The diving fitness examination is a good opportunity to ensure, before a planned diving holiday, that there are no relevant medical problems that could lead to unnecessary dangers or problems when diving.
The diving medical examination, or “GTÜM examination”, serves this purpose.

The examination interval depends on the age.

up to 15 years: annually
16-39 years: every 3 years
from 40 years: annually

This recommendation is the maximum time between examinations. The diving doctor decides on the basis of the constitution of the person to be examined whether, if necessary, an examination is also indicated before the end of this maximum period.
The diving medical examination includes an anamnesis and physical/neurological examination as well as an apparative diagnosis consisting of ECG, lung function and laboratory. In addition, the ENT tract is examined very carefully to ensure that there are no problems with pressure equalisation. The examinations take place according to the GTÜM guidelines.

https://www.gtuem.org/tauchmedizin/sh-15.html