Oral anticoagulant therapy with warfarin is monitored by the prothrombin time. Because different thromboplastins vary in sensitivity to plasma levels of factors II, VII, and X, the international normalized ratio (INR) was introduced to allow comparison of prothrombin times obtained with different reagents (see the glossary for definitions) [6]. The INR is calculated by using the international sensitivity index, a correction factor that correlates a particular thromboplastin to a standard reference preparation [6]. The international sensitivity index corrects for differences in sensitivities to vitamin K-dependent coagulation factors but not to other plasma components that may alter the prothrombin time. For example, the INR is invalid for patients with severe liver disease, who frequently have coagulopathies involving clotting factors that are not dependent on vitamin K [7].

The optimal therapy for patients with lupus anticoagulants who have sustained a thromboembolic event is controversial [4, 8-10]. Rosove and Brewer [8] recommended that the INR in these patients be maintained at 2.6 or greater because a higher recurrence rate was seen in patients whose INR was less than 2.6. For the same reason, Khamashta and colleagues [9] recommended high-intensity anticoagulation with INRs of 3.0 or greater for patients with antiphospholipid antibodies and thrombotic events. In contrast, Ginsberg and associates [4] suggested that an INR of 2.0 to 3.0 was sufficient because they saw no recurrent thromboses during conventional-intensity warfarin therapy.

None of these studies considered the observation that patients with lupus anticoagulants may have a prolonged prothrombin time [11-13]. Lupus anticoagulants are antiphospholipid antibodies that interfere with phospholipid-dependent coagulation reactions in vitro (Figure 1). They are frequently associated with a prolonged activated partial thromboplastin time or dilute Russell viper venom time [14] (Figure 1). Although it has been stated that a prolonged prothrombin time is "uncommon" in patients with lupus anticoagulants [17], Horellou and coworkers [13] described 57 patients with lupus anticoagulants, of whom 31 had a prolonged prothrombin time (53%). Because some patients with lupus anticoagulants have a prolonged prothrombin time as a result of an antibody-mediated decrease in factor II levels [18, 19], Horellou and colleagues [13] measured these levels and found that they were normal in 30 of these patients. Fleck and coworkers [12] reported that the prothrombin time was prolonged by more than 2 seconds in 12 of 42 (28.6%) patients with lupus anticoagulants who had normal factor II levels.

View larger version (20K): [in this window] [in a new window]   Figure 1. Coagulation cascade showing the coagulation reactions of the activated partial thromboplastin time, the dilute Russell viper venom time, the prothrombin time, and the prothrombin-proconvertin time. Phospholipid-dependent reactions (PL) include activation of factor X to factor Xa by factor VIIa-tissue factor (extrinsic factor X-ase complex), the activation of factor X by factor IXa in the presence of factor VIIIa and calcium ions (intrinsic factor X-ase complex), and the activation of prothrombin by factor Xa in the presence of factor Va and calcium ions (prothrombinase complex). Antiphospholipid antibodies have been shown to interfere in vitro with the activation of factor X by the intrinsic factor X-ase complex and the activation of prothrombin by the prothrombinase complex [15, 16].

It is essential to determine whether use of the INR is valid for patients with lupus anticoagulants, because following generalized recommendations may otherwise result in inappropriately low or high levels of anticoagulation in the individual patient. Thus, our objectives were to 1) use several thromboplastins to determine the prevalence of abnormal baseline prothrombin times in patients with lupus anticoagulants, 2) determine whether standardization of prothrombin time by conversion to INRs is valid and accurate for patients with lupus anticoagulants who receive warfarin, and 3) investigate alternate assays as ways to monitor the intensity of anticoagulation with warfarin in these patients.

Methods

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Patients

Patients were identified through review of lupus anticoagulant panels performed by the Duke University Medical Center Clinical Coagulation Laboratory between April 1993 and February 1996. To confirm the presence of lupus anticoagulants, we used the following criteria, recommended by the Subcommittee on Lupus Anticoagulants/Antiphospholipid Antibodies of the International Society of Thrombosis and Haemostasis [20]: 1) prolongation of a phospholipid-dependent screening assay, 2) lack of correction of a prolonged screening assay after a 1:1 mix with pooled normal plasma, and 3) correction of a prolonged screening assay by the addition of excess phospholipid. Screening assays were the activated partial thromboplastin time and dilute Russell viper venom time (DVV test, American Diagnostica, Inc., Greenwich, Connecticut). Confirmatory assays were DVV Confirm (American Diagnostica, Inc.) and the platelet neutralization procedure [20]. Additional studies included a prothrombin time and thrombin clot time. Anticardiolipin antibody IgG and IgM levels were determined as described by Loizou and colleagues [21].

