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PubMed1998

A cross-Canada survey of prothrombin time testing: Does the establishment of local ISI values improve the accuracy of international normalized ratio reporting? Thrombosis Interest Group of Canada.

The international normalized ratio (INR) was established as a means of standardizing the prothrombin time regardless of the thromboplastin used in the individual laboratories. The INR is the prothrombin time ratio of the sample raised to the power of the International Sensitivity Index (ISI). Traditionally, the ISI is determined by using a manual clotting technique by comparing the test thromboplastin with a World Health Organization international reference thromboplastin with results from 60 patient samples standardized on warfarin, and 20 samples from normal volunteers. Most laboratories no longer perform prothrombin time testing by a manual technique but instead automate the procedure by using a variety of coagulation instruments. Thromboplastin ISI values have the potential to be modified considerably by instrumentation, and as a consequence, they, may result in INRs that are unreliable when compared to those obtained by the traditional manual method. It has been proposed that the use of calibrant plasmas tested with the laboratory's thromboplastin and instrument may overcome this problem and a local ISI value could be established. Our study objective was to determine whether a local ISI calibration with the plasma calibrants could reduce the variation of the INR over a wide range of thromboplastin/ instrument combinations. A total of 58 laboratories from across Canada, including community hospitals, private laboratories, and large tertiary care units, participated. The findings indicate that the use of calibrant plasmas to calculate the local ISI does improve the accuracy of INR reporting in the majority of thromboplastin/instrument combinations.

Autoanalysis↗

PubMed2000

Phytomenadione or menadiol in the management of an elevated international normalized ratio (prothrombin time).

AIM: To evaluate the efficacy of oral menadiol compared to intravenous phytomenadione when correcting coagulopathies associated with cholestasis. METHODS: A total of 26 patients with cholestasis and an international normalized ratio (prothrombin time) greater than 1.2, were randomized to receive either 20 mg o.d. for 3 days of oral menadiol (n=12), or 10 mg o.d. of intravenous phytomenadione (n=14) prior to endoscopic retrograde cholangeopancreatography. Liver function tests and international normalized ratio were measured daily for 3 days. RESULTS: Liver function tests and international normalized ratio were comparable between groups at entry into the study (P > 0.05), but serum albumin was significantly lower in the intravenous phytomenadione group following treatment (P < 0.05). A decrease in international normalized ratio occurred in both groups following administration of vitamin K (P < 0.05). Two patients in the intravenous group required fresh frozen plasma, as failure to normalize international normalized ratio was observed. No adverse drug reactions were observed in either group, and no patient required re-admission for bleeding during a 4-week follow-up period after cholangeopancreatography. CONCLUSION: Oral menadiol appears to be an effective alternative to intravenous phytomenadione in the correction of coagulopathies associated with obstructive liver disease. This simplifies the care of patients with deranged clotting times requiring cholangeopancreatography, particularly those to be managed as out-patients.

Aged↗

PubMed2004

Effect of warfarin nonadherence on control of the International Normalized Ratio.

OBJECTIVE: The frequency and causes of ab- errant International Normalized Ratios (INRs) in warfarin recipients and the percentage explainable by warfarin nonadherence were studied. METHODS: The medical records of patients whose warfarin therapy was monitored by a telephone-based anticoagulation service in a Midwestern urban hospital between March 2000 and March 2001 were reviewed for causes of out-of-range INRs, the percentage of out-of-range INRs attributable to warfarin nonadherence, and demographic and clinical variables predictive of nonadherence. RESULTS: Data from 347 patients were studied. The cohort yielded 4305 INRs, of which 1002 (23%) were out of range (lower than 1.8 or higher than 3.4). Thirty-six percent of the out-of-range INRs were due to warfarin or dietary nonadherence, 9% were due to medical systems problems (including drug interactions), 18% were due to a change in clinical status, and 38% were idiopathic. Age younger than 65 years, age greater than 80 years, and living closer to the laboratory were predictive of warfarin nonadherence. CONCLUSION: Warfarin nonadherence was the most common cause of explainable aberrant INRs in patients taking warfarin. Age younger than 65 years, age greater than 80 years, and living close to the laboratory were predictive of warfarin nonadherence.

