Lipid class composition and heparin sensitivity in the activated partial thromboplastin time

In an APTT reagent, prepared from purified lipids, the role of phosphatidyl serine (PS) in determining the sensitivity of the APTT test system to measurement of the effect of heparin in plasma has been evaluated. As the concentration of PS decreases sensitivity to heparin increases but procoagulant activity decreases. Dilution of the test liposome over a wide range (1 g/l to 30 mg/l) had a minimal effect on the clotting time. At levels below 30 mg/l, however, the amount of total lipid appeared to be rate limiting; a loss of procoagulant activity being paralleled by an increase in heparin sensitivity. Phosphatidyl inositol (PI) was not a satisfactory substitute for PS in the APTT method studied. The degree of unsaturation of test liposomes appeared to have no effect on either procoagulant activity or sensitivity to heparin at the lipid concentration employed. In the light of these findings, a more critical appraisal of the phospholipid components of APTT reagents should facilitate the development of more reliable reagents for heparin control. A further benefit of this type of approach should be a reduction in the acknowledged wide variations in sensitivity to heparin which exist between available APTT reagents.     

Monitoring heparin therapy: relationships between the activated partial thromboplastin time and heparin assays based on ex-vivo heparin samples

Five different APTT-reagents, two amidolytic anti-IIa assays, one amidolytic anti-Xa assay, and one coagulometric anti-Xa/anti-IIa assay were used to assess the effect of heparin in patients treated for venous thromboembolic disease. Good correlations were observed between log-transformed APTTs determined with the various reagents (correlation coefficients: 0.92-0.96). Nevertheless there were important differences in the slopes of the lines of relationship between the APTT reagents. Good correlations were observed between the anti-Xa and anti-IIa assay results (correlation coefficients: 0.92-0.97). However, the amidolytic anti-Xa activity was significantly higher (p less than 0.001) than the two amidolytic anti-IIa activities. Less good correlations were observed between the log-transformed APTTs and the anti-Xa or anti-IIa activities (correlation coefficients: 0.64-0.78). The correlations were improved by transforming the APTT into APTT-ratio, i.e. the ratio of the patient's APTT to the same patient's APTT after removal of heparin from the plasma sample by means of ECTEOLA-cellulose treatment. The correlation coefficients of log (APTT-ratio) with anti-Xa or anti-IIa ranged from 0.76 to 0.87. For both APTT and amidolytic heparin assay, the response to in vitro heparin was different from the response to ex vivo heparin. Therefore, equivalent therapeutic ranges should be assessed by using ex vivo samples rather than in vitro heparin. Because of the response differences between the APTT reagents, it is not adequate to define a therapeutic range for heparin therapy without specification of the reagent.     

Analysis of the activated partial thromboplastin time test using mathematical modeling

Activated partial thromboplastin time (APTT) is a laboratory test for the diagnosis of blood coagulation disorders. The test consists of two stages: The first one is the preincubation of a plasma sample with negatively charged materials (kaolin, ellagic acid etc.) to activate factors XII and XI; the second stage begins after the addition of calcium ions that triggers a chain of calcium-dependent enzymatic reactions resulting in fibrinogen clotting. Mathematical modeling was used for the analysis of the APTT test. The process of coagulation was described by a set of coupled differential equations that were solved by the numerical method. It was found that as little as 2.3 x 10(-9) microM of factor XIIa (1/10000 of its plasma concentration) is enough to cause the complete activation of factor XII and prekallikrein (PK) during the first 20 s of the preincubation phase. By the end of this phase, kallikrein (K) is completely inhibited, residual activity of factor XIIa is 54%, and factor XI is activated by 26%. Once a clot is formed, factor II is activated by 4%, factor X by 5%, factor IX by 90%, and factor XI by 39%. Calculated clotting time using protein concentrations found in the blood of healthy people was 40.5 s. The most pronounced prolongation of APTT is caused by a decrease in factor X concentration.     

