Novel biosensors, based on immobilized creatininase, creatinase and urease are developed, using ISFETs with weak inversion at pH 6-8 and 37.0C. The ISFETs with circuitry, demonstrate a linear relationship of urea and creatinine at the range of 0-200 mM and 0-20 mM, respectively. Preliminary results show that biosensors operating in weak inversion mode can eliminate many of the disadvantages of ISFETs operating in the strong inversion region, providing a wide dynamic range output in nanoAmp. Such characteristics fit the analytical requirements for improving real-time monitoring in Peritoneal Dialysis (PD). Further work covers stability of ISFET sensors biased with CMOS circuits in the weak inversion mode working in the room temperature of 15°C to 40°C.