Correlation of point-of-care International Normalized Ratio to laboratory International Normalized Ratio in hemodialysis patients taking warfarin

Abstract

Background and objectives: To determine whether point-of-care (POC) International Normalized Ratio (INR) test results correlate with plasma INR measures in intermittent hemodialysis (IHD) patients on warfarin. Anemia is thought to reduce the accuracy of POC INR assay results. Whether POC INR testing could be implemented for hemodialysis patients on chronic warfarin, who are often anemic despite hematopoietic therapy, has not been established.

Design, setting, participants, & measurements: Thirty-seven chronic hemodialysis patients on warfarin contributed sets of three consecutive blood samples for INR comparison immediately before hemodialysis: one finger stick, two from hemodialysis access (arteriovenous graft, fistula, or catheter). POC INR testing was performed using CoaguChek S device. Anemia was defined as hematocrit < 32%.

Results: Pairwise comparison and correlation of 258 INR results showed high correlation for POC versus laboratory INR (r = 0.94; P < 0.001). Of these, 16 (6%) differed by >0.6 INR units, four (1.6%) differed by >0.8 INR units, and one differed by >1.0 INR units. Resulting pairwise correlation analyses between samples were: for anemic patients (0.96; P < 0.001), nonanemic patients (0.93; P < 0.001), and for those obtained from arteriovenous grafts (0.94; P < 0.001). POC INR samples from dialysis catheters correlated poorly with laboratory INR results.

Conclusions: POC INR correlates well with plasma INR measures in IHD patients requiring chronic warfarin, and anemia did not influence this reliability. Blood sampling from finger stick or arteriovenous graft or fistula showed excellent correlation with laboratory INR, whereas sampling from dialysis catheters was unsatisfactory, likely from heparin contamination.

Trial registration: ClinicalTrials.gov NCT00660946.

Figures

Figure 1.

Point-of-care (POC) and plasma International Normalized Ratio (INR) comparisons. (A) Scatterplot of standard plasma laboratory INR (Lab INR) versus POC INR with line of equality for the entire cohort of 258 samples. (B) Bland–Altman plot of the difference of Lab INR − POC INR versus the mean of the two measurements. Horizontal lines represent the mean difference = 0.08 and 95% limits of agreement −0.54 to 0.70. Correlation between Lab INR and POC INR: r = 0.937, P < 0.0001.

Figure 2.

Effect of anemia on POC INR measurements. (A) Side-by-side box plots showing the distribution of differences between Lab and POC INR measurements for anemic and nonanemic patients (anemic patients defined as hematocrit n = 84; nonanemic defined as hematocrit ≥ 32%, n = 174). (B) Plot of difference of plasma INR − POC INR measured over range of measured hematocrit in all 258 pairs (r = −0.219, P = 0.004).

Figure 3.

Sampling source and POC INR accuracy. (A) Scatterplot of Lab INR versus POC INR samples taken from dialysis arteriovenous (AV) fistula or graft with the line of equality (n = 96). Lab INR graft-obtained POC INR analysis: Mean 0.0573, SD 0.253, 95% confidence interval 0.006 to 0.109. (B) Scatterplot of Lab INR versus POC INR. Both samples taken from dialysis catheter (usually an Ash-split catheter) with the line of equality (n = 139). Catheter-obtained POC INR results showed poor agreement with Lab INR results.