Instability equation

Hi all,
I am currently doing a research about certain blood protein and am having problems with calculation of its stability.
I know I should use instability equation but it seems I can't find the right formula.
I had 20 blood samples that were measured in duplicates in three time points (t0, t1 and t2). The results were shown in u/L.
I have already calculated averages for all three time points for all blood samples, t1 bias (%), t2 bias(%), t1 bias (u/L) and t2 bias (U/L).
Additionally, I had 2 blood samples that were done in 20 ''repetitions'' in each time point for which I have calculated average, SD and CV (%) - obviously I will use it for calculating precision for my method.
I would like to make instability equation calculation using the least squares adjustment with confidence intervals for the slope (in my graph), as well as stability equation calculation using the point to point estimation with confidence intervals for the mean of patients at every study time.
What is my next step? Is it even possible? Am I missing something important and fairly obvious?
As you can see I am quite new to this whole field but am eager to learn.
Thank you in advance!