Comparison of a Chromogenic Factor X Assay to International Normalized Ratio for Monitoring Oral Anticoagulation Therapy David L. McGlasson; Major Benjamin.

Comparison of a Chromogenic Factor X Assay to International Normalized

Ratio for Monitoring Oral Anticoagulation Therapy

David L. McGlasson; Major Benjamin G. Romick†, †, Bernard J. Rubal‡

†Wilford Hall US Air Force Medical Center, Lackland Air Force Base, TX

‡ Brooke Army Medical Center, Fort Sam Houston, TX

Background

• The International Normalized Ratio (INR) is the primary method for monitoring patients on oral anticoagulation therapy (OAT).

• Numerous confounding variables limit monitoring of INR in patients on OAT– Lupus Anticoagulant (LA)– Converting direct thrombin inhibitors to OAT

Background

• Chromogenic Assay for Factor X (CFX) has been validated for use in monitoring patients with LA who are on OAT

• CFX is not affected by presence of LA and other variables that affect INR

Introduction

• CFX has not been compared to INR in monitoring a broad population of patients on OAT

• No universally defined range of values of CFX for:– Therapeutic anticoagulation– Supratherapeutic anticoagulation– Subtherapeutic anticoagulation

Introduction

Study (n) INR CFX

Rosborough 2-3 22-44%

Moll 2-3.5 11-42%

Moll, et al. 2-3 20-40%

Sanfellipo 2-3.5 17-43%

Robert, et al. 2-3 28-44%

Introduction

• Objectives of the present study:

1. Define the therapeutic range for CFX in this population

2. Assess the relationship between CFX and INR values in an outpatient “Coumadin Clinic” setting

Methods

• INR and CFX levels were evaluated in randomly selected excess specimens from the coumadin clinic and normal subjects not receiving anticoagulant therapy

• INR values were correlated to CFX for determining the following ranges for OAT patients:– Normal or Subtherapeutic INR < 2.0– Therapeutic INR 2.0-3.0– Supratherapeutic INR > 3.0

Methods

• Instrumentation:– STA-R Evolution automated coagulation

analyzer (Diagnostica-Stago, Inc.®, Parsippany, NJ)

– Chromogenic Factor X assay (Diapharma, Inc.®, Westchester, OH)

– INR performed with PT method using Neoplastine Cl+ with ISI of 1.28 and geometric mean of 13.8 seconds

Data Analysis

• Overall correlation of INR to CFX assessed with inverse square method (Sigmaplot version 9.01; Systat Software, Inc, San Jose, Ca)– Goodness of fit expressed as the coefficient of

determination: R2

• Receiver Operator Characteristic (ROC) curves used to discriminate therapeutic ranges (SPSS version 11.5, SPSS, Inc. Chicago, IL) – ROC area > 0.900 → highly discriminative– ROC area > 0.800 → good discrimination

Data Analysis

• CFX ranges consistent with INR therapeutic ranges defined by plots of sensitivity and specificity versus CFX

• Kolmogorov-Smirnov test employed to assess normality of CFX distributions among therapeutic subsets

• Non-normally distributed data presented as median and 25th and 75th percentiles

Data Analysis

• One-way Analysis of Variance on Ranks (Sigmastat version 3.11; Systat Software, Inc., San Jose, CA) used to assess differences in CFX among INR therapeutic ranges

• Differences between groups assess used Dunn’s post hoc test

• P values < 0.05 are considered significant

Results

• 309 randomly selected OAT patients were tested

• 30 normal subjects not on anticoagulants tested for comparison

• Range of INR & CFX in OAT patients:– INR 0.92 – 12.76– CFX 9 – 132%

Results

INRCFX Range

(Mean)N

< 2.032 - 132%

(53%)70

2.0 – 3.018 – 48%

(28%)135

> 3.09 – 46%

(21%)104

Normals72 – 131%

96%30

Figure 1. Good model fit between INR and CFX when expressed as a second order inverse function (N = 339, r2 = 0.929; P<0.001). Open circles represent samples from normal control group (CFXn), closed circles represent patients receiving Coumadin therapy (CFXc).

Figure 2A. ROC curve using INR ≥ 2.0 as criteria for the threshold of therapeutic anticoagulation. Figure 2B (arrow) is a plot of sensitivity and specificity over the range of CFX values tested (N = 339). The arrow indicates the CFX value ≤ 35.5% that has maximum combined sensitivity and specificity for the INR therapeutic threshold (INR ≥ 2.0).

Figure 3A is a ROC curve for the patients with INR ≥ 2.0 (N = 240) using an INR value >3.0 for discriminating therapeutic form surpratherapeutic ranges of CFX. Figure 3B is a plot of sensitivityand specificity vs CFX values. Arrow indicates that CFX ≤ 23.5% is consistent with an INR >3.0.

Figure 4 depicts box plots (median: solid line, mean: dotted line, whiskers: 10th and 90th percentile) for CFX values categorized by INR therapeutic ranges. Significant differences were noted between all groups. Dashed lines indicate the CFX range (23.5%-35.5%) is equivalent to the INR therapeutic range (INR 2.0-3.0).

Conclusion

• CFX correlates well with INR in a randomly selected group of OAT patients at varying levels of anticoagulation– R = 0.964 by inverse square function

• CFX is highly discriminative for patients in therapeutic and subtherapeutic INR ranges– ROC curve area 0.948, p < 0.0001, n = 339

• CFX has good discrimination for patients in therapeutic and supratherapeutic INR range– ROC curve area 0.864, p < 0.0001, n = 240

Conclusion

• CFX < 35.5% appears to be equivalent to INR > 2.0– Sensitivity 91.7%– Specificity 91.9%

• CFX < 23.5% suggests supratherapeutic anticoagulation equivalent to INR > 3.0– Sensitivity 78.2%– Specificity 84.6%

Conclusion

• Current findings suggest usefulness of CFX for monitoring oral anticoagulation in broad groups of patients seen in a coumadin clinic

Conclusion

• Further study warranted to compare outcomes in OAT patients monitored with CFX and conventional INR methods– Bleeding events– Thromboembolism– Cost– Convenience– Availability

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