Monitoring of unfractionated heparin with the activated partial thromboplastin time: determination of therapeutic ranges

The purpose of the present study was to determine therapeutic ranges for unfractionated heparin therapy using the activated partial thromboplastin time (APTT) by calibration against anti-Xa concentration. APTT assays were performed locally, i.e. at the institution of blood collection, on fresh plasma samples from patients treated with intravenous unfractionated heparin. The measurements were performed by 25 Dutch clinical laboratories using 11 different APTT reagents and 10 different types of coagulometers. After the local APTT measurement, the samples were frozen and transported to a central laboratory for measurement of anti-Xa activity. The number of samples from the participating laboratories ranged from 10 to 48. Local APTT results were correlated with the central anti-Xa measurements. Orthogonal regression analysis of log-transformed values was used to calculate APTT therapeutic ranges corresponding to anti-Xa concentrations of 0.29-0.47 IU/ml. The calculated APTT ranges were different between laboratories, even when the same reagent was used. In many laboratories, the therapeutic APTT range in use was much wider than the calculated range. Imprecision of the calculated APTT range was influenced by the wide scatter of the measurement points and by the selection of samples for the orthogonal regression equation. The present results show that, if anti-Xa concentrations of 0.29-0.47 IU/ml reflect the true therapeutic range, many laboratories do not use the proper therapeutic APTT range.     

Monitoring of heparin therapy with the activated partial thromboplastin time and chromogenic substrate assays

Heparin therapy was monitored with the activated partial thromboplastin time (APTT) and with chromogenic substrate assays (factor Xa and factor IIa inhibition) in 100 plasma samples from 47 patients. Heparin concentrations were classified as being below, within or above a defined therapeutic range (TR; 0.2-0.55 units heparin/ml). In a first group of patients (A), all three assays allocated the plasma heparin levels to the same concentration interval with respect to the TR. The most frequent diagnoses in group A were uncomplicated arterial or venous thromboembolism, myocardial infarction with limited tissue necrosis, cardiac surgery without major complications and successfully treated infectious disease. In a second group of patients (B), the results of APTT suggested higher heparin concentrations with respect to the TR than the chromogenic assays. Predominant diagnoses were severe infectious diseases, severe liver disorders, extensive myocardial infarction and postoperative complications after cardiac surgery. The discrepancy between heparin concentrations determined by either APTT or the chromogenic substrate assays is most likely due to a non-heparin related prolongation of APTT caused by the underlying disease.     

A comparison of heparin potency estimates obtained by activated partial thromboplastin time and british pharmacopoeial assays

Heparin samples from five manufacturers were assayed by the revised British Pharmacopoeia (BP) heparin assay and the results compared with those obtained using the activated partial thromboplastin time (APTT) assay. The United States Pharmacopoeia (USP) reference heparin preparation and the 4th International Standard (IS) for heparin were also assayed by the two methods relative to the 3rd IS. The results obtained by the revised BP assay were in close agreement with those obtained by the APTT assay for all the heparins that were tested. The assays revealed that there is at least a 10% discrepancy between the International Unit for heparin and the USP unit.     

The reliability of activated partial thromboplastin time methods and the relationship to lipid composition and ultrastructure

Wide variations in procoagulant properties, lipid composition and ultrastructure of five commonly used APTT methods have been demonstrated. Performance of the methods with a range of coagulation abnormalities has been ranked. Most of the reagents obtained a high score with one or more defects, but a low score with others. A consistent good ranking throughout was only observed with one reagent. The number of significant correlations between the reagents' procoagulant activities and lipid content confirms the view that the performance of an APTT method is largely dependent upon its lipid composition. Marked differences in concentration and distribution of phospholipids, fatty acids and neutral lipids were evident. The importance of the concentration of phosphatidyl serine in regulating the procoagulant activity of an APTT method has been demonstrated. Electron microscopy provides evidence of the contrasting composition of the reagents from the more discrete uniform liposomes present in the more reliable reagents, to more ill-defined components present in those reagents which performed less well. The study highlights the need for standardisation of the APTT.     

