Methodologic Considerations in Calculating and Analyzing Proportion of Time Covered as a Measure of Longitudinal...

Proportion of time covered (PTC, or "covered time") is a longitudinal measure of adherence to preventive health services, the use of which has increased in recent years. This measure is helpful for evaluating the success of delivering screening interventions over time. However, there are challenges and nuances in computing and interpreting PTC.

In this manuscript, we describe some desired properties of PTC measures, challenges in achieving those, and potential solutions using hypothetical examples.

We propose a modified PTC measure (mPTC) to complement the standard, existing PTC measure. The mPTC measure focuses on screening completion rather than initiation when a screening modality requires more than one step; is affected less by loss to follow-up, death, or cancer during covered time than the standard PTC measure; and is not sensitive to screening episode results. We propose weighting strategies to ensure that the average PTC and mPTC are more heavily influenced by individuals who were observed for longer and are thus more informative. We further describe how PTC and mPTC measures can incorporate test indication to focus specifically on screening.

We recommend that studies of covered time present ample descriptive information, calculate both PTC and mPTC, describe how symptoms and indication are handled, and present multiple complementary measures, such as the proportion never screened and the proportion in need of screening.

Common approaches, terminology, and reporting practices for covered time measures have the potential to improve the study of longitudinal cancer screening adherence.