Creatinine enzymatic field effect transistors (C-EnFETs) realised from a pH-sensitive field effect transistor (pH-ISFET) have been modelled by taking into account the enzymatic reaction, the diffusion phenomena of the main chemical species in the electrolyte, the related acid/basic chemical reactions in watery solution and the detection properties of the pH-ISFET sensitive gate. Thus, the concentrations of the main chemical species have been characterized near the sensitive gate, and the C-EnFET micro sensor detection properties have been studied according to the most influential parameters. A good fit has been shown between modelling and experimental results. The model has been developed for the optimization of creatinine-EnFETs in the field of haemodialysis, but is also fully compatible for other EnFETs based on pH-ISFET-metry.