The SDTMIG’s description of time point variables covers two different use cases:
1. A planned set of findings scheduled relative to a reference time point, usually a dose of study treatment. This is the use case for pharmacokinetic samples with results represented in the Pharmacokinetic (PC) domain, but also applies to pharmacodynamic measurements represented in Vital Signs (VS) or other domains. The purpose of such a schedule of findings is to understand how the body responds to the study drug as a function of time. In this use case, the time points are described by the following:
a. A reference time point, with a name in --TPTREF and a date/time in --RFTDTC
b. The planned interval between the time point and the reference time point, in --ELTM
c. The time point, with a number in --TPTNUM and, optionally, a name in --TPT
2. A planned number of repeated measurements. The classic example of this situation is a set of three blood pressure measurements taken 10-15 minutes apart. The purpose of these repeated measurements is to obtain the most accurate estimate of blood pressure, since blood pressure measurements are more variable than many other measurements. In this case, the exact timing of the measurements may not be collected, and the schedule for taking them is not based on a reference time point and may be only loosely defined. Therefore, the times of the repeated measurements are described by:
a. The time point, with a number in --TPTNUM and, optionally, a name in --TPT
In the SDTMIG v3.3, a new findings variable for Repeat Number, --REPNUM, was introduced to address pulmonary function tests. Pulmonary function tests can be performed multiple times in a session, not only to address variability, but also because some tests may need to be disregarded due to issues during the testing. There is a need to distinguish between different instances of the test during a session, but the number of instances can vary, therefore use case #2 doesn't work.
Now that --REPNUM is available, its use is preferable for representing repeated tests as time points that don't have reference time points or planned elapsed times, as in use case #2. --REPNUM values are generally the same as --TPTNUM values, and the variable name (Repeat Number) is a clearer description of what the number represents. Using --REPNUM for repeated measurements also clearly distinguishes between tests that are merely repeated, and those with a planned schedule based on a reference time point.