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

Comparison of two point-of-care international normalized ratio devices and laboratory method

: A number of factors contribute to interlaboratory variation of international normalized ratio (INR) results. This variability can lead to differences in clinical decision-making. This prospective cohort study evaluated the accuracy, precision, and clinical decision-making variability of two point-of-care (POC) devices (CoaguChek XS and the Coag-Sense) to a clinical laboratory method of INR measurement as the 'reference' and by using the laboratory's accuracy standard (±25% of the reference INR). Study subjects included adults taking warfarin undergoing a POC INR with each device and a laboratory INR on the same day and evaluated differences in clinical decision-making using a warfarin nomogram. A correction factor was derived for each POC method to estimate the corresponding laboratory INR for POC INRs more than 3.0. A total of 100 patient encounters (76 unique patients) included INR testing by all three methods. Both POC devices demonstrated excellent precision across the entire range (r = 0.98), but poor accuracy relative to the laboratory standard for POC INRs greater than 3.0 (accuracy CoaguChek 19% and Coag-Sense 35%). A correction factor applied to POC INRs greater than 3.0 improved accuracy to 99% for both devices. Applying the correction factor also significantly reduced differences in clinical decision-making (49-28% for CoaguChek and 56-40% for Coag-Sense, P < 0.001). Both POC devices demonstrated poor accuracy for INRs over 3.0. A device-specific, institution-specific correction factor applied to POC INRs greater than 3.0 can provide better accuracy of INR measurement and can significantly reduce variability in warfarin dosing decisions. More study is needed to assess impact on clinical adverse events.

Precision and accuracy of point-of-care testing coagulometers used for self-testing and self-management of oral anticoagulation therapy

BACKGROUND

Oral anticoagulation therapy is monitored by the use of the International Normalized Ratio (INR). Patients who perform self-testing or self-management use a point-of-care testing (POCT) coagulometer (INR monitor) to estimate their INRs. A precondition for a correct dosage of coumarins is a correct INR estimation, and the method and apparatus used for providing the INR measurements are crucial in this context. Several studies have been published regarding the precision and accuracy of these POCT coagulometers, and have led to diverse conclusions. It is difficult and challenging to perform an overview of the literature, owing to the vast amount of papers, with differences in design, statistical analysis, etc.

OBJECTIVES

The aim of this systematic review was to analyze the current literature, especially regarding the precision and accuracy of the POCT coagulometers, to provide recommendations for clinical use and quality control, and to point out areas for future research.

METHODS

We included a total of 22 studies, of which four were characterized as high-quality studies.

RESULTS

The precision of the POCT coagulometers was generally adequate for clinical use. Their performance in terms of accuracy has to be viewed in the context of the inherent inaccuracies of INR measurements.

CONCLUSIONS

The accuracy of POCT coagulometers seems, in this respect, to be generally acceptable, and they can be used in a clinical setting.

Accuracy of a portable international normalized ratio monitor for patients receiving a low molecular weight heparin as a bridge pending full oral anticoagulant efficacy

BACKGROUND

Point of care (POC) devices measuring the international normalized ratio (INR) are accurate for patients with stable disease, but their efficiency has not been prospectively assessed during the "bridging period" when patients are receiving a low molecular weight heparin (LMWH) on top of a vitamin K antagonist (VKA) until the target INR is reached.

METHODS

188 dual INR measurement using the POC (INR(POC)) and the laboratory (INR(lab)) at the same time were consecutively determined : 69 in patients receiving LMWH+VKA (bridging group) and 119 in patients receiving only a VKA (control group). INRpoc was compared to INR(lab).

RESULTS

Test strip failure rate was higher in the bridging group than in the control group (29% vs 4%; p<0,001). In successful tests, POC accuracy was not modified by LMWH administration: the correlation coefficients between POC and lab INR values for the bridging group and the control group were 0,81 and 0,87 respectively, and the relative measure of divergence (RMD=INR(lab) - INR(poc)/INR(lab)) was lower in the bridging group than in the control group (4+/-7% vs 10+/-14%; p=0,02). Finally, clinically relevant agreement between POC and laboratory was of 90% in the bridging group and 92.1% in the control group (p=0.6).

