Patients with atrial fibrillation have a 5- to 6-fold increased risk of stroke, and numerous studies have attempted to define clinical criteria that may be used to classify patients as low or high risk. Such risk stratification may aid in estimating prognosis and in selecting appropriate candidates for therapies such as warfarin.

Thomas J. Wang, M.D., of the Framingham Heart Study, and his American colleagues derived clinical risk scores with a focus on 2 outcomes: stroke alone and stroke or death. The study included 868 participants of the Framingham Heart Study who had new-onset atrial fibrillation, 705 of whom were not treated with warfarin at baseline.

Risk scores for subsequent stroke (ischemic or hemorrhagic) and stroke or death were developed. During a mean follow-up of 4.0 years free of warfarin use, stroke alone occurred in 83 participants and stroke or death occurred in 382 participants.

"A risk score for stroke was derived that included the following risk predictors: advancing age, female sex, increasing systolic blood pressure, prior stroke or transient ischemic attack, and diabetes. With the risk score, 14.3 percent of the cohort had a predicted 5-year stroke rate of 7.5 percent or less (average annual rate 1.5 percent or less), and 30.6 percent of the cohort had a predicted 5-year stroke rate of 10 percent or less (average annual rate of 2 percent or less). Actual stroke rates in these low-risk groups were 1.1 and 1.5 per 100 person-years, respectively. Previous risk schemes classified 6.4 percent to 17.3 percent of subjects as low-risk, with actual stroke rates of 0.9 to 2.3 per 100 person-years," the authors wrote.

They added, "These risk scores can be used to estimate the absolute risk of an adverse event in individuals diagnosed with atrial fibrillation, which may be helpful in counseling patients and in making treatment decisions. Our data indicate that although atrial fibrillation is associated with a high overall risk of stroke or death, risk factors can be used to easily stratify patients at particularly high or low risk"

The authors noted that it will be important to test the performance of this risk score in other cohorts.

"An understanding of absolute risk is fundamental to many clinical decisions involving patients with atrial fibrillation, such as the decisions to initiate anticoagulant therapy or temporarily stop anticoagulation for surgical procedures. Anticoagulation therapy may not be justified in individuals with low predicted rates of stroke," they concluded.

In an accompanying editorial, Albert L. Waldo, M.D., wrote that determining which patients with atrial fibrillation should receive oral anticoagulation will always be a critical question.

He noted that warfarin may be underused because of the risk of hemorrhage. "Thus, for each patient, clinicians must strike an acceptable balance between their patients' risk of ischemic stroke and the risk of bleeding. In the absence of an absolute or important relative contraindication, the data seem compelling that warfarin therapy should be offered to patients with atrial fibrillation at risk of stroke. The difficulty is to know what threshold of stroke risk is low enough so that the potential risks of warfarin therapy outweigh its potential benefits. The risk scoring system of Wang et al should be most helpful in determining benefit vs. risk."