Re: Covariate Models Using Weight

Date: Fri, 19 Nov 1999 10:09:16 -0600 From: James Gallo <JM_Gallo@fccc.edu> Subject: Re: Covariate Models Using Weight I would be hesistant to use CLCR [creatinine clearance] as a covariate for a drug that undergoes appreciable renal excretion, particularly if CRCL is estimated from a standard formula. These calculated formulas are derived [depending on the particular formula, Cockcroft-Gault, etc...] from, if my memory is correct, a relatively small populations of subjects. Moreover, its been shown that other measures [such as 51Cr-EDTA clearance] are better predictors of renal function or glomerular filtration rate than the 'calculated' formulas that suggest CRCL is of lower mechanistic value. These latter methods [51Cr-EDTA, etc..] of estimating renal function are not always readily available, however, I believe [I think someone has published on this as well] estimation of renal function/GFR by serum creatinine plus other covariates is a better predictor than the 'calculated' formulas. You also raise the issue of correlation [collinearity] amongst covariates. There are a variety of objective criteria that can indicate such a problem that you are probably much more familiar with than myself. Not to deny you artisitc freedom as a modeler, but I would like to think objective criteria can direct your selction of covariates in most cases. At the same time, if I interpret your message correctly, I agree that selection of covariates is problematic even given objective criteria. jim gallo