Age Factors↗

PubMed2002

Evaluation on the use of a portable unit versus the laboratory for the monitoring of International Normalized Ratio (INR) in orally anticoagulated patients.

INTRODUCTION: Measuring the International Normalized Ratio (INR) through a laboratory requires venipuncture and takes about 1 hour for results to be available. A portable monitor (CoaguChek Plus System), which measures the INR using fingerstick samples, is evaluated in this study to determine its clinical significance in anticoagulated and non-anticoagulated individuals. The hospital's outpatient and inpatient laboratories were also compared in the study. MATERIALS AND METHODS: Paired venous and capillary blood INRs were performed on anticoagulated patients using the monitor and the Singapore General Hospital (SGH) outpatient and inpatient laboratories (OPS and IPS labs). Paired INRs of control samples were also performed using the monitor and the IPS lab. RESULTS: We plotted the difference in INR by the 2 methods (monitor and OPS lab, n = 91) against their mean, and calculated the limits of agreement (95% of the difference would lie between -0.90 to 0.70). After a logarithmic transformation on the data, we found that for 95% of the cases, the OPS lab would differ from the monitor by 13% below to 14% above. There was also a marginal difference (95% limits of agreement of -0.14 to 0.10) when we compared the INR obtained from OPS and IPS laboratories (n = 43). Our control sample (n = 19) showed that the 95% confidence interval for the bias was -0.04 to 0.10. CONCLUSION: The monitor should be used with caution in patients with INR > 3. We suggest use of the monitor in situations where the non-anticoagulated state of a patient needs to be measured. There is a difference in INR measured by laboratories within the same institution.

Anticoagulants↗

PubMed2000

Acetaminophen causes an increased International Normalized Ratio by reducing functional factor VII.

Acetaminophen may increase International Normalized Ratio (INR) in patients taking anticoagulation medication, and in patients with acetaminophen poisoning without hepatic injury. The objective of this study was to describe and investigate the effect of acetaminophen on INR. The authors studied patients admitted to a regional toxicology treatment center with acetaminophen poisoning with INR and without potentially confounding coingestion or hepatic injury. Exposed and nonexposed (control) cohorts were recruited from admissions with acetaminophen poisoning and psychotropic drug poisoning, respectively. From 1,437 acetaminophen poisonings, after exclusions, there were 143 admissions with 205 estimations of INR. INR showed a time-dependent increase. Fifty percent of all patients and 66% of those with an extrapolated 4-hour acetaminophen concentration > or = 150 mg/L had an abnormal INR at some time. Dose ingested (p = 0.01) and nomogram-based risk (p for trend = 0.005) were correlated with the effect. N-acetylcysteine had a protective effect. Functional factor VII was lower (p = 0.005) in exposed patients (n = 30) than controls (n = 8), and less than antigenic factor VII in exposed patients (p = 0.03). Factor IX was lower (p = 0.02). Factor VIIIc was not significantly different. The authors concluded that an isolated, small rise in INR is common after acetaminophen poisoning without hepatic injury. It appears to be caused by inhibition of Vitamin K-dependent activation of coagulation factors. This effect suggests a possible mechanism for the observed interaction between acetaminophen and warfarin.

Acetaminophen↗

PubMedDate not supplied

Sampling for international normalized ratios in patients on hemodialysis with central venous catheters.

