Time in Therapeutic Range Calculator

Calculator Input

Therapeutic minimum INR

Therapeutic maximum INR

Maximum interpolation gap in days

Days before first reading

Days after last reading

Patient or case label

INR readings as date,value pairs

Enter one reading per line using this format: YYYY-MM-DD,INR

Clinical notes

Example Data Table

Date	INR	Interpretation
2026-01-03	1.80	Below target
2026-01-10	2.30	Within target
2026-01-18	2.70	Within target
2026-01-27	3.40	Above target
2026-02-04	2.90	Within target
2026-02-15	2.10	Within target
2026-02-24	1.90	Below target

Formula Used

This calculator uses an interval-based therapeutic range method inspired by Rosendaal interpolation. It assumes INR changes linearly between two consecutive valid readings when the gap stays within the selected maximum interval.

TTR % = (Interpolated days within range / Total evaluable interpolated days) x 100

It also reports time below range and time above range. Long gaps can be excluded because sparse testing can distort control estimates. Optional lead and trail days extend the first and last reading as flat values.

How to Use This Calculator

Enter the therapeutic INR minimum and maximum.
Choose the maximum allowed gap for interpolation.
Paste INR data as date,value pairs, one line each.
Optionally add lead or trail days for boundary coverage.
Submit the form to calculate TTR and interval details.
Review percentages, evaluable days, and excluded gaps.
Use the CSV or PDF buttons to export results.

FAQs

1. What does time in therapeutic range measure?

It estimates how much time anticoagulation values stayed inside the desired INR range. A higher percentage usually suggests steadier control between test dates.

2. Why are long gaps excluded?

Long gaps weaken the assumption that INR changed smoothly between tests. Excluding them prevents sparse data from creating a misleading control estimate.

3. What is a typical target INR range?

Many warfarin patients use 2.0 to 3.0, but some valves or conditions may need different targets. Always follow the treating clinician’s documented range.

4. Does this replace medical advice?

No. This tool summarizes data mathematically. Dose changes, bleeding risk, clotting risk, and treatment decisions still require clinical judgment.

5. Can I use dates in any order?

Yes. The calculator sorts valid dates automatically before interpolation. Duplicate dates keep the last entered value for that day.

6. What do lead and trail days do?

They extend the first and last INR reading as constant values outside the observed dates. This can help when you need a fixed reporting window.

7. How should I interpret a low TTR?

Low TTR may suggest unstable anticoagulation, missed testing, adherence issues, interacting medicines, diet changes, or an unsuitable dose plan.