We identified 161 patients with lupus anticoagulants. We reviewed medical records to identify patients who were followed by a physician at Duke University in Durham, North Carolina, or asked patients to travel to Duke for additional testing. Inclusion in the study was therefore based on the confirmation of the presence of a lupus anticoagulant and the ability to obtain plasma samples for study. Informed consent was obtained for all patients, as approved by the Institutional Review Board of Duke University Medical Center. By applying these criteria, we enrolled 34 patients during a 6-month period from September 1995 through February 1996 (Table 1).

Twenty-two patients with lupus anticoagulants (who are designated with an L) were not receiving anticoagulation when plasma samples were obtained. Half of the patients had a prothrombin time greater than 1 second above the upper limit of the normal range, and half had a normal prothrombin time or prolongation less than 1 second (Table 1). Baseline factor II levels were measured in 18 patients and were normal in all but 1 patient.

Five of these patients also had samples obtained during warfarin therapy. Twelve additional patients who had started receiving warfarin before the study were also investigated. Baseline clinical laboratory data from the medical records were summarized when available (Table 1).

Blood was collected from 25 normal donors to establish mean prothrombin times and normal ranges. Blood was also collected from five normal persons and from 10 patients without lupus anticoagulants who were receiving stable warfarin therapy (patients in both groups are designated with an N). Normal donors were physicians and laboratory personnel. Venous blood was collected into Vacutainer tubes (Becton Dickinson, Rutherford, New Jersey) that contained 3.2% sodium citrate. Platelet-poor plasma was prepared by centrifugation (final platelet count < 20 000 cells/mm³) and frozen at –75°C until used.

Thromboplastins

Nine commercially available thromboplastins were used (Table 2). Thromboplastins were selected to cover a wide range of international sensitivity index values and included several reagents commonly used in the United States (based on the 1996 College of American Pathologists Comprehensive Coagulation Survey [22]). Simplastin, Simplastin Excel, Simplastin Excel S, and MDA Simplastin L were all provided by Organon Teknika Corp. (Durham, North Carolina). Dade Innovin, Dade Thromboplastin IS, and Dade Thromboplastin C Plus were obtained from Baxter Diagnostics (Deerfield, Illinois). Thromboplastin with calcium was obtained from Sigma Diagnostics (St. Louis, Missouri). Thromboscreen (Pacific Haemostasis, Huntersville, North Carolina) was evaluated only in the patients who were not receiving warfarin.

Coagulation Testing

Prothrombin times were measured in duplicate on an ST4 mechanical coagulometer (Diagnostica Stago, Asnieres-sur-Seine, France). The two measurements differed by less than 10% or were repeated. Normal ranges were established for each thromboplastin (Table 2), and prothrombin time ratios were calculated by dividing the patient prothrombin time by the mean normal prothrombin time [23]. For patients receiving warfarin, INRs were calculated as described in the glossary. Samples were obtained at different time points from five patients receiving warfarin. These samples were evaluated by using three thromboplastins (Simplastin Excel S, Dade Innovin, and thromboplastin with calcium) and the prothrombin-proconvertin time.

Prothrombin-proconvertin times were determined on the ST4 coagulometer by using Simplastin A (Organon Teknika Corp.) according to the manufacturer's instructions [24]. Simplastin A is a tissue thromboplastin obtained from rabbit brain and lung that is supplemented with bovine fibrinogen and factor V plus calcium chloride. Plasma samples were diluted into saline, and clotting times were determined. A log-log plot of clotting times against dilutions of control pooled plasma generated a linear standard curve. The clotting time of the 1:10 dilution was assigned a value of 100%, and clotting times of patient plasma dilutions were converted into a percentage of control pooled plasma activity [24]. An INR of 2.0 to 3.5 correlated to a prothrombin-proconvertin time result of 9% to 27% in patients without lupus anticoagulants (Ortel TL. Unpublished data), which correlates well to the published therapeutic range [24]. A supratherapeutic result is less than 9%, and a subtherapeutic result is greater than 27%.