Development of a photometric assay for activated partial thromboplastin time and its application to the cobas biocentrifugal analyzer

We describe a two-step procedure for APTT that can be performed on photometric devices. It includes preincubation of diluted plasma with ellagic acid and phospholipids and a starting reagent that contains calcium and a chromogenic peptide substrate for thrombin, Tos-Gly-Pro-Arg-pNA. Reaction time is recorded from addition of the starting reagent until thrombin formation occurs, and a prefixed amount of substrate is cleaved. The pattern of sensitivity to clotting factors and heparin was similar to clotting assays and the substrate used did not interfere with the activity of factor Xa. An application of the method was made for the Cobas^(R) Bio centrifugal analyzer. Absorbance readings were sent to an external computer and were transformed into reaction times by a computer program. Although the results are independent on fibrinogen concentrations, from kinetic data of the reaction curve fibrinogen concentrations can be estimated. Correlation studies showed good correspondence to clotting methods (r = 0.92, N = 53) as well as an excellent precision (CV 3% for inter-assays, N = 15) and high throughput of samples (>100/h) in the automated assay.     

A shortened activated partial thromboplastin time predicts the risk of catheter-associated venous thrombosis in cancer patients

Introduction

Hypercoagulability due to high coagulation factor levels resulting from host inflammatory response to cancer contributes to an increased risk of venous thromboembolism (VTE) in cancer patients. Central venous catheters (CVCs) further heighten this risk. Activated partial thromboplastin time (aPTT) can be used to broadly screen for elevated levels of relevant coagulation factors. Our objective was to determine if a shortened aPTT ratio (coagulation time of test- to- reference plasma) was a predictor of CVC-associated VTE in cancer patients.

Materials and Methods

We performed a retrospective case–control study on cancer patients undergoing tunneled CVC insertion at our center from 1999 to 2006 and identified 40 patients who had CVC-associated VTE. VTE was confirmed with color duplex ultrasonography or computed tomography scan. For each case, we obtained 5 controls that had the same cancer diagnosis and were matched on the following factors: age, chemotherapy, hormone therapy (if applicable), tobacco use, TNM staging and year of diagnosis. All patients had aPTT testing within 30 days prior to surgery. We compared aPTT and aPTT ratio between cases and controls using Wilcoxon two sample test.

Results

aPTT ratio was significantly shorter in patients with CVC-related VTE as compared to controls [0.86 (95% confidence interval (CI) 0.78, 0.94) vs. 0.98 (0.94, 1.01), p = 0.0003]. Mean aPTT was also significantly shorter. [25.6 seconds (95% CI 23.2, 27.9) vs. 28.1 (26.9, 29.3), p = 0.001] aPTT ratios of the controls tended to spread across larger aPTT ratio values whereas those of cases tended to clustered around the mean.

Conclusions

Cancer patients undergoing catheter placement who develop CVC-associated VTE have a shorter aPTT and aPTT ratio than those who do not develop VTE. aPTT, a simple and inexpensive test might be useful as a predictor of CVC-associated VTE risk in cancer patients.     

Heparin, brain thromboplastin and the insensitivity of the prothrombin time to heparin activity

This report confirms previous observations of the insensitivity of the prothrombin time to heparin. It was shown that the heparin inhibitory activity of brain thromboplastin paralleled its tissue factor component being heat labile, non-dialysable, destroyed by techniques disrupting lipid protein interactions and of high molecular weight. The high molecular weight brain thromboplastin component was shown to inhibit the heparin activation of antithrombin III. These observations in addition to explaining in vitro phenomena may also indicate an in vivo role of heparin inhibition by tissue thromboplastin.     

