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PubMed1998

Comparison of a portable capillary whole blood coagulation monitor and standard laboratory methods for determining international normalized ratio.

The international normalized ratio (INR) is the current standard for monitoring anticoagulation therapy. Although simple to determine, it normally requires venipuncture and extensive laboratory resources for specimen handling and analysis. The portable capillary whole blood coagulation monitor is an alternative to laboratory venipuncture. Its promoted advantages are: it obtains a blood sample by finger-stick versus venipuncture; rapid turnaround time for results; resultant dosage adjustments (as appropriate) performed in minutes versus hours or days after testing; relative ease of use by nonlaboratory personnel; and potential for home monitoring. This project compared the results of INRs obtained through the venipuncture/laboratory process to INRs obtained by the portable monitoring process at the National Naval Medical Center. A correlation coefficient of 0.97 was determined. The difference in the mean INR results of the two testing methods was not clinically significant (p = 0.269). The portable monitor was determined to be a viable alternative to laboratory testing.

Adult↗

PubMed1995

The value of plasma calibrants in correcting coagulometer effects on international normalized ratios. An international multicenter study.

A study of lyophilized plasma calibrants in correcting for coagulometer effects on International Normalized Ratios (INR) has been conducted in an international survey. Prothrombin times were performed with the calibrants on 2 "common" thromboplastins and in-house thromboplastins in 3 brands of coagulometer at 37 centers. The International Sensitivity Indices (ISI) of 95 local systems with the calibrants were derived. The "true" INR of 10 test plasmas from warfarin-treated patients were established centrally using thromboplastin International Reference Preparations (IRP). International Normalized Ratios of these plasmas for each center were calculated using both the manufacturers' stated and the local ISI. The mean deviation of the 95 systems from the "true" INR of the warfarin plasmas was +14.4% with the manufacturers' ISI, but reduced to +1.04% with the local ISI. Local ISI determination with the calibrants avoids many of the difficulties of conventional thromboplastin calibrations. Plasmas from patients on warfarin and parallel manual PT with thromboplastin IRP are not required.

Blood Coagulation Tests↗

PubMed1996

Calibration verification of the International Normalized Ratio.

The International Normalized Ratio (INR) system for reporting the prothrombin time (PT) is essentially a calibration activity intended to standardize PT reporting across various reagent/instrument systems. However, complete standardization of PT reporting through the INR has been difficult to achieve for a variety of reasons, including inaccurate assignment of thromboplastin International Sensitivity Indexes (ISIs) and specific (local) reagent/instrument effects. Until now, the individual laboratory has not been able to easily verify the accuracy of its INR. Using standard lyophilized plasmas with INR values assigned against IRP RBT/90 rabbit thromboplastin, the authors present a method that allows a laboratory to locally verify its range of accuracy for the INR. The method is illustrated on a single coagulometer with two thromboplastin lots of differing sensitivity (Pacific Hemostasis Thromboplastin-DS and Thromboplastin-D from rabbit sources, with respective International Sensitivity Indexes of 1.20 and 1.97). In this illustration of the method, the accuracy of Thromboplastin-DS was superior to that of Thromboplastin-D. Interpretation of the data and cautions regarding the use of standard plasmas for calibration verification are discussed. Using this method, a reportable range of accuracy at a given error tolerance can be established locally for INR measurements within a laboratory. Laboratories of any size can apply this method to study the accuracy of their INR reagent/instrument systems, thus performing calibration verification. When used in conjunction with assessments of assay precision, this method can help laboratories to select better reagent/instrument systems and thereby produce more accurate and more clinically meaningful INR results.

Animals↗

PubMed1994

The influence of the reference mean prothrombin time on the international normalized ratio.

The International Normalized Ratio (INR) is a mathematical transformation of the prothrombin time (PT). The transformation requires a laboratory to compute the geometric mean of its own reference population. In this paper, the authors examine how the reference mean PT influences the INR accuracy and precision using a validated probabilistic model. The variance of the geometric mean of reference populations in three laboratory settings was determined. Because the variance of an individual laboratory geometric mean is not directly determinable by simple parametric equations, its variance is estimated using bootstrap analysis. The geometric mean is compared to the computationally simpler arithmetic mean for effects on accuracy and precision of the resulting INR. The study shows mathematically and empirically that using the arithmetic mean biases INR determinations so that patients tend to be over-anticoagulated. However, in the laboratory settings examined, the amount of bias was both statistically and clinically insignificant. An analysis of the effect on the INR of errors in estimating the geometric mean reference PT also is performed. For large biases in estimating the reference mean, the INR can be significantly affected and can trigger inappropriate clinical actions in patients. The authors demonstrate empirically and mathematically that biases in the geometric mean reference PT do not affect the INR coefficient of variation. However, they produce significant differences in confidence intervals for INR determinations. Laboratories must exercise care in determining specific reference means to ensure that biases do not occur in geometric mean reference PT determinations. This can be achieved by circumspection in the selection of normal subjects for the reference population, carefully reviewing the data, and performing the proper calculations on the data.

