Endogenous material - C0 is not eq zero

Dear Users! I tried to describe the PK of an endogenous material. I started from the example in UG VIII but after I realize that this method is not so good for me, because the amount measured in the central compartment is not the object of the DADTs. Instead of, the dadt describes a system that adjust the amount in the compartment against the so called base line level, therefore A(1) can be negative!!! Is there any method that treats situation when the C0 is not zero (I also interested to estimate C0 somehow). Thanks, István István Németh PhD Biostatistician Developmental Drug Metabolism & Pharmacokinetics Pharmacological and Drug Safety Research Gedeon Richter Plc Tel: 00 36 1 505 7082 Fax: 00 36 1 889 8526