How to Calculate Time in Therapeutic Range (TTR)
Time in Therapeutic Range (TTR) is a control-quality metric used in anticoagulation programs and other therapeutic monitoring contexts where maintaining values within a target interval is essential. At an operational level, TTR estimates the proportion of monitored time that remains inside the prescribed range. Because treatment risk can rise when values drift high or low, TTR is often tracked as a quality KPI for clinics, care pathways, and payer programs.
This article explains a day-count TTR workflow suitable for quality dashboards, program benchmarking, and audit documentation. For complementary analytics, compare with medication possession ratio, use the MPR calculator for refill coverage, and review clinical enrollment timing when designing monitoring resources.
What TTR measures and what it does not
TTR is the proportion of observed days during which a patient or population remained in the therapeutic interval. It is expressed as a percent and interpreted as control quality over time. Higher TTR usually indicates more stable management, while lower TTR signals greater volatility and potential risk exposure.
However, TTR does not explain the clinical reason for excursions, and it does not replace event-based outcomes such as bleeding or thrombosis rates. Use it as one layer in a broader governance framework.
Variables, symbols, and units
- Din: days inside the therapeutic interval (days).
- Dtotal: total monitored days in the period (days).
- TTR: time in therapeutic range (percent).
- TTRtarget: benchmark value for quality evaluation (percent).
Maintain day-based units across the dataset and ensure Din never exceeds Dtotal. If monitoring coverage is interrupted, document whether gaps are excluded or included in Dtotal.
Formulas and practical calculation steps
TTR = (Din / Dtotal) × 100
Target gap = TTR − TTRtarget
1. Define the therapeutic interval
Use a clinically approved range for the treatment context and keep the same range across the analysis period unless the care plan formally changed.
2. Count monitored days in range
Classify each monitored day as in-range or out-of-range based on available measurements and your interpolation policy. Sum in-range days as Din.
3. Determine total monitored days
Count all eligible monitoring days in the observation window as Dtotal. Exclude administrative duplicates and document treatment interruptions.
4. Compute TTR and compare with target
Divide Din by Dtotal, multiply by 100, and round to two decimals. Compare with a target benchmark such as 65.00% to quantify control gap.
Validation checks and methodological limits
Validate completeness first: missing observation days can overstate TTR if only stable intervals are recorded. Reconcile clinic logs, laboratory feeds, and data warehouse counts before publishing KPI values.
This implementation is a day-count method, not Rosendaal interpolation. If your protocol or publication standard requires interpolation, use a dedicated method and report it explicitly. Also note that TTR is sensitive to target range selection; changing the range can move the KPI even if patient behavior is unchanged.
Worked examples
Example A: Din = 195 days and Dtotal = 270 days. TTR = 72.22%. Against a 65.00% target, the gap is +7.22 percentage points, indicating acceptable control in many quality programs.
Example B: Din = 84 days and Dtotal = 180 days. TTR = 46.67%. This is below both a 65.00% target and a 50.00% alert threshold, so it warrants process review, follow-up cadence checks, and dose-management protocol assessment.
Embed: Time in therapeutic range calculator
Use the calculator below to compute TTR, target gap, and threshold flagging with consistent percentage formatting.