Update on the Methods of the U.S. Preventive Services Task Force: Linking Intermediate Outcomes and Health Outcomes in Prevention

The U.S. Preventive Services Task Force (USPSTF) is an independent body of experts who make evidence-based recommendations about clinical preventive services using a transparent and objective process. Developing recommendations on a clinical preventive service requires evidence of its effect on health outcomes. Health outcomes are symptoms, functional levels, and conditions that affect a patient's quantity or quality of life and are measured by assessments of physical or psychologic well-being. Intermediate outcomes are pathologic, physiologic, psychologic, social, or behavioral measures related to a preventive service. Given the frequent lack of evidence on health outcomes, the USPSTF uses evidence on intermediate outcomes when appropriate. The ultimate goal is to determine precisely a consistent relationship between the direction and magnitude of change in an intermediate outcome with a predictable resultant direction and magnitude of change in the health outcomes. The USPSTF reviewed its historical use of intermediate outcomes, reviewed methods of other evidence-based guideline-making bodies, consulted with other experts, and reviewed scientific literature. Most important were the established criteria for causation, tenets of evidence-based medicine, and consistency with its current standards. Studies that follow participants over time following early treatment, stratify patients according to treatment response, and adjust for important confounders can provide useful information about the association between intermediate and health outcomes. However, such studies remain susceptible to residual confounding. The USPSTF will exercise great caution when making a recommendation that depends on the evidence linking intermediate and health outcomes because of inherent evidence limitations.