This paper explains what a diagnostic test interval (DTI) is, how it is measured, and how it impacts the safety performance of a safety instrumented function (SIF). In applications where a dangerous condition occurs infrequently, the DTI has an insignificant impact on PFDavg. For example if a dangerous condition occurs once a week, then a DTI less than 1.68 hours will be insignificant. This calculation uses the 100X factor described in IEC 61508:2010. In applications with more frequent demands the impact can be determined as described below. However, the 100X factor is not always applicable. More accurate factors can be calculated even when the factor is as low 2X.