Effects of unfractionated heparin, low-molecular-weight heparin, and heparinoid on thromboelastographic assay of blood coagulation.

Thromboelastography (TEG) has been used increasingly as an intraoperative hemostasis monitoring device. Low-molecular-weight heparins are given increasingly to reduce the development of antibodies against the heparin-platelet factor 4 complex, and heparinoids are given to patients who have developed the antibody. We studied the effect of unfractionated heparin, a low-molecular-weight heparin (enoxaparin sodium [Lovenox]), and a heparinoid (danaparoid sodium [Orgaran]) on blood clotting assayed with TEG (TEG clotting) in vitro and the efficacy of protamine sulfate and heparinase for reversing the effect. Heparin, enoxaparin, and danaparoid all caused a dose-dependent inhibition of TEG clotting of normal blood. Concentrations of enoxaparin and danaparoid that totally inhibited TEG clotting only minimally prolonged the activated partial thromboplastin time. While inhibition of TEG clotting by heparin and enoxaparin was reversed by protamine sulfate and heparinase, inhibition by danaparoid was reversed only by heparinase. Abnormal TEG clotting was observed in patients receiving enoxaparin whose plasma level of the drug was more than 0.1 antiXa U/mL. However, the degree of TEG abnormality did not always coincide with plasma levels of the drug.

Stability of plasma for add-on PT and APTT tests.

We conducted studies to determine at what time point an add-on prothrombin time (PT) or activated partial thromboplastin time (APTT) test can be honored on specimens that have been received in the laboratory hours earlier without yielding results with clinically significant differences from those if the test had been performed on the original unstored plasma. PT and APTT tests were performed on blood samples from 20 healthy subjects, 30 patients receiving warfarin, and 30 patients receiving heparin anticoagulation therapy. The tests were performed on plasma prepared initially after the samples were obtained. The same tests were assayed on plasma that had been left on spun-down blood cells at room temperature for 2, 4, and 8 hours. We found that the PT of the majority of plasma samples from healthy subjects and from patients receiving oral anticoagulant therapy tended to become shorter on storage. However, the difference in PT values was small and had no clinical significance. In most cases, the APTT values for the stored plasma from healthy subjects tended to increase with time. Except in one specimen in which the 8-hour add-on APTT was 1.2 seconds longer than the APTT result for the original sample, all others had APTT results less than 1.2 seconds longer than the original values. In patients receiving heparin, the differences in APTT values between the initial and add-on tests were larger than those observed for healthy subjects. However, those differences are not beyond what we would accept for duplicate checks for heparinized samples with high APTT values. Unlike samples from healthy subjects, there was no obvious trend of time-related prolongation of the APTT in heparinized plasma. These results led us to believe that within an 8-hour period and with plasma on spun-down cells at room temperature, add-on tests for PT and APTT could be performed with results similar to what would be obtained from testing unstored samples.

Implications of use of low international sensitivity index thromboplastins in prothrombin time testing.

The recent introduction of thromboplastin reagents with low international sensitivity index (ISI) values into the US market for the purpose of generating a more precise international normalized ratio than high ISI thromboplastins could has necessitated an evaluation of the impact of the low ISI reagents on prothrombin time (PT) testing in general. In this study, PT testing with three thromboplastin reagents, one of which (presently used in our laboratory) has an ISI of 2.10 and the other two ISI values of 0.92 and 1.06, respectively, was performed on normal individuals, on quality control reference plasma specimens and single-factor-deficient plasma specimens, and on patients with liver disease, intravascular coagulation, and receiving oral anticoagulant therapy. We found that PTs of normal individuals determined by all three thromboplastins were virtually identical. The thromboplastins with a low ISI generated much longer PTs on abnormal reference plasma specimens than did the high ISI product. Low ISI reagents also produced longer PTs in all three groups of patients. However, the degree of prolongation was far greater for patients receiving warfarin than for the other two groups of patients. Conversion of the PT to an international normalized ratio minimized the discrepancy seen in the PT ratio in patients receiving oral anticoagulants. The two low ISI thromboplastins did not produce near-identical values of PT, PT ratio, or international normalized ratio on plasma specimens obtained from patients who received warfarin therapy. The critical value set for PT with a high ISI thromboplastin would not be adequate if the reagent is to be replaced with a low ISI product.