Female↗

PubMedDate not supplied

Quality control of the prothrombin time and international normalized ratios. National and international schemes.

The international normalized ratios (INR) system allows valid comparisons in results and quality of performance to be made between users of different thromboplastin reagents. In the international quality control surveys currently over 80% of the 53 countries participating report INR. Stated local international sensitivity index (ISI) values show fair agreement with values calculated from quality control returns obtained with local and reference reagents. The coefficient of variations (CV) of the INR in these surveys are between 11-22% depending upon INR values. In comparison, CV of the UK national scheme are currently 7-13%. However, analysis of UK results has shown high CV with high ISI reagents. This is due to the ISI effect as CV of INR is CV of prothrombin ratio (PR) multiplied by the ISI. Ideal thromboplastins should show good precision of PR and a low ISI to prevent this apparent deterioration when PR results are transformed into the INR scale. Instrumentation has a further effect on the INR result. Unfortunately, the effect is not uniform even within instrument type and model or even between normal and therapeutic results. Local instrument adjustment or local calibration is therefore necessary. Thus, quality control surveys continue to highlight problems in prothrombin time standardization.

Automation↗

PubMed2004

In-house calibration of the international sensitivity index or calibration curve for determination of the international normalized ratio.

CONTEXT: The international normalized ratio (INR) has been used since 1983 to standardize prothrombin time results for patients on oral anticoagulants. However, significant interlaboratory variations have been noted. Attempts have been made to address these differences with the use of instrument-specific International Sensitivity Index (ISI) values and in-house calibration of ISI values. OBJECTIVE: To assess the performance of laboratories using a calibration curve for INR testing. DESIGN: Attempts to improve performance of the INR include the use of instrument-specific ISI values, model-specific ISI values, in-house calibration of ISI values, and more recently, the preparation of a calibration curve. Several studies have shown an improvement in performance using these procedures. In this study of licensed laboratories performing routine coagulation testing in the Province of Ontario, Canada, the determination of the INR by a calibration curve was compared with the laboratories' usual method of assessment. These methods were subsequently analyzed by comparing the results to instrument-specific ISI, model-specific ISI, and in-house calibrators. International normalized ratios derived by both methods were analyzed for accuracy and precision. The stability of a calibration curve was also investigated. RESULTS: Performance of INR testing has improved with use of a calibration curve or in-house calibrators. CONCLUSION: The results confirm that either in-house calibrators or the calibration curve improve performance of INR testing. The calibration curve may be easier to use and appears stable up to 4 months.

Calibration↗

PubMed1996

Reliability of the international normalized ratio for monitoring the induction phase of warfarin: comparison with the prothrombin time ratio.

The International Normalized Ratio (INR) was introduced to reduce the variability of prothrombin time (PT) reporting. One potential problem with the use of the INR is the assumption that its reliability is reduced when it is used to monitor patients during the induction phase of treatment. This shortcoming arises because the model used to establish the INR system is based on the use of pooled plasma from patients stabilized on warfarin for at least 6 weeks. Because the prolongation of the PT by warfarin during the induction phase mainly reflects reduction in factor VII levels (whereas the prolongation of the PT after 6 weeks of stabilization reflects reductions in factors X, II, and VII), there exists a potential for loss of accuracy of the INR during warfarin induction. To overcome this potential problem, it has been suggested that the PT ratio should be used to report results during the induction phase of treatment and that the INR system should be reserved for reporting results after the patient has been stabilized. This approach is confusing to the clinician. In addition, the validity of this approach has never been demonstrated in a clinical study. To address this issue, we studied 43 patients for the first 5 days after they started warfarin therapy. We measured the PT in the same plasma samples from each patient with five different commercial thromboplastins. The variance in the PT ratios among the five thromboplastins was compared with the variance obtained with the INR values derived from the PT ratios when using the international sensitivity indexes provided by the manufacturer. Our results indicate that, even during the induction phase, there is less variance with the INR system than with the PT ratio system.

