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Conclusion: Time in Therapeutic Range does not always lead to lower stroke rate
In a group of patients I recently studied, patients with the lowest TTR % (as calculated using the Rosendaal Method in the patient's first year on Warfarin) had the lowest stroke risk.
Also, I found that patients with the most frequent testing had the lowest TTR.
This study consisted of 210 NVAF patients who are managed by our HMO. Fifty-seven of the patients (37.25%) have their INR testing and Warfarin dosing adjustments through our Anticoagulation Clinic while the other 153 patients have this choreographed by their PCP's office.
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In addition to INR testing, the Anticoagulation Clinic ("the Clinic") also offers patient education and patients who are managed through the Clinic attend these classes.
Patients who were managed in the Clinic had more frequent testing (every 8.0 days on average) and 83% of these patients had "Frequent testing" (less than 10 days between tests on average). Patients who were managed through their doctor's office went an average 13.8 days between INR tests and only 30% of those patients had "Frequent testing."
The average time in therapeutic range (TTR) was 63.4% in the first year for the Clinic patients and 53.3% for the non-Clinic patients. These numbers were 56.3% and 47.0% if calculated using the Traditional Method for TTR calculation, respectively.
Clinic patients received on average 37.93 INR tests in the first year on Warfarin and the non-Clinic patients only received 18.14. This corresponds to an average 38 AC Clinic visits per year for the Clinic patients.
Medicare pays $5.40 per Prothrombin Test (to assign a monetary value, though our tests are point-of-care and thus not reimbursed by Medicare) which equates to a total of $20,482.11 per 100 patients per year for Clinic patients and only $9,797.14 for non-Clinic patients. Operating costs for the Clinic were not determined and were outside of the scope of the requested analysis.
Having stated that Clinic patients had (a) more frequent testing, (b) better TTR %, I was initially led to conclude 1 of two things:
However, when I went to study these conclusions, I found that "Frequent testing" alone did not equate to higher TTR. AC Clinic patients with "Frequent testing" had an average TTR of 62.7% and the AC Clinic patients with "Infrequent testing" had an average TTR of 71.2%. In non-clinic patients, those with "Frequent testing" had an average TTR of 44.2% and those with "Infrequent testing" had an average TTR of 57.5%.
Moreover, I found no correlation between higher TTR and a reduction in stroke rates. Patients with "Good" INR control (defined as TTR > 75%) had a 7.9% stroke rate; patients with "Moderate" INR control (defined as TTR between 60-75%) had a 6.8% stroke rate; and patients with "Poor" INR control had a 3.1% stroke rate.
I devised a few possible explanations. Good INR control correlating to higher stroke risk could indicate that the patients with the highest stroke risk are the best-managed with regards to INR control. Patients with the lowest risk of stroke are not as well-managed with regards to INR control. And also that perhaps patients who have previously experienced a stroke (and are thus at higher risk) are the most compliant with regards to INR management.
My final conclusions were (1) AC Clinic patients are not experiencing fewer strokes due to more frequent INR testing and better TTR %. (2) Clinic patients may be experiencing fewer strokes because of the other education and more frequent touch-points they receive through the Clinic. And (3) Time in Therapeutic Range is not a good indicator of risk of stroke.
CHADS-VASc Score is a better stroke risk assessment tool
A better indicator of stroke risk in this population was the CHADS-VASc score.
The risk of stroke in the patients studied follows the risk predicted by the CHADS-VASc Score much more closely than the time in therapeutic range.