if you assume exponential  clearance C(t) / C(0) = exp (-Kt) then you could numerically fit an exponential curve directly using all the timepoint data and directly obtain K.. Any math/stats package could do it.

HI James,

Thanks for your reply. I had the drug administered in ip route hence had Tmax at 45 min (bell shaped plasma concentration). In this case,

1. Is it okay to assume exponential clearance in which only the the last three points are used to plot the exponential curve to obtain the Kel? 

2. Should I extrapolate exponential curve before converting it to linear regression? Or it does not matter if I extrapolate in exponential curve or linear regressive line?

Regards, 

Sophie

1. yes

2.extrapolate directly from fitted exponential

Thank you so much for the comments Prof James.