Anticoagulants↗

PubMed1994

Assessing random error in the international normalized ratio.

Changes in the international normalized ratio (INR) following warfarin administration may be explained not only by the attendant anticoagulant effect but also, in part, by random errors normally associated with laboratory assays. Thus, a patient's true INR will differ from the reported value by some random error related to the variability in the prothrombin time (PT) assay. By employing a statistical technique known as the delta method, the error expected in an INR for a particular institution can be estimated by taking the product of the international sensitivity index (ISI) and the coefficient of variation (CV) for the PT assay.

Drug Monitoring↗

PubMed1996

Prothrombin time in liver failure: time, ratio, activity percentage, or international normalized ratio?

Prothrombin time (PT) is a universal indicator of liver disease severity. However, variability in thromboplastin reagents leads to large interlaboratory differences in PT results. The aim of this study was to determine whether the use of the international normalized ratio (INR) or other modes of expression might achieve PT standardization in patients with liver failure. PT was measured with seven thromboplastin reagents with different sensitivities in plasmas from 27 patients with miscellaneous chronic and acute liver failure and, as a control population, 29 patients on oral anticoagulation therapy. PT was expressed in seconds, ratio, activity percentage, and INR. In patients with liver failure, only activity percentage expression eliminated variability in PT results obtained with the seven thromboplastins while INR, seconds, and ratio values remained significantly different (P < .01). In patients on oral anticoagulant therapy, only INR normalized PT results. We conclude that, in patients with liver failure, INR fails to yield a PT expression independent of the thromboplastin used and only activity percentage expression may provide a common international scale of PT reporting.

Analysis of Variance↗

PubMed1997

Effect of citrate concentration in specimen collection tubes on the International Normalized Ratio.

PROBLEM: Does citrate concentration in specimen collection tubes affect the International Normalized Ratio? METHODS: The International Normalized Ratio was determined on quadruplicate plasma specimens from 32 patients treated long term with oral anticoagulants-two from tubes with 3.2% citrate and two with 3.8% citrate. Two laboratories, using two different coagulometers, tested the specimens. RESULTS: International Normalized Ratios of plasma from tubes with 3.8% citrate were significantly higher than those from tubes with 3.2% citrate when tested with either coagulometer. Patients given adequate anticoagulation on the basis of the International Normalized Ratio at one concentration of citrate appeared either overanticoagulated and at risk of bleeding or underanticoagulated and at risk of thromboembolism at the other concentration of citrate. CONCLUSION: Results emphasize the need for using a single concentration of citrate for prothrombin time testing. We recommend 3.2% citrate.

Anticoagulants↗

PubMed1999

Multicenter evaluation of lyophilized and deep-frozen plasmas for assignment of the International Normalized Ratio.

The interlaboratory variation of the International Normalized Ratio (INR) in various external quality assessment schemes is still relatively high. This is partly caused by inaccuracy of manufacturers' stated International Sensitivity Index (ISI) and/or local instrumentation effects. The interlaboratory variation and accuracy of INR determinations may be improved by a local calibration procedure based on lyophilized plasmas with assigned INRs. The purpose of the present study was to determine INR values for different types of lyophilized plasmas to be used for local calibration. A total of 13 lyophilized plasmas (one normal, six from coumarin-treated patients, six artificially depleted) were analyzed by 10 laboratories, each using five calibrated prothrombin time (PT) systems. INRs were calculated for each plasma using each laboratory's specific ISI and mean normal prothrombin time values. In the same way, five deep-frozen pooled plasmas from coumarin-treated patients were analyzed. There were significant INR differences for the lyophilized plasmas between the prothrombin time systems. The differences were relatively small for the deep-frozen coumarin plasmas (CV 2.6-3.3%) and three lyophilized coumarin plasmas from one manufacturer (CV 3.7-4.8%). Important INR differences were observed for three lyophilized coumarin plasmas from another manufacturer (CV 9.5-14.1%) and several artificially depleted plasmas (CV 5.3-12.8%). The citrate concentrations in the artificially depleted plasmas were lower than those in the normal and coumarin plasmas. These differences should be considered in the selection and certification of plasmas as calibrants for local calibration of PT systems. The lyophilized plasmas' INR values obtained in the present study will be used for a field study of local PT calibration to assess their efficacy.

