Time in therapeutic range: what TTR means and how to calculate it

What TTR actually measures

Imagine two people, both targeting an INR of 2.0 to 3.0, both averaging exactly 2.5 across a year of testing. The first sits between 2.3 and 2.7 all year. The second swings from 1.5 to 3.6 and back. Their averages are identical. Their risk is not remotely the same — the second person spent weeks under-protected and weeks over-anticoagulated, and the average hid all of it.

Time in therapeutic range fixes that blind spot. It asks a different question: on what percentage of days was this person's INR inside the target range? Days, not tests. That distinction is the whole point, because most of your days are days you were not tested.

What counts as a good TTR

For context, mean TTR in published warfarin studies commonly lands in the low-to-mid sixties. If your figure is above 70% you are doing better than the average trial participant. Below 60% is not a personal failing — it is usually a sign that something identifiable is interfering, and it is exactly the sort of thing worth raising at your next appointment.

The Rosendaal method, explained

The standard approach is Rosendaal linear interpolation. The reasoning is straightforward. Warfarin changes your INR gradually rather than in jumps, so if you were 2.0 on the 1st and 3.0 on the 11th, the most reasonable assumption is that you passed through roughly 2.1, 2.2, 2.3 and so on across those ten days.

The method therefore fills in a value for every single day between two tests, checks each of those days against your target range, and divides the in-range days by the total. The result is your TTR.

Two details matter. Interpolation only happens between two real readings — the method never extends past your last test into the unknown. And the earliest weeks of warfarin, while a dose is still being found, are often excluded from formal assessments because that period is expected to be unstable and says little about long-term control.

A worked example

Target range 2.0 – 3.0. Three readings:

• 1 March — INR 2.0
• 11 March — INR 3.0 (10 days later)
• 21 March — INR 4.0 (10 days later)

Across the first interval, the interpolated values run 2.0, 2.1, 2.2 … 2.9 — all ten days sit inside 2.0 to 3.0. Ten in-range days.

Across the second, they run 3.0, 3.1, 3.2 … 3.9. Only the first day is still at the 3.0 boundary; the remaining nine are above range. One in-range day.

That gives 11 in-range days out of 20, so TTR = 55% — with 45% of the time spent above range. Note that the average of the three readings is 3.0, which would have looked merely borderline. TTR tells the more honest story.

Why long gaps get excluded

Interpolation is a reasonable guess over a few weeks and a fantasy over several months. If you were tested in January and again in September, drawing a straight line between the two and calling the intervening 240 days in range would be inventing data.

Good implementations therefore apply a gap cap and drop any interval longer than it. Coagly and the calculator on this site use 56 days, which comfortably covers normal monthly and six-weekly testing while excluding stretches where nothing is genuinely known. If a long gap is dropped, you are told, rather than being quietly handed a flattering number.

Calculate your TTR free

Our free TTR calculator runs the Rosendaal method in your browser. Enter your target range and your past INR results with their dates, and you get your time in range, the split above and below, your variability, and a chart of your readings against your target band. Nothing is uploaded — the calculation happens on your own device, and your readings stay in your browser.

How to improve your TTR

The levers that actually move TTR are unglamorous:

• Take every dose, on time. A single missed dose can pull your INR down for the better part of a week. Adherence is the largest controllable factor by a distance.
• Test on schedule. Longer gaps mean drift goes unnoticed for longer. Monthly testing keeps roughly half to sixty per cent of patients in range; weekly testing pushes that markedly higher.
• Keep vitamin K steady rather than low. Consistency beats avoidance. See the vitamin K guide.
• Flag every new medication. Antibiotics in particular are a frequent cause of a sudden spike — more here.
• Write down what changed. A note attached to an odd reading is what lets you find the pattern three months later.

Coagly keeps your TTR updated automatically as you log results, so you can see the effect of a change rather than waiting to be told at a review months later. It also reminds you of your dose and your next blood draw, which addresses the two biggest causes of a poor score directly.

Common questions