The primary objectives of this study were to evaluate the effect of varying heparin concentrations on International Normalized Ratio values and to assess the accuracy of these values deter mined through the arterial line of the dialysis circuit Twenty-two patients on hemodialysis with central venous catheters were studied After a peripheral venipuncture, timed samples from the arterial line of the hemodialysis circuit were obtained after dialysis was initiated and prior to initiating heparin. Assays for coagulation parameters were performed. There was no signifcant difference in any coagulation parameter measured from the arterial line samples compared to the venipuncture samples. Serial heparin dilutions in vitro demonstrated that residual amounts of heparin in the central venous catheter may falsely elevate International Normalized Ratio values. This study demonstrates that accurate International Normalized Ratio values in patients on hemodialysis with heparinized central venous catheters can be obtained ericiently and cost-effectively from the arterial line within 1 minute of dialysis initiation.

Anticoagulants↗

PubMed2002

Discrepant sensitivity of thromboplastin reagents to clotting factor levels explored by the prothrombin time in patients on stable oral anticoagulant treatment: impact on the international normalized ratio system.

BACKGROUND AND OBJECTIVES: We tested the principle of local International Normalized Ratio (INR) calibration using INR calibrator plasmas (PT Calibration Plasma Kit, Behring), two thomboplastin reagents (Neoplastin plus, rabbit brain, Stago, and Recombiplastin, recombinant human tissue factor, Ortho Diagnostics) and the same coagulometer (STA, Stago) on 92 patients on stable oral anticoagulant treatment. DESIGN AND METHODS: A four-point calibration was obtained with each reagent by linear regression (sec/INR) on a log-log scale (r > or = 0.999). The bias between the two reagents (Recombiplastin - Neoplastin Plus) was reduced from 31.7% to 17.5% and 7.5% (p=0.001) when results were expressed, respectively, as PT ratio (using the mean normal prothrombin time as denominator term), INR (using instrument-specific ISI supplied by the manufacturers) and calibrated INR, but there was a consistently significant regression of the differences over the average values even after log transformation (r > or = 0.586). The bias between the reagents was reduced to 1% (p=ns) when assuming Recombiplastin as the reference thromboplastin and applying Tomenson's correction, but limits of agreements were as large as 20%. Factor VII, X, V and II activity was measured with the two thromboplastin reagents in all plasma samples using immunodepleted plasmas (Stago). RESULTS: Statistically significant biases were observed for all clotting factors with the two reagents (Recombiplastin Neoplastin Plus) and ranged from 3.5 % (FII) to 37.2% (FVII). In addition, for FVII and FV there was a significant regression of the difference over the average value (after log-transformation, r > or = 0.282). The patients were divided into 3 groups according to their degree of anticoagulation (INR <2.0; INR between 2.0 and 3.5; INR >3.5). Factor levels differed significantly with the two reagents throughout the 3 groups of patients. In addition, the relative distributions of the 3 vitamin K-dependent factors also differed in the 3 groups with the two thromboplastin reagents. INTERPRETATION AND CONCLUSIONS: The discrepant sensitivity to factor VII, X and V levels of the two thromboplastin reagents explored in this study prevents INR calibration with commercially available calibrator plasmas and is responsible for a significant variability in INR values even under optimal conditions of INR calibration.

Adult↗

PubMed1999

Transbronchial biopsy in the presence of profound elevation of the international normalized ratio.

STUDY OBJECTIVE: To identify a level of coagulopathy, reported as the international normalized ratio (INR), that predicts hemorrhage following transbronchial forceps biopsy (TBBx) in an animal model. DESIGN: Crossover blinded study using Yucatan mini-swine (Sus scrofa). SETTING: Tertiary medical center with a dedicated animal research facility. STUDY DESIGN: A two-stage study. In stage 1, flexible fiberoptic bronchoscopy with TBBx was performed to establish the amount of bleeding in animals with normal coagulation systems. Animals then were administered escalating dosages of warfarin to obtain one of several increased INR levels. The endpoint of stage 1 was defined as the INR that resulted in a blood loss of > or = 100 mL in > or = 50% of the study animals. In stage 2, all the animals were to be anticoagulated to the INR level determined in stage 1. Topical and systemic measures would then be administered in an attempt to decrease postprocedure hemorrhage, and the results were recorded. RESULTS: Eighteen animals were enrolled in the study. Despite INR levels > 10, no animals developed a hemorrhagic complication of the transbronchial forceps biopsy (TBBx). Eleven animals had INRs > 7. Four animal deaths were recorded, with three animal deaths attributed to nonpulmonary hemorrhage, each due to a ruptured ovarian cyst. One death was anesthesia related. Stage 2 of the study was not performed due to the extreme INR levels reached in the animals during stage 1 and to the lack of a procedure-related complication. CONCLUSIONS: Our study suggests that INR elevation does not correlate with an increased risk of bleeding following TBBx in this animal model.