Animals↗

PubMed2005

Low-molecular-weight heparin may alter point-of-care assay for international normalized ratio.

STUDY OBJECTIVE: To compare the international normalized ratio (INR) measured by a point-of-care testing device with that measured by a reference laboratory method for patients receiving either warfarin only or warfarin plus low-molecular-weight heparin (LMWH). DESIGN: Retrospective study. SETTING: Outpatient anticoagulation clinic. SUBJECTS: Ninety-one patients receiving warfarin for various indications; 59 of them receiving only warfarin and 32 receiving warfarin plus LMWH. INTERVENTION: Capillary blood was obtained for INR determination by a point-of-care device, and venous blood was obtained for INR determination in a standard reference laboratory. MEASUREMENTS AND MAIN RESULTS: Ninety-one patients had INR pairs run on a point-of-care device and by the laboratory. In both the patients receiving only warfarin and in those receiving warfarin plus LMWH, the mean INR as determined by the point-of-care testing device was statistically significantly higher than the mean INR determined by the laboratory. Although the differences were statistically significant in both groups, the clinical significance of this difference was accentuated in the patients receiving warfarin plus LMWH. The measure of divergence between the point-of-care and laboratory methods was greater in the group receiving warfarin plus LMWH than in the warfarin-only group, with a mean +/- SD percent change between the INR values of 24.19 +/- 27.54% in the warfarin plus LMWH group and 7.21 +/- 17.73% in the warfarin-only group. In assessing the clinical impact of such variability, a greater degree of discordance in dosing adjustment decisions was noted for patients receiving warfarin plus LMWH. In this group, a 25% rate of discordance was noted compared with 8% in the warfarin-only group. Such discrepancy in dosing decisions based on the point-of-care INR would have resulted in discontinuation of LMWH therapy before the patient acquired a true therapeutic INR, with use of the laboratory measurement. CONCLUSION: The INR measured with the point-of-care device in patients receiving concurrent LMWH and warfarin therapy may be inaccurate. Patients receiving LMWH in addition to warfarin should have INRs checked by means of the standard reference laboratory method.

Adult↗

PubMed2004

Patient satisfaction with point-of-care international normalized ratio testing and counseling in a community internal medicine practice.

Point-of-care international normalized ratio (POC INR) testing is increasingly used to monitor anticoagulant therapy. This study assessed patient satisfaction with registered nurse--managed POC INR testing in a primary care internal medicine practice. One hundred eighty-seven of the first 216 (87%) patients using the program responded to the survey. The respondents were surveyed for satisfaction after they had experienced the nurse-managed POC INR system at the Mayo Clinic, Rochester, Minnesota, for one month. Eighty-eight percent of patients indicated that they were very satisfied or satisfied with the POC INR system. The authors conclude that nurse-managed POC INR testing is quick, convenient, less painful, and more satisfying for patients compared with traditional venipuncture and telephone follow-up.

Anticoagulants↗

PubMed1990

Evaluation of international normalized ratios by a controlled field survey with 4 different thromboplastin reagents.

A nationwide survey has been performed in Japan involving 75 laboratories to assess the relative reliability of different methods of reporting prothrombin time results in anticoagulant control. The interchangeability of results using prothrombin time, prothrombin activity percentage, prothrombin ratio and international normalized ratios (INR) were compared with four different thromboplastin reagents and a range of coagulometers. A secondary batch of reference thromboplastin of human brain origin (BCT/454) was used to calibrate the local thromboplastins and for comparison of methods of reporting. The study revealed the closest agreement of the results between BCT and the other reagents, and the regression lines of these reagents were almost identical, when the results were reported as INR. Box-Whisker plot analysis showed that the distribution of the results was large with the more deficient plasmas with all methods of reporting. It was found by this analysis that the interchangeability of the results was greatest when the results were expressed by INR, because the mean values obtained of each plasma using different thromboplastin reagents gave the lowest CV and the frequency of the far-out data was least, compared with the other methods of expression. On the other hand, the type of coagulometer had almost as much effect as the thromboplastin reagent on the prothrombin time, even if INR was used. Interchangeability of INR would be further improved by providing ISI values for each reagent/instrument combination.

Humans↗

PubMed1997

Using the international normalized ratio to standardize prothrombin time.

The international normalized ratio, or INR, was introduced in 1983 by the World Health Organization, or WHO, Committee on Biological Standards to more accurately assess patients receiving anticoagulation therapy. The INR mandates the universal standardization of prothrombin time. This article describes the method used to calculate INR, as well as its clinical relevance to the practice of dentistry.

