Monitoring metabolism fluctuations inside a cell culture is a valuable method for assessment of the cells vitality. Enzyme-based biosensors can provide selective measurement of metabolites such as glucose, lactate, glutamate and choline. However, integration of these biosensors inside a cell culture is a challenging issue that can disrupt the properties of the cells microenvironment or influence the biosensors' enzyme functioning. Herein, a technique for measuring the abovementioned metabolites in a cell culture without affecting the enzