Testing Methods for Your INR

Publication Date: 
Tue, 11/08/2011
By: Alere Staff

Warfarin levels are determined by a blood test that measures prothrombin time and is most often reported as an International Normalized Ratio (INR) value. Testing methodology, or the manner by which blood is collected and analyzed for the INR test, is one of the variables that can lead to INR instability.

Advances in technology have resulted in a variety of methods to draw blood and determine warfarin levels. There are two primary methods of obtaining the blood sample, either venous draw or finger stick. INR blood testing can be performed in the laboratory, at the point of care, or at home.

Venous Draw (Laboratory Testing)

The traditional means of obtaining a blood sample for INR testing is a venous draw, also called a venipuncture, usually from the arm. This procedure requires that a trained healthcare professional collect the blood sample. After blood is drawn, it is typically sent to an outside laboratory for analysis. INR results are reported back to the healthcare provider by telephone, fax, or email. 

Reagents are materials used in INR testing, and they can vary in sensitivity. Less sensitive reagents are often less expensive than, but lack the accuracy of, more sensitive reagents. Variability in reagent sensitivity is a factor that can influence the accuracy of an INR blood test result. Despite the importance of high-quality reagents, patients and physicians are usually unaware of the sensitivity of reagents used in any given laboratory. Some patients may find recurrent blood draws undesirable.

Finger Stick (Point of Care Testing)

A growing number of warfarin patients have blood collected by the finger stick method, followed by analysis at the point of care. A drop of blood is obtained from the finger and placed into a small, portable INR testing device. Test results are available within two minutes or less. Point of care testing is often described as the preferred method of INR testing because it allows the healthcare professional to read and interpret the results while the patient waits which saves time and facilitates effective counseling.

The reagents on the testing strips used in point of care office-based monitors offer the highest sensitivity available and provide accurate test results. Unlike outside laboratories, where reagent sensitivity can vary, point of care testing benefits from consistent, high-quality reagents.

Finger Stick (Home Testing)

Home INR Monitoring, or self-testing, is the newest method of INR monitoring. Blood is also collected by finger stick, and the same high-quality monitors and testing strips used for professional point of care testing are used for home testing. The benefits of patient self-testing include the ability to test more frequently, which is a proven strategy to decrease bleeding and stroke risk.1, 2

It is important to remember that each testing method introduces some variability based on different methodologies used for clot detection (. i.e. optical, impedance) different thromboplastin reagents and different sample types (i.e. capillary whole blood, plasma). The best approach to maintain a stable INR is to test more frequently and to use the same testing system for each INR.  If periodic evaluations of a home device are desired, then it should be understood that there are rules by the International Standards Organization (ISO) that provide guidelines for acceptable variance.  Results may not match, but they should correlate. 

  1. Heneghan C., et al. Self-monitoring of oral anticoagulation: a systematic review and meta-analysis. 2006. The Lancet, 367, 404-11.
  2. Am J Manag Care. 2014;20(3):202-209.