Chromogenic factor X levels were measured on an ACL 300 Plus automated coagulation analyzer (Instrumentation Laboratory, Lexington, Massachusetts) by using the Coatest factor X kit (Chromogenix AB, Molndal, Sweden). First, Russell viper venom is used to activate factor X in test plasma samples in the presence of calcium ions [25]. In the second step, activated factor X hydrolyzes the chromogenic substrate S2337, which is detected by monitoring absorbance at 405 nm [26]. A 1:10 dilution of control pooled plasma was assigned an activity value of 100%, and measurements of test plasma dilutions were converted into a percentage of the control pooled plasma activity. An INR of 2.0 to 3.5 correlated to a chromogenic factor X level of 11% to 42% in patients without lupus anticoagulants (Ortel TL. Unpublished data), which corresponds to the published therapeutic range [27]. A supratherapeutic result is less than 11%, and a subtherapeutic result is greater than 42%.

Factor II levels were measured on an MDA 180 coagulation analyzer (Organon Teknika Corp.). Factor II-depleted plasma (Diagnostica Stago) was used in a prothrombin time-based assay with MDA Simplastin L. Test plasma samples were serially diluted, and factor II activity was calculated by interpolation from a standard reference curve. An INR of 2.0 to 3.5 correlates to a factor II level of 5% to 35% in patients without lupus anticoagulants (Ortel TL. Unpublished data), which corresponds to published therapeutic ranges [24, 28]. A supratherapeutic result is less than 5%, and a subtherapeutic result is greater than 35%. All correlation studies for each patient were performed on the same plasma sample.

Statistical Analysis

The frequency of abnormal prothrombin times obtained with multiple thromboplastins for normal persons and patients with lupus anticoagulants who were not receiving warfarin was evaluated by using the Fisher exact test for the individual thromboplastins and for all thromboplastins combined. For patients receiving warfarin, the variation in INRs obtained with different thromboplastins was compared by using separate two-way analyses of variance on the logarithm of the INR for patients with and those without lupus anticoagulants. The two factors were individual patients and the specific thromboplastins. International normalized ratios obtained with different thromboplastins were correlated to each other and to factor II levels, chromogenic factor X levels, and prothrombin-proconvertin times by determining best curve fits and Pearson correlation coefficients.

Role of Funding Source

The agencies that supported this project had no direct role in gathering, analyzing, or interpreting the data and played no role in the decision to publish the study findings.

Results

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Influence of Lupus Anticoagulants on the Prothrombin Time in Patients Not Receiving Warfarin

In the five normal persons, all prothrombin times were normal and the mean prothrombin time ratio for each thromboplastin was close to 1.0 (Table 3). In contrast, depending on the thromboplastin used, prothrombin times were prolonged in five or more patients with lupus anticoagulants (Table 3). In comparisons of the numbers of abnormal prothrombin times between normal persons and patients with lupus anticoagulants, P values were significant for three thromboplastins (Dade Thromboplastin C Plus [P = 0.0031], Simplastin Excel [P = 0.047], and Simplastin [P = 0.0087]). A pooled chi-square analysis for the number of abnormal results with all nine thromboplastins was highly significant (P < 0.001).

For the patients with lupus anticoagulants, the mean prothrombin time ratio obtained with each thromboplastin was greater than any of the mean ratios obtained for the normal persons (Table 3). In addition, the variation in prothrombin time ratios was greater for patients than for normal persons (Table 3). All 22 patients had a prolonged prothrombin time with at least two thromboplastins (range, two to nine thromboplastins; data not shown). Furthermore, when prothrombin time ratios were converted to INRs, 11 patients had INRs greater than 2.0 with one or more thromboplastins (data not shown).

Influence of Lupus Anticoagulants on the International Normalized Ratio in Persons Receiving Stable Warfarin Therapy

For the 10 persons without lupus anticoagulants who were receiving warfarin therapy, the difference between the highest and lowest INRs obtained for any single patient sample ranged from 0.5 to 1.2 (Figure 2). In contrast, the INRs for patients with lupus anticoagulants varied widely; the difference between the highest and lowest value for any single patient plasma sample ranged from 0.4 to 6.5 (Figure 2).

View larger version (22K): [in this window] [in a new window]   Figure 2. International normalized ratios of 16 patients (designated by L) who had lupus anticoagulants and were receiving warfarin and 10 patients (designated by N) who did not have lupus anticoagulants and were receiving warfarin. The ratios were obtained by using eight different thromboplastins (white square = Dade Innovin, black square = Simplastin Excel, white triangle = Dade Thromboplastin C Plus, black triangle = Simplastin Excel S, white circle = Dade Thromboplastin IS, black circle = Simplastin, white diamond = MDA Simplastin L, and black diamond = Sigma thromboplastin).

