Correlation of two dependent variables

Hi all!

I have a small data set which contains two tissue samples from ~20 patients. Each patient has contributed a tissue sample from left and right hip joint. My aim is to study certain cell amounts in these samples. My aim is to investigate whether findings of the hips in each patients correlates, ie. that both hips have similar cell counts (=there is only a minimal within-subject variation).

This can be simply done creating two variables, Var1 and Var2. Var1 is the cell count of right hip and Var2 left hip. If there is significant correlation, it is shown that cell counts in same patient does not vary in random.

However I have two covariates. First one, Cov1, is a blood marker, ie. each patient contributes one value, ie. there is no within-subject variation. The other variable (Cov2) is hip related variable, (time interval between primary surgery operation and point where the tissue sample was retrived), ie. there is within-subject variation.

How can i investigate the correlation between Var1 and Var2 and adjust with Cov 1 and/or Cov2?