Days in therapeutic INR range/Days INR result determined

Therapeutic INR range (TTR) is a way of summarizing international normalized ratio (INR) control over time. This term identifies a patient's TTR when the Rosendaal method is used. The Rosendaal method uses linear interpolation to assign an INR value to each day between successive observed INR values. Gaps of greater than 56 days between INR values are not interpolated. After interpolation, the percentage of time during which the interpolated INR values lie within the target therapeutic range is calculated. The formula for TTR for a patient is: [Number of days that a patient's interpolated INR result is in therapeutic range] / [Number of days for which the patient has an interpolated INR result]. (See http://www.qualitymeasures.ahrq.gov/content.aspx?id=32739)