If her specimen is processed using a less-sensitive reagent, her PT would be lower. Leveling the playing field Designed to standardize values, the INR provides a more accurate assessment of a patient's anticoagulation intensity level regardless of the reagent used. The mean normal PT is determined by measuring the PT on at least 20 fresh plasma samples obtained from healthy subjects. To establish the ISI, WHO designated a batch of human brain thromboplastin as the first international reference preparation for all thromboplastin reagents. Since 1977, each new batch of thromboplastin reagent is compared with this reference preparation, then assigned a sensitivity index number.
Higher values indicate decreasing reagent sensitivity.
Most thromboplastin reagents used in the United States range from 1.4 to 2.8 on the ISI scale. Using the ISI to calculate the INR allows accurate comparisons of PT and PTR values between labs and even between batches of reagent produced by the same manufacturer. Mrs. Jones leaves on her extended trip on November 1. The physician tells her to have a PT in 4 weeks and to call the office on the following day to receive dose instructions and the date for her next PT test. On November 29, Mrs. Jones reports to a hospital in Florida to have a PT specimen drawn and calls her physician the next day.