The use and limitations of the INR system.

This study was undertaken to answer some practical questions physicians and medical directors of clinical laboratories face when they contemplate replacing their high international sensitivity index (ISI) thromboplastins with low ISI ones and using international normalized ratio (INR) in place of prothrombin time ratio (PTR) for monitoring warfarin therapy. To the question of whether low-ISI thromboplastins would produce a prolonged PT on normal patients, the answer is probably no. To the question of the extent of normalization of disparate PTs, determined by high and low ISI thromboplastins, of patients on oral anticoagulants upon the conversion of PTR to INR, the answer is a mixed one. For those whose PTs were 14-20 sec, conversion of PTR to INR would markedly, but not completely, normalize the PTR values. In other words, there would be a lessening of disparity of the PTR after the conversion. For patients whose PTs were > 20 sec, conversion of PTR to INR could even widen the disparity seen with the PTR. Finally, when PTs were assayed on different coagulation devices with the same reagent, highly congenial results were obtained.

Modified APC-resistance test: variable ratios with respect to source of factor V-deficient plasma.

A single point mutation of the factor V (FV) gene, leading to the substitution Arg506Gln in the FV molecule (FV-Leiden) and hence resistance to its breakdown by activated protein C (APC), is the most prevalent risk factor for venous thrombosis in the Caucasians. A ratio determined by activated partial thromboplastin time (APTT) of test plasma in the presence or absence of exogenous APC (the APC ratio), is the method widely used to screen individuals with this risk factor for thrombosis. Because of functional defects of vitamin K-dependent clotting factors in patients on oral anticoagulant therapy, this method cannot be applied to those patients without modification. One modification is to mix test plasma (1:5 or 1:10) with FV-deficient plasma so that 80-90% of functioning vitamin K-dependent factors are supplied by the FV-deficient plasma. Even with 10-20% of FV in the mixture, APC-resistance still can be demonstrated. In this report, we present our results of the modified APC-sensitivity assay using FV-deficient plasma from different commercial sources. APC ratios determined by the original method in which test plasma is not mixed with FV-deficient plasma can be significantly different from those determined by the modified method in which test plasma is diluted 1:5 with FV-deficient plasma. This difference between methods was observed not only in normal individuals, but also in FV-Leiden positive individuals, and in patients on warfarin therapy. Further, APC ratios varied significantly depending on the commercial source of the FV-deficient plasma. The modified method is apparently suitable to identify APC-resistance in patients on warfarin therapy, as well as in individuals not receiving anticoagulant treatment. However, one must be aware that APC-resistance ratios obtained with the modified method are likely to be different from those established with the original method, and the source of FV-deficient plasma can be a factor influencing the ratios in the former cases.

Performance characteristics of a new synthetic APTT reagent.

Performance characteristics of a totally synthetic activated partial thromboplastin time (APTT) reagent, recently available commercially, were evaluated and compared with a rabbit-brain extracted reagent. We found that the synthetic reagent, Synthasil, returned significantly higher normal APTT values than the brain-extracted reagent, Thrombosil. APTT ratios (APTT patients/normal mean APTT), yielded by Synthasil were higher in the majority of patients receiving heparin therapy. Synthasil also returned longer APTT values than did Thrombosil on normal plasma spiked with heparin. On patients with lupus anticoagulants, APTTs assayed with Synthasil were generally longer than with Thrombosil. However, the differences disappeared when APTT values were converted to ratios. Factors XII-, XI-, IX- and VIII-deficient plasmas supplemented with normal plasma to yield activities of 2-50%, generally gave longer APTTs with Synthasil than with Thrombosil. However, this was not always the case on plasmas from haemophilias A and B patients. No reduction in Synthasil activity was noted after the reagent had been left at 24 degrees C for 28 days.