Effect of covariances on repeatability of result

#1
I am fairly new to statistics and hoping someone can provide some advice on some analysis I'd like to do. I am looking to analyse the effects of covariates on the repeatability/variability of a continous variable. The continuous variable is a blood test and is repeated on a yearly basis for 5 subsequent years for each subject (total number of subjects approximately 5000).

To try and make my question clearer I'll give an examples; a covariate might be gender of patient, presence of comorbidity, taking a certain medication- does taking this medication influence the variability of the test result for each subject on subsequent years etc.

Thanks in advance
 
#2
This is a copy from a paper. I find it might be the easiest way to handle this type of data. Creating summary measures. Then compare the difference of this summary measure between the subgroups.

A summary measure that captures the important aspects of the data, and calculate this summary measure for each individual. Typical summary measures are:
change from baseline at a pre-determined timepoint;
maximum (peak) or minimum (nadir) value reached;
time to reach the maximum (or minimum) value;
time to reach some other pre-specified value;
average value;
area under the curve