Goldstone and colleagues conducted an observational study that addressed an important question that is not well suited to randomization. The authors acknowledge limitations, including residual confounding, but the possibility that these limitations explain the survival difference favoring mechanical over biologic prosthetic mitral valves is inadequately explored.

The 30-day mortality among patients 40 to 49 years of age was higher among recipients of a biologic prosthesis than among recipients of a mechanical valve (5.6% vs. 2.2%, P=0.009), which indicates that the patients who received a biologic prosthesis were substantially sicker. The reported survival benefit was driven by higher mortality among the relatively few patients who received biologic prostheses between 1996 and 2001, before mitral repair supplanted replacement and when biologic prostheses were reserved for selected patients who had limited life expectancy or were unlikely to adhere to anticoagulation therapy.¹ The authors only partially adjust for these practice changes by including surgery year in the model, and they could not adjust for the most common indications for biologic prostheses, including frailty, nonadherence to medical therapy, poor access to health care, or intravenous drug abuse. Individualized decisions that are supported by consensus guidelines recommending mechanical or biologic prosthetic valves at any age according to the patient’s preference and adherence to anticoagulation therapy are the appropriate standard of care.