To test whether this variation in INR was statistically significant, we performed separate two-way analyses of variance. Under the null hypothesis, the ratio of these two variances (mean squared error) would follow an F-distribution. The mean squared error for the patients without lupus anticoagulants was 0.0041 for 63 degrees of freedom, corresponding to an average variation for any particular measurement in this group of 6.4%. In contrast, the mean squared error for patients with lupus anticoagulants was 0.0406 for 101 degrees of freedom, corresponding to an average variation of 20.2%. The F-ratio for these two errors was 9.92, with 101 and 63 degrees of freedom, respectively (P < 0.001). This difference is highly significant; the lupus anticoagulant group had more than three times the average variation of the group without lupus anticoagulants.

For several patients with lupus anticoagulants, the variation in INR was small, similar to that in patients without lupus anticoagulants. (Figure 2). Anticardiolipin antibody titers, dilute Russell viper venom time ratios, and baseline prothrombin times were not good predictors of a greater variation in INRs (r = 0.49, 0.29, and 0.65, respectively). For example, of the five patients who were tested before and after initiation of warfarin treatment, one had a relatively narrow range of the prothrombin time ratio before treatment began but a wide INR range after treatment began (Figure 2).

Definition of Therapeutic Anticoagulation in Patients with Lupus Anticoagulants Receiving Warfarin

All but one patient who had lupus anticoagulants and received warfarin were also investigated by using the prothrombin-proconvertin time, chromogenic factor X, and factor II assays. Chromogenic factor X and prothrombin-proconvertin time assays correlated well with each other (r = 0.850 [Figure 3, left]). By using the therapeutic ranges for these two assays, therapeutic and nontherapeutic results matched in 14 of 16 patients (87.5%). Six results were clearly subtherapeutic according to both assays, and an additional result was subtherapeutic according to the chromogenic factor X assay but was at the low end of the therapeutic range according to the prothrombin-proconvertin assay (Figure 3, left).

View larger version (15K): [in this window] [in a new window]   Figure 3. Therapeutic and nontherapeutic results obtained by using the prothrombin-proconvertin time, chromogenic factor X, and factor II assays and by using international normalized ratios. Values are for 16 patients who had lupus anticoagulants and were receiving warfarin. The results of the prothrombin-proconvertin time and chromogenic factor X assays are compared in the left panel, results of the factor II and chromogenic factor X assays are compared in the middle panel, and the international normalized ratios obtained by using Simplastin Excel S (black square) and Dade Thromboplastin IS (white circle) and results of the chromogenic factor X assay are compared in the right panel. The horizontal and vertical lines mark the following therapeutic ranges for the individual assays: prothrombin-proconvertin time, 9% to 27%; chromogenic factor X, 11% to 42%; factor II, 5% to 35%; and international normalized ratio, 2 to 3.5. Values for the factor II, chromogenic factor X, and the prothrombin-proconvertin time assays are the percentage of control pooled plasma activity.

In contrast, although factor II levels correlated well with results of the prothrombin-proconvertin time (r = 0.832) and chromogenic factor X assays (r = 0.750), the factor II assay overestimated the extent of anticoagulation when compared with either of these assays. Seven of 16 patients (44%) had therapeutic anticoagulation according to the factor II assay but clearly had subtherapeutic anticoagulation according to the chromogenic factor X assay (Figure 3, middle). Similar results were obtained when factor II was compared to the prothrombin-proconvertin time (data not shown).

We also compared the results of the chromogenic factor X, prothrombin-proconvertin time, and factor II assays with the INRs obtained by using the thromboplastins. Although some thromboplastins correlated well with these assays (correlation coefficients ranged from 0.434 to 0.878), the INRs tended to overestimate the extent of anticoagulation. For example, 11 of 16 INRs obtained by using Dade Thromboplastin C Plus exceeded 3.5, although three of these patients actually had subtherapeutic anticoagulation according to the chromogenic factor X or prothrombin-proconvertin time assay (data not shown). The best correlations were seen with Simplastin Excel S and Dade Thromboplastin IS (Figure 3, right); even these reagents, however, identified several patients as having "therapeutic" anticoagulation according to INRs even though the patients were shown to have subtherapeutic anticoagulation according to the chromogenic factor X or prothrombin-proconvertin time assay.

