Historically, patients with CHADS₂ scores of 0 or 1 have been considered to be at relatively low risk of a future stroke, whereas at a score of ≥2 the patient is at moderate-to-high risk.[6] Of course, this is a continuum of risk, and there has been some blurring of the exact definitions of what constitutes low vs higher risk, with a consequent blurring of exactly which patients should be considered appropriate for anticoagulation.

It is important for physicians and patients to realize that when a risk score such as CHADS₂ calculates a high level of risk, this means that there is an extremely elevated risk of stroke per year. Thus for patients with a CHADS₂ score in the 5–6 range there is a double-digit risk of stroke per year, meaning that anywhere between 12% and 18% of these patients will probably have a stroke (Figure 10).[10]

On the other hand, for patients with a score of 0, that risk of stroke per year is much lower, approximately 2%. But here again it is important for patients and physicians to understand that this does not mean that the risk is 0. In other words, a risk score of 0 does not mean 0% risk of stroke.

In other words, there is really a continuum of risk, and it must be carefully considered in an individual patient exactly when anticoagulation should be started across this spectrum of risk scores.[11]  Even though historically the threshold has been that anticoagulation needs to be started only at a risk score of >1 – that is, ≥2 – but in fact this is really a moving target. It depends on the patient, and now that there are novel anticoagulants other than warfarin (see Figure 15), it also depends on the type of oral anticoagulation being prescribed.