The effect of fluorescent humic substances existing in the well water of Blackfoot disease endemic areas in Taiwan on prothrombin time and activated partial thromboplastin time in vitro

Fluorescent humic substances (FHS) in well water of Blackfoot disease endemic areas were purified and fractionated by Sephadex G-25 column chromatography. Four fractions of the purified FHS were isolated. The purified FHS and their fractions were then added to normal human pool plasma in vitro separately to detect the abilities of the effects on prothrombin time (PT) and activated thromboplastin time (APTT). Results showed that all of the four fractions of the purified FHS prolonged both PT and APTT in the higher concentration ranges (10 mg/ml - 20 mg/ml), but shortened both PT and APTT in the lower concentration ranges (0.5 mg/ml - 5 mg/ml). Among the four fractions of the FHS, the fraction 1, the humic substance with the highest molecular weight among all the four FHS, showed the most obvious effects. Owing to the effects of the FHS on PT and APTT values, we supposed that there is a close relationship between the FHS and the cause of Blackfoot disease.     

The effect of lysed platelets on neutralization of heparin in vitro with protamine as measured by the activated coagulation time (ACT)

The effect of lysed platelets on the activated coagulation time (ACT) was studied in heparinized whole blood during titration with protamine. Frozen-thawed washed platelet suspension, or a chromatography fraction thereof, or autologous frozen-thawed platelet-rich plasma was added in various dilutions to freshly drawn blood anticoagulated with 3,000 USP units/l heparin. After a 10 min incubation, the amount of protamine needed to restore the ACT to baseline ("protamine titration dose") was determined. We found that the protamine titration dose decreased in proportion to the amount of lysed platelet material added; expressed as a percentage of the total number of platelets present, each unit increase in lysed platelets produced a 1.7% +/- 0.8 (SD) reduction in the protamine dose needed to normalize the ACT. A heparin activity assay showed that this effect was not due to anti-heparin activity of lysed platelets such as platelet factor 4 (PF4). Our data indicate that the procoagulant activity of platelet membranes reduced the sensitivity of the ACT to heparin. These findings suggest that membranous platelet microparticles may cause an inaccurate calculation, based on the ACT, of a protamine dose to reverse heparin anticoagulation in cardiopulmonary bypass procedures.     

Automated mixing studies and pattern recognition for the laboratory diagnosis of a prolonged activated partial thromboplastin time using an automated coagulation analyzer

Introduction

The objective of this study was to explore whether an automated coagulation analyzer could be applied to normal plasma mixing studies for the assessment of blood samples showing a prolonged activated partial thromboplastin time (APTT).

Materials and methods

Ten laboratory staff members performed normal plasma mixing studies and evaluated plasma samples using 3 different methods: (1) manual dilution and analysis, (2) manual dilution and automatic analysis with STA-R^®, and (3) automatic dilution and analysis with the Coapresta^® 2000 (CP2000). The time from the start of the analysis to the generation of the result plots and the plasma volumes required were determined. We analyzed patient plasma samples showing a prolonged APTT using the CP2000, and the result plots were categorized into 3 curve patterns based on the area ratio values: the inhibitor type (convex pattern), deficiency type (concave pattern), and suspicious inhibitor type (approximately straight pattern).

Results

When pooled patient plasma was used, the same patterns were obtained from normal plasma mixing studies using the 3 different methods. The time required to complete the mixing studies and the plasma volumes required were 28.2 ± 2.4 min and 350 μL for manual analysis, 23.2 ± 2.1 min and 875 μL for STA-R^®, and 8.5 ± 0.1 min and 175 μL for CP2000, respectively. Of 31 patient samples, 9 were categorized into the inhibitor type, 15 were categorized into the deficiency type, and 7 were categorized into the suspicious inhibitor type.

Conclusions

The CP2000 analyzer is applicable to the laboratory diagnosis of a prolonged APTT using pattern recognition, as it requires a shorter time to complete mixing studies and a smaller plasma volume in comparison with manual analysis.     