Temporal Relation of International Normalized Ratios and Prothrombin-Proconvertin Time

We also investigated whether the variations in INRs that were obtained with different thromboplastins were consistent over time in individual patients. At different time points, similar relations were seen among INRs obtained by using different thromboplastins and between INRs and results of the prothrombin-proconvertin time assay (data not shown). However, relative differences among INRs obtained by using different thromboplastins were magnified with an increasing intensity of anticoagulation. By comparing individual INRs with the results of the prothrombin-proconvertin time assays, it was possible to establish individual therapeutic INRs for each patient-thromboplastin combination. In one patient, for example, a prothrombin-proconvertin time assay with activity values of 27% to 9% corresponded to the following INRs: 2.6 to 3.5 with Simplastin Excel S, 4.0 to 6.6 with Dade Innovin, and 7.0 to 12.1 with Sigma thromboplastin (data not shown).

Discussion

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Our data show that the prothrombin time is frequently prolonged in patients with lupus anticoagulants. Although certain thromboplastins seem to be less sensitive to the effect of a lupus anticoagulant than others, none of the reagents was insensitive to the presence of a lupus anticoagulant in all patient plasma samples. In particular, we found that "conventional" (that is, nonrecombinant) thromboplastins were affected as profoundly as recombinant thromboplastin. This finding differs from the observation of Khamashta and colleagues [17].

Our data also demonstrate that the INR does not standardize for the variability in the responsiveness of thromboplastins in patients with lupus anticoagulants who receive warfarin (Figure 2). In the most extreme cases, the INR was subtherapeutic with one thromboplastin (INR < 2.0), therapeutic with another (INR, 2.0 to 3.5), and supratherapeutic with a third (INR > 3.5). In addition, the INR frequently overestimated the extent of anticoagulation in these patients. The most reliable results were obtained with the chromogenic factor X and prothrombin-proconvertin time assays; the factor II assay also overestimated the intensity of anticoagulation.

Current therapeutic recommendations for patients with lupus anticoagulants who have had a thromboembolic event are controversial. Several researchers recommend high-dose warfarin (maintaining an INR 3.0) [8, 29]. This recommendation is not accepted by others, however, and hemorrhagic complications related to high-dose warfarin are not uncommon [30, 31]. Other researchers claim that warfarin therapy often fails in patients with antiphospholipid antibodies and venous thromboembolic disease and that these patients "are best managed by use of long-term ... heparin therapy" [32]. The fact that the INR is not valid in many of these patients explains the difficulties encountered in managing their warfarin therapy.

Two limitations of our study are the relatively small number of patients studied and the fact that many of the patients had been referred to a tertiary care center for management of recurrent thrombotic events. Consequently, our observations may not be applicable to all patients with antiphospholipid antibodies and thrombosis. For example, we did not include patients with anticardiolipin antibodies who did not have laboratory evidence of a lupus anticoagulant. We believe, however, that the persons studied are representative of the patients with lupus anticoagulants and thrombosis who are the most difficult to manage and who often present with recurrent thrombosis or a hemorrhagic complication related to anticoagulant therapy.

The most important clinical implication of our study is that the INR cannot be relied on to accurately monitor oral anticoagulant therapy with warfarin in many patients with lupus anticoagulants. Consequently, all previous therapeutic recommendations based on the INR must be reconsidered, and no single recommendation will probably be true for all patients. For some patients, particularly those in whom the baseline prothrombin time is prolonged, the INR will not be reliable. Optimal therapeutic management for these patients would be established by the prothrombin-proconvertin time or chromogenic factor X assay. For others, the INR obtained by using a thromboplastin that is relatively insensitive to lupus anticoagulants may be sufficient. However, a baseline normal prothrombin time does not guarantee that the INR will be accurate in an individual patient. For example, one of our study patients (L23) had a normal baseline prothrombin time (Table 1), but the INR during warfarin therapy varied greatly (Figure 2).