CONCLUSION

With the POC used (INRatio), in patients receiving LMWH when the POC gives a result, it is as accurate as in patients not receiving a LMWH.

Anticoagulation: Low-Molecular-Weight Heparins in Renal Impairment and Obesity: Available Evidence and Clinical Practice Recommendations Across Medical and Surgical Settings

Objective

To develop practical recommendations for the use of low-molecular-weight heparins (LMWHs) as prophylaxis and treatment of venous thromboembolism and acute coronary syndromes in patients with impaired renal function or obesity.

Data Sources

Multiple MEDLINE searches were performed (November 2008) to identify studies for inclusion, using a comprehensive list of search terms including, but not limited to, LMWH, enoxaparin, dalteparin, tinzaparin, obesity, weight, renal, kidney, elderly, monitoring, and anti-Xa.

Study Selection And Data Extraction

Only articles published in English that were relevant for this review were included.

Data Synthesis

In the majority of patients, standardized prophylaxis or treatment doses of LMWHs can be used without the need for monitoring and adjusting regimens. For patients with severe renal impairment (estimated creatinine clearance [CrCl] <30 mL/min), doses of some LMWHs should be adjusted or unfractionated heparin should be used instead. CrCl should be estimated using the Cockcroft-Gault method. Differences are noted in the degree of accumulation of various LMWHs in patients with moderate-to-severe renal impairment, and thus, the degree of dose adjustment may differ among the various LMWHs. Increasing the prophylactic doses of LMWH may be appropriate in morbidly obese patients (body mass index ≥40 kg/m2). The use of total body weight is appropriate for therapeutic doses of LMWH in obese patients. Laboratory monitoring of the anticoagulation effect of LMWHs is generally not necessary, but should be considered in patients with morbid obesity (weight >190 kg), those with severe renal impairment, and those with moderate renal impairment with prolonged (>10 days) LMWH use. When anti-Xa activity is monitored, it should be determined using a chromogenic method and a calibration curve based on the LMWH used.

Conclusions

Additional data are needed for specific dose guiding in obese and renally impaired patients, who are often excluded from larger clinical trials. Practice recommendations are made based on available evidence and authors' clinical opinions.

Clinical factors influencing normalization of prothrombin time after stopping warfarin: a retrospective cohort study

Background

Anticoagulation with warfarin should be stopped 4–6 days before invasive procedures to avoid bleeding complications. Despite this routine, some patients still have high International Normalized Ratio (INR) values on the day of surgery and the procedure may be cancelled. We sought to identify easily available clinical characteristics that may influence the rate of normalization of prothrombin time when warfarin is stopped before surgery or invasive procedures.

Methods

Clinical data were collected retrospectively from consecutive cases from two cohorts, who stopped warfarin 6 days before surgery. An INR value of 1.6 or higher on the day of surgery or requirement for reversal with vitamin K the day before surgery were criteria for slow return (S) to normal INR.

Results

Of 202 patients, 14 (7%) were classified as S. Eight of the S-patients required reversal with vitamin K one day before surgery and in another case surgery was cancelled due to high INR. Baseline INR was the only variable significantly associated with classification as S in stepwise logistic regression analysis (p = 0.003). The odds ratio for being in the normal group was 0.27 (95% confidence interval 0.12–0.62) for each unit baseline INR increased. The positive predictive value of baseline INR with a cut off at > 3.0 was only 15% and for INR > 3.5 it was 33%.

Conclusion

Baseline INR, but not the size of the maintenance dose, is associated with the rate of normalization of prothrombin time after stopping warfarin, but it has limited utility as predictor in clinical practice. Whenever normal hemostasis is considered crucial for the safety, the INR should be checked again before the invasive procedure.