Animals↗

PubMed2000

Effect of three loading doses of warfarin on the international normalized ratio for dogs.

OBJECTIVE: To evaluate effect of 3 loading doses of warfarin sodium on the international normalized ratio (INR) for dogs. ANIMALS: 18 dogs weighing between 25 and 30 kg. PROCEDURE: Dogs were randomly allocated into 3 groups and received 2, 4, or 6 mg of warfarin administered orally once a day for 2 days after surgery for bilateral iliac artery grafting. Activated partial thromboplastin (APTT) and prothrombin times (PT) were measured before and after treatment. Prothrombin time also was reported as an international normalized ratio. RESULTS: The APTT were not significantly different among groups before or after treatment. The INR and PT were significantly increased in all groups after treatment. The INR and PT of the 6-mg group were significantly greater than those of the 2-mg and 4-mg groups. None of the dogs had clinical evidence of bleeding. CONCLUSIONS AND CLINICAL RELEVANCE: A warfarin loading dose of 6 mg/d can be safely administered for 2 days to dogs weighing between 25 and 30 kg. Anticoagulation can be achieved safely in dogs in 2 days by use of warfarin. The effects of warfarin can be monitored with the INR.

Administration, Oral↗

PubMed1998

The effect of recombinant factor VIIa (NovoSeven) in healthy volunteers receiving acenocoumarol to an International Normalized Ratio above 2.0.

Vitamin K antagonists are most commonly used in long-term thrombosis prophylaxis and the use in patients with cardiovascular disease seems to be increasing. By interfering with the normal hemostatic mechanism, an increased risk of bleeding will arise and administration of human plasma or prothrombin complex concentrates may be necessary. It can be difficult to normalize hemostasis using plasma and prothrombin complex concentrates, because these may be associated with thromboembolic side-effects. The level of factor VII, one of the vitamin-K-dependent coagulation factors, decreases during oral anticoagulant therapy and the administration of recombinant factor VIIa normalizes the prolonged prothrombin time in warfarin-treated rats. After administration of acenocoumarol (International Normalized Ratio > 2), decreased levels of factor X and factor IX (19-46%), protein C (2-20%) and factor VII (4-17%) were found in 28 healthy volunteers. After one dose of recombinant factor VIIa (5, 10, 20, 40, 80, 120, 160, 240, or 320 microg/kg) the International Normalized Ratio and prothrombin time normalized, which may imply an effect on bleeding in individuals receiving oral anticoagulant therapy. The lowest dose (5 microg/kg) normalized the International Normalized Ratio for 12 h and doses > 120 microg/kg normalized it for 24 h. Fragment 1+2 stayed within its normal range in all dose groups, indicating that no systemic coagulation occurred.

Acenocoumarol↗

PubMed1995

Temporary discontinuation of warfarin therapy: changes in the international normalized ratio.