Transition from argatroban to oral anticoagulation with phenprocoumon or acenocoumarol: effects on prothrombin time, activated partial thromboplastin time, and Ecarin Clotting Time

Treatment with the direct thrombin inhibitor argatroban (ARG) is often followed by vitamin K-antagonist treatment (VKA). Phenprocoumon (PC) and acenocoumarol (AC) are frequently used in Europe. The standard monitoring test for VKA, pro-thrombin time (PT), is prolonged by direct thrombin inhibitors. Therefore the International Normalized Ratio (INR) obtained during combined treatment does not reflect the true effect of the VKA. A similar interference of the VKA on the activated partial thromboplastin time (aPTT), a monitoring assay for direct thrombin inhibitors, can occur. In 39 healthy volunteers the effect of ARG alone or combined with PC or AC on PT, INR, aPTT, and Ecarin Clotting Time (ECT) was investigated. 6 groups each of 6-8 volunteers received a 5-hour infusion of either 1.0, 2.0 or 3.0 microg/kg/min ARG (days 1, 3, 4 and 5) before initiation of either PC or AC (day 1) and during continued VKA dosing (target INR 2-3). A linear relationship (INR(ARG+VKA) = intercept + slope * INR (VKA alone)) was observed between the INR measured "on" and "off" ARG. The slope depended on the argatroban dose and on the International Sensitivity Index (ISI) of the PT reagent, the steepest slope (i.e., the largest difference between INR (ARG+VKA) and INR (VKA alone)) was seen with the highest ARG dose and the PT reagent with an ISI of 2.13. There was a close correlation between plasma levels of ARG and aPTT or ECT. Under VKA the ARG-aPTT relationship indicated an increased sensitivity of the aPTT to ARG, VKA treatment had no effect on the prolongation of the ECT induced by argatroban. In conclusion, ARG at doses up to 2 microg/kg/min can be discontinued at an INR of 4.0 on combined therapy with VKA, as this would correspond to an INR between 2.2 and 3.7 for the VKA. If it is necessary to monitor ARG in the critical transition period, the ECT which is not influenced by VKA can be used as an alternative to the aPTT.     

Prolonged activated partial thromboplastin time and deficiency of high molecular weight kininogen in brown Norway rat mutant (Katholiek strain)

Activated partial thromboplastin time (APTT) was examined in Brown Norway (B/N) Katholiek rat, which was previously reported as high molecular weight kininogen deficient. APTT of B/N Katholiek was prolonged to 35 sec in comparison with B/N Kitasato and SD rat, showing APTT of 22-24 sec. The mixture of B/N Katholiek plasma and B/N Kitasato plasma (1:1) showed normal APTT value. B/N Katholiek plasma corrected the abnormally prolonged human coagulation factor deficient plasmas, such as XI, XII and prekallikrein deficient plasmas, while it did not correct the APTT of HMW kininogen deficient, Fitzgerald plasma. Intravenous injection of bromelain, which was previously reported to produce prolonged hypotension through the activation of factor XII to release bradykinin, induced slight effect in Katholiek rat, while in Kitasato rat it showed prolonged hypotension in similar degree as SD rat. Contents of coagulation factors in B/N Katholiek thus measured as well as the values of prekallikrein and HMW kininogen previously reported were summarized and suggested that B/N Katholiek rat could be similar deficiency as Fitzgerald trait.     

Identification of two distinct heparin cofactors in human plasma. II. Inhibition of thrombin and activated factor X

Heparin alone has two direct effects on the thrombin clotting time. First, there is an immediate prolongation of the clotting time, which can be fully reversed upon neutralization of the heparin by polybrene or protamine. Second, there is a slow, time-dependent increase in the clotting time, representing an irreversible decrease in the concentration of active thrombin. Increasing the heparin concentration above 10^?2 Units/ml leads to an increase in the rate of thrombin inactivation in the presence of cofactor A. This lost thrombin cannot be recovered by neutralization of the heparin. In the absence of heparin, cofactor A has only slight progressive antithrombin activity. Cofactor B by itself has marked progressive antithrombin activity, the rate of which is further enhanced by the addition of increasing concentrations of heparin above 10^?2 Units/ml. However, heparin diminishes the ultimate thrombin capacity of cofactor B. Cofactor B also inactivated activated Factor X (Factor Xa). The effect is greatly accelerated in the presence of heparin. Cofactor A has no effect on Factor Xa, even in the presence of heparin.