Anticoagulants↗

PubMed2003

Acute GI bleeding in the setting of supratherapeutic international normalized ratio in patients taking warfarin: endoscopic diagnosis, clinical management, and outcomes.

BACKGROUND: Acute GI bleeding is a life-threatening complication of warfarin therapy. Acute GI bleeding in patients with an international normalized ratio of 4.0 or greater (supratherapeutic) is often attributed to trivial mucosal lesions. The aim of the study was to determine the frequency of potentially significant lesions that would warrant endoscopy in this setting. METHODS: A retrospective review was conducted of patients treated with warfarin who were admitted to a single Veterans Affairs hospital from 1996 to 2000 with acute GI bleeding. Endoscopic findings, clinical management, and outcomes are reviewed for patients with a supratherapeutic international normalized ratio (>or=4.0) and compared with patients with an international normalized ratio in the therapeutic range (2.0-3.9). RESULTS: Fifty-five patients with an international normalized ratio of 4.0 or greater (mean 8.4 [3.9]) and 43 patients with an international normalized ratio between 2.0 and 3.9 (mean 2.9 [0.6]) were hospitalized with acute GI bleeding. Thirty-seven patients (67%) with a supratherapeutic international normalized ratio and GI bleeding underwent upper endoscopy. Of these, 81.1% had positive findings, 18.9% had peptic ulcer disease, and 7.2% required endoscopic treatment. Thirty-eight percent of the patients with a supratherapeutic international normalized ratio underwent lower endoscopy; of these, 57.1% had abnormal findings and 9.5% required endoscopic treatment. Four patients (7.3%) in the supratherapeutic international normalized ratio group died during the index hospitalization. When patients with GI bleeding and a therapeutic international normalized ratio were compared with those with a supratherapeutic international normalized ratio, there were no significant differences between the two groups with regard to days of hospitalization, units of blood transfused, frequency of recurrent bleeding, need for surgery, or in-hospital deaths. CONCLUSIONS: The high frequency of clinically significant lesions in patients taking warfarin with an international normalized ratio in the supratherapeutic range and acute GI bleeding supports a role for endoscopic evaluation.

Acute Disease↗

PubMed2006

Effects of prednisone on the International Normalized Ratio.

PURPOSE: The effects of prednisone on the International Normalized Ratio (INR) values of a patient were examined. SUMMARY: A 66-year-old white man with a history of multiple myeloma was treated in an ambulatory care anticoagulation clinic for deep vein thrombosis. His INR values were normal during therapy with warfarin 14 mg weekly and thalidomide 300 mg daily. His INR values began to increase after three months of starting prednisone 10 mg daily. His weekly dose of warfarin was changed over the next two years, and his dietary intake of vitamin K was increased. For every INR value that was below the therapeutic goal, the patient was not taking prednisone; every time the INR value was above the therapeutic goal, he was taking prednisone. In November 2004, the prednisone and thalidomide were stopped and only the warfarin was continued. After a few dosage increases, ending with a weekly warfarin dose of 21 mg, the patient's INR values remained in the therapeutic range. Multiple variables must be examined when assessing INR values, as many things interact with warfarin. For example, tobacco use, alcohol consumption, and changes in vitamin K intake can affect the INR. Since this patient did not use tobacco or consume alcohol and had a fairly consistent dietary intake of vitamin K, these variables were ruled out as influencing the INR. In this case, the changes in his INR values corresponded to the addition or deletion of prednisone. CONCLUSION: A patient's INR values increased after the addition of prednisone to his warfarin regimen.

Aged↗

PubMed2004

Low-dose oral vitamin K therapy for the management of asymptomatic patients with elevated international normalized ratios: a brief review.

Asymptomatic elevation of the international normalized ratio (INR) is a common problem associated with hemorrhage. Evidence from randomized controlled trials supports the use of low-dose oral vitamin K therapy as a treatment that promptly reduces the INR. Vitamin K given orally is more effective than subcutaneous vitamin K injection, and as effective as intravenous administration when INR values are compared 24 hours after administration. A 1.0-mg vitamin K dose is likely most appropriate for patients with INR values between 4.5 and 10. The fear of over-correction of the INR has limited the widespread use of vitamin K; however, our review suggests that this occurs infrequently when small doses are administered orally.

Administration, Oral↗