OBJECTIVE: To measure the rate of decrease of the international normalized ratio (INR) after temporary discontinuation of warfarin therapy. DESIGN: Prospective evaluation of an outpatient cohort. SETTING: University medical center anticoagulation clinic. PATIENTS: 22 patients receiving a fixed evening dose of warfarin for whom temporary discontinuation of therapy was deemed safe. MEASUREMENTS: Serial plasma samples were drawn for INR measurements approximately 20, 65, 115, and 185 hours after patients received the last dose of warfarin. In five patients, INR was measured twice daily for 5 days. RESULTS: For patients with a mean steady-state INR of 2.6, the mean INR 65 hours (2.7 days) after discontinuation of warfarin therapy was 1.6 (range, 1.11 to 2.16); 20 of 22 patients (91%) had an INR greater than 1.2. The mean INR 115 hours (4.7 days) after discontinuation of warfarin therapy was 1.1; 5 of 22 patients (23%) had an INR of 1.2 or greater. In 5 patients studied in detail, the INR decreased exponentially and had a half-life that ranged from 0.52 to 1.2 days; the onset of maximal decrease began 24 to 36 hours after discontinuation of warfarin therapy. In the total cohort, age was a significant (P < 0.005) independent predictor of smaller decreases in the INR between day 1 and day 3 (regression coefficient = -6.8% +/- 2%/2 days per decade of age; R2 = 0.34). CONCLUSIONS: By simulating preoperative discontinuation of warfarin therapy, we found that the INR decreases exponentially, with wide interpatient variation in the rate of decrease. Age is associated with a slower rate of decrease. To be certain that the INR at the time of the surgery is less than 1.2, warfarin should be withheld for 96 to 115 hours (4 doses) in patients with a steady-state INR between 2.0 and 3.0. For patients with a higher steady-state INR, a longer wait is necessary.

Adult↗

PubMed2001

Clinical impact of interlaboratory variation in international normalized ratio determinations.

Variation among laboratories and point-of-care (POC) devices in International Normalized Ratio (INR) determinations for individual patients in the same community was studied. For each patient, blood samples obtained from a single venipuncture followed by a finger stick were tested at three laboratories and with two POC devices. A nonparametric analysis of variance and regression analysis were used to compare INRs across sources. The potential clinical impact of differences between pairs of INR readings (i.e., whether a difference was likely to result in a change in therapy) was evaluated. Complete data were obtained and analyzed for 41 patients. Of the five sources of INRs, results from two laboratories and one POC device were significantly similar. Results from one laboratory and one POC device were significantly different from the other results. Variation between INR readings from two different sources might have resulted in a change in therapy 10-30% of the time. Variations occurred in INR readings among three laboratories and two POC devices and could have led to a substantial number of inappropriate changes in therapy.

Anticoagulants↗

PubMed2002

Effect of stress on international normalized ratio during warfarin therapy.

OBJECTIVE: To discuss the effect of stress on the international normalized ratio (INR) when patients are taking warfarin. CASE SUMMARY: Two patients at a pharmacist-managed anticoagulation clinic who were stable with anticoagulation developed elevated INR values after a stressful event occurred. All other factors known to elevate the INR were unchanged; furthermore, the INR values returned to the prior level of control after resolution of the stressful events. DISCUSSION: Management of anticoagulation with warfarin requires the knowledge of factors that may alter an INR. Many of these factors, such as dietary changes, illnesses, drug interactions, patient compliance, and physical activity, have been described. In spite of this understanding, many patients continue to experience variability in their INR values, suggesting there are other factors that can alter the INR that have not been fully described. The cases presented here demonstrate that stressful events, physical or psychological, can elevate the INR. The mechanism for this occurrence is unknown, but may be related to decreased metabolism of warfarin during stress. CONCLUSIONS: When an unexplained INR value exists, a stressor should be evaluated as a potential cause.

Aged↗

PubMed2001

Clinical predictors of prolonged delay in return of the international normalized ratio to within the therapeutic range after excessive anticoagulation with warfarin.