Because neither the chromogenic factor X nor the prothrombin-proconvertin time assay is widely available to many clinicians, alternate approaches to the management of anticoagulant therapy in these patients must be identified. Rapaport and Le [10] recommended generating individualized INRs by comparing the INRs to results of the prothrombin-proconvertin time test. Two limitations to this approach are the individual variation in antiphospholipid antibody activity over time [33] and the fact that many hospital laboratories change thromboplastins at least annually. This may necessitate frequent reevaluation of the patient. Other investigators have proposed monitoring the levels of activation markers (for example, prothrombin fragment 1 + 2) to monitor warfarin therapy [14, 34]. These assays are also not widely available, however, and would not be useful for distinguishing therapeutic anticoagulation from supratherapeutic anticoagulation. Still other investigators have recommended long-term heparin therapy in these patients [32], but long-term complications and difficulties with monitoring limit this approach. Although low-molecular-weight heparins may decrease the risk for osteoporosis [35] and eliminate the need for monitoring, the cost of these medications currently precludes their long-term use in most patients.

Our study shows that use of the INR in patients with lupus anticoagulants receiving warfarin has limitations. Future studies must define the optimal approach to managing anticoagulant therapy in these patients.

Glossary

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Antiphospholipid antibodies: A heterogenous family of autoantibodies, frequently identified in patients with systemic lupus erythematosus and related rheumatic disorders, that recognize various phospholipid and protein complexes [36]. Antiphospholipid antibodies include lupus anticoagulants (identified by clotting assays) and anticardiolipin antibodies (identified by enzyme-linked immunosorbent assay).

Chromogenic factor X assay: A two-stage amidolytic assay specific for factor X. Factor X in test plasma samples is converted to factor Xa by Russell viper venom. Only factor X that has undergone vitamin K-dependent carboxylation can be used as a substrate [25]. This reaction does not require a phospholipid membrane surface. Factor Xa then catalyzes hydrolysis of the chromogenic substrate S2337, which is detected by monitoring absorbance at 405 nm [26].

Dilute Russell viper venom time: Phospholipid-dependent screening assay for detecting lupus anticoagulants. Russell viper venom contains two enzymes that activate factor X to factor Xa and factor V to factor Va, thereby initiating coagulation at the level of the common pathway (Figure 1).

Gla domain: Homologous domains located within the amino-terminal region of vitamin K-dependent proteins that contain 9 to 12 -carboxyglutamic acid residues in the fully modified protein. Warfarin produces its anticoagulant effect by interfering with the carboxylation of glutamic acid residues in these domains [6].

International normalized ratio: The INR is derived from the prothrombin time ratio by the following equation:

INR = (prothrombin time ratio) international sensitivity index

By using this equation, the prothrombin time ratio is converted to the ratio that would have been obtained if the World Health Organization reference thromboplastin had been used with a manual prothrombin time technique [6]. This enables standardization of many thromboplastin reagents.

International sensitivity index: A measure of the sensitivity of any given thromboplastin to a reduction of the vitamin K-dependent coagulation factors compared to the international reference preparation of the World Health Organization [6]. The lower the international sensitivity index, the more sensitive the particular reagent is to decreases in the vitamin K-dependent factors.

Lupus anticoagulant: A subtype of antiphospholipid antibodies that interferes with phospholipid-dependent coagulation assays.

Platelet neutralization procedure: An activated partial thromboplastin time-based (or dilute Russell viper venom time-based) assay used to confirm the presence of a lupus anticoagulant [37]. Activated partial thromboplastin time for screening purposes is performed with and without platelet extract added. This extract provides excess phospholipid. A shortening of the activated partial thromboplastin time in the presence of added platelet extract confirms the presence of a lupus anticoagulant.

Prothrombin time ratio: The ratio of the patient prothrombin time to the geometric mean of prothrombin times obtained from 20 or more normal donors [6].

Prothrombin-proconvertin time assay: A thromboplastin-based clotting assay similar to the prothrombin time. This assay differs from the prothrombin time in that the specific thromboplastin is supplemented with bovine factor V and fibrinogen [38]. Plasma samples are diluted into normal saline before analysis, making the assay particularly sensitive to minor changes in the levels of the vitamin K-dependent factors VII, X, and II.

Thromboplastin: A phospholipid-protein extract of tissues (usually tissues from the brain, lung, or placenta) that contains tissue factor and phospholipids. In the presence of calcium ions, thromboplastin initiates the extrinsic pathway of coagulation, promoting the activation of factor X by factor VIIa, and is used to determine the prothrombin time [6]. The thromboplastin used in the prothrombin-proconvertin time assay is supplemented with bovine factor V and fibrinogen.

Dr. Ortel: Box 3422, Department of Medicine, Division of Hematology, Duke University Medical Center, Durham, NC 27710.