) platelet aggregation in whole blood and aVP from seven patients undergoing elective hip surgery. Samples were taken at set points over the operative period. FVIII:C increased during the operation from a geometric mean of 0.7 iU/ml pre-operatively to 1.09 iU/ml (p less than 0.05) post-operatively. vWF:Ag and FVIII R:Co rose in a similar manner. PAA (10(6)/ECLT2) rose from 12 units pre-operatively to 167 units (p less than 0.001) at prosthesis cementing, and post-operatively fell to subnormal levels. FPA increased from 13 pmol/ml to 58 pmol/ml (p less than 0.05) at prosthesis cementing, and fell to 9 pmol/ml post-operatively. Plasma XL-FDP rose from 115 ng/ml pre-operatively to 456 ng/ml at skin closure (p less than 0.05). Plasma aVP rose from 0.5 pg/ml pre-operatively to 40 pg/ml (p less than 0.01) at division of the femoral neck. There were no changes in platelet aggregation using 1.5 microM ADP. The results demonstrate activation of coagulation and fibrinolysis during the operative procedure. The mechanisms involved in these changes are complex, but the results support the hypothesis that aVP has effects on factor VIII and fibrinolysis similar to those described for abdominal surgery.

Monoclonal antibodies to crosslinked fibrin degradation products (XL-FDP). II. Evaluation in a variety of clinical conditions

Plasmas from patients with a wide variety of thrombotic and presumed prethrombotic conditions were examined for high molecular weight crosslinked fibrin degradation products (known as X-oligomers) using a two-site enzyme-linked immunospecific assay (ELISA). This assay employed a catcher-tag principle using two monoclonal antibodies (mabs) directed towards different epitopes on the complex X-oligomer fraction. In general, thrombotic events (pulmonary embolism, PE, myocardial infarction, MI, peripheral vascular disease, PVD, and disseminated intravascular coagulation, DIC) were accompanied by elevated levels of X-oligomers in the plasma. During pregnancy the value of X-oligomer assays was demonstrated to be a clear-cut marker for pre-eclampsia. Patients following a variety of forms of surgery present with heterogeneous plasma levels of X-oligomers and this may merely reflect the formation and lysis of the fibrin formed during and after surgery. The possible value of this ELISA procedure in monitoring thrombolytic therapy is discussed with a critical analysis of the data presented herein. While the assay of X-oligomer was demonstrated to be a valuable marker of fibrinolysis in plasma, more extensive data are required in order to assess whether such an assay is of diagnostic value in thrombosis-related conditions.