BACKGROUND: An elevated international normalized ratio (INR) increases the risk for major hemorrhage during warfarin therapy. Optimal management of patients with asymptomatic elevations in INR is hampered by the lack of understanding of the time course of INR decay after cessation of warfarin therapy. OBJECTIVE: To identify predictors of the rate of INR normalization after excessive anticoagulation. DESIGN: Retrospective cohort study. SETTING: Outpatient anticoagulant therapy unit. PATIENTS: Outpatients with an INR greater than 6.0 were identified from August 1993 to September 1998. Patients in whom two doses of warfarin were withheld and a follow-up INR was obtained on the second calendar day were enrolled. No patient received vitamin K(1). MEASUREMENTS: The INR was measured 2 days after an INR greater than 6.0 was recorded. RESULTS: Of 633 study patients with an initial INR greater than 6.0, 232 (37%) still had an INR of 4.0 or greater after two doses of warfarin were withheld. Patients who required larger weekly maintenance doses of warfarin were less likely to have an INR of 4.0 or greater on day 2 (adjusted odds ratio per 10 mg of warfarin, 0.87 [95% CI, 0.79 to 0.97]). Other risk factors for having an INR of 4.0 or greater on day 2 included age (odds ratio per decade of life, 1.18 [CI, 1.01 to 1.38]), index INR (odds ratio per unit, 1.25 [CI, 1.14 to 1.37]), decompensated congestive heart failure (odds ratio, 2.79 [CI, 1.30 to 5.98]), and active cancer (odds ratio, 2.48 [CI, 1.11 to 5.57]). CONCLUSIONS: Steady-state warfarin dose, advanced age, and extreme elevation in INR are risk factors for prolonged delay in return of the INR to within the therapeutic range. Decompensated congestive heart failure and active cancer greatly increase this risk.

Age Factors↗

PubMed2001

[The factors that influence the consistency of international normalized ratio of prothrombin time].

OBJECTIVE: To investigate the factors that influence the consistency of international normalized ratio (INR) of prothrombin time. METHODS: Using the same automated blood coagulation analyzer and INR calibration plasma with two different types of prothrombin time (PT) reagent to assay the INR values of 19 controls (group 1), 12 patients who took long-term anticoagulant orally (group 2), 12 patients who took it at the first time (group 3) and 8 patients who took long-term anticoagulant together with cephalosporin N (group 4). RESULTS: The two kinds of reagents gave quite different INR values in all four groups (P < 0.01). The INR difference in group 3 (0.23 +/- 0.07, n = 12) was larger than that in group 2 (0.51 +/- 0.20, n = 12) (P < 0.01). The difference in group 4 (0.61 +/- 0.21, n = 8) was larger than that in group 2 (P < 0.01). CONCLUSIONS: The factors that may influence INR consistency are the source and sensitivity of the reagents, as well as the period of the anti coagulant been taken and the other drugs taken at the same time.

Administration, Oral↗

PubMed1994

The international normalized ratio (INR) for monitoring warfarin therapy: reliability and relation to other monitoring methods.

OBJECTIVE: To enhance understanding of the reliability of the international normalized ratio (INR) for monitoring warfarin therapy and its relation to other monitoring techniques. DESIGN: Prospective cohort study. SETTING: A university hospital. PATIENTS: 79 patients attending an anticoagulation clinic. MEASUREMENTS: International normalized ratios obtained with a portable capillary monitor (Coumatrak) and the following from a simultaneous plasma sample: INRs from prothrombin times done with six thromboplastins, prothrombin-proconvertin (P&P) test activity, specific prothrombin activity, and native prothrombin antigen. RESULTS: Converting to INRs failed to standardize prothrombin time results obtained with high- and low-sensitivity thromboplastins. Coumatrak INRs correlated best with INRs obtained with high-sensitivity thromboplastins. The INR range of 2.0 to 3.0 corresponded to a P&P range of 30% to 13%, a native plasma prothrombin antigen range of 56 to 24 micrograms/mL, and a specific prothrombin activity range of 43% to 21%. CONCLUSIONS: Low-sensitivity thromboplastins may give erroneously high INRs in the upper therapeutic range. Plasma prothrombin times should be done with a high-sensitivity thromboplastin, particularly in patients maintained at the upper limit of the therapeutic range. An INR so obtained correlated well with an INR obtained with a portable capillary blood monitor.

