TTR and Point Prevalence - Making Sense of Anticoagulation Control

We spoke to our Clinical Director, Dr Robert Treharne Jones, about these two main methods of assessing anticoagulation control.

“Point prevalence is a statistical tool which checks the most recent INR blood result against the target INR for each patient in the assessment group. Those results which fall within the +/- 0.5 range are deemed as showing good control, while other results may fall above or below the upper and lower limits for each patient and do not form part of the PP result. Point prevalence results for groups of patients being managed using INRstar typically have a median value of about 80%, according to INRstar PP feedback data meaning that 4 out of every 5 patients had an in-range blood result on the most recent occasion their INR was checked. This compares well with results for patients managed using manual methods which typically fall between 55%-60% according to reports from our training and installations team.
Because PP gives an indication of only the most recent test result, rather than an overall degree of control, it has now been largely superseded by TTR, but both assessment tools are now available in INRstar.
TTR works out the estimated time each patient is spending in-range over a much longer period of time and relies on the supposition that there is a linear relationship between successive INR blood results. For instance, if a patient with a target of 2.5 has one INR of 2.5 followed by another of 3.5 then the TTR for that patient (the iTTR) during that short period would be assumed to be 50%, since half the intervening time is spent in-range between 2.5 and 3.0 and the other half out of range between 3.0 and 3.5.
It is almost impossible to calculate TTR and iTTR quickly and easily without the aid of a computer programme such as INRstar.
TTR results are useful, not only to give an overall assessment of the efficacy of a clinic but also to indicate whether patients may benefit from a switch to one of the new oral anticoagulant drugs (NOACs or ODIs). An iTTR figure of more than 65% generally implies good control, and patients may be left on warfarin so long as both they and their clinician are happy with the current situation. Patients whose iTTR falls below 60% may be considered for a switch to one of the NOACs, but if the iTTR figure falls below 40% then action is required. The evidence suggests that such patients are at greater risk of another thrombotic event than if they were to take no warfarin at all! In these circumstances, if the patient merits continued anticoagulation, then a switch to a NOAC is recommended.”

Dr Robert Treharne Jones, Clinical Director