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In Reply: Dr Suissa raises a question about immortal time bias in our intent-to-treat analysis. This bias is a result of incorrectly classifying the person-time before the first exposure to a medication as being "on therapy."¹ However, our analysis was not performed in this manner. Follow-up started when each participant met the combination of criteria for heart failure and Adult Treatment Panel III criteria to receive cholesterol-lowering therapy, and this defined the participant's index date. After the index date, we structured our data in 2 ways for different analyses: intent-to-treat and time-varying exposure to statin therapy. In both data structures, person-time before the first-identified exposure to statins during follow-up was correctly classified as being "off therapy."

The primary difference between the intent-to-treat and time-varying classification to statin therapy is that in our intent-to-treat approach, the participants were considered "on therapy" once they were exposed during follow-up; in the latter, participants . . . [Full Text of this Article]

Alan S. Go, MD
alan.s.go@kp.org

Jingrong Yang, MA
Kaiser Permanente of Northern California
Oakland

Jerry H. Gurwitz, MD
Meyers Primary Care Institute
Worcester, Mass

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