Autoantigens↗

PubMed1999

Clinically significant differences in the International Normalized Ratio measured with reagents of different sensitivities. SPAF Investigators. Stroke Prevention in Atrial Fibrillation.

The International Normalized Ratio (INR) system was introduced a decade ago as a way of standardizing the results of prothrombin time testing for patients taking oral anticoagulants. A strong emphasis has been placed upon using thromboplastin reagents that are very sensitive to the effects of oral anticoagulants upon the prothrombin time [i.e. reagents with low International Sensitivity Index (ISI)]. In order to assess how well the INR system functions as currently used in clinical laboratories, we compared the INRs determined using thromboplastins of differing ISIs in samples collected during a large clinical trial of oral anticoagulation for atrial fibrillation (Stroke Prevention in Atrial Fibrillation III trial). Frozen plasma was subjected to prothrombin time testing using thromboplastins with ISIs ranging from 0.97 to 2.49. INRs were calculated using machine-specific ISIs and Westgard's rules were followed to maintain quality control. An unanticipated coagulometer failure allowed a determination of the effect of machine recalibration upon the INR of control plasmas. The correlation between each pair of INRs obtained from 1181 plasmas was high (> 0.9), but the differences between reagents were statistically different from zero (P<0.001 for pairwise comparisons). Plasmas had INRs within the therapeutic range (2.0-3.0) with one reagent but not with another in an average of 20% of instances. Among the 20% discordant pairings, 43% (8.5% of the total tested) showed a difference in INR of more than 0.2 INR units above or below the target range. Low ISI thromboplastins did not perform better in this pairwise comparison than other reagents or the locally determined INR. Recalibration of a coagulometer resulted in a significant change in the INRs obtained from control plasmas (P<0.0001), which confirms and extends the observations of other authors concerning the sensitivity of the INR to coagulometer-related variables. There was a clinically significant difference in the INRs obtained with different thromboplastins, and low ISI reagents did not perform better than others. Since the risk of thrombosis rises sharply below the lower limit of the currently recommended target ranges, consideration should be given to narrowing the recommended range, or advising clinicians to aim for its mid-point. These findings illustrate the difficulties in imposing standardization upon coagulation testing after a test is in widespread use.

Administration, Oral↗

PubMed2004

Unreliability of international normalized ratio for monitoring warfarin therapy in patients with lupus anticoagulant.

STUDY OBJECTIVE: To compare the international normalized ratios (INRs) of patients positive for lupus anticoagulant and the INRs of control patients receiving warfarin therapy with equivalent therapeutic chromogenic factor X levels. DESIGN: Prospective case series. SETTING: A 625-bed, adult, private, tertiary care teaching hospital. PATIENTS: Sixty-eight outpatients positive for lupus anticoagulant and 57 control patients receiving long-term warfarin therapy. MEASUREMENTS AND MAIN RESULTS: Concomitant INR and chromogenic factor X activity were measured in all patients. In 44 control patients (77%) and 46 patients with lupus anticoagulant (68%), chromogenic factor X activity was 22-40% of normal, which is therapeutic. Of the 44 control patients, 4 (9%) had an INR above 3.0, and none had an INR above 4.0. In contrast, 18 (39%) of the 46 patients with lupus anticoagulant had an INR above 3.0, and 5 (11%) had an INR above 4.0. CONCLUSION: At least 10% of patients with lupus anticoagulant receiving long-term warfarin therapy may have falsely high INR values, which could lead to inappropriate warfarin dosage reduction. Monitoring warfarin therapy by chromogenic factor X activity in patients with lupus anticoagulant avoids this INR artifact.

Adult↗