How sparse is too sparse?

From: "Robert L. James" rjames@rhoworld.com Subject: [NMusers] How sparse is too sparse? Date: Thu, June 10, 2004 2:26 pm Dear group, I am a NONMEM user with some modeling experience (perhaps 8 PK publications mostly 2-cpt models with time varying covariates). A Pharm company has just approached me to do a simple PK analysis (Cmax, Tmax, half-life, and AUC) on some VERY sparse data. Although there are 150 patients, each patient has only one plasma conc sample following an oral dose on two separate occasions spaced 2 weeks apart (the expected half life of the drug is a few hours). The timing of the single plasma sample following the oral dose was appears to be randomly timed. I don't have the data and so I don't know how well spaced these single samples are. Is this hopeless? The data has already been 80% collected. There are no previous human PK studies. The only plan of action I can think of is to pool the data across patients and fit a simple one-compartment model with either zero- or first-order absorption following an absorption lag. I could also model interoccasion variabilility into the model. Has anyone tried to fit models to data this sparse (1 sample following each of two dosing occasions). Would I be just spinning my electrons to accept this assignment? ---Robert Robert L. James, M.S., M.Stat. Senior Biostatistician Rho, Inc. 100 Eastowne Drive Chapel Hill, NC 27514 (919) 408-8000 x 468 (919) 408-0999 (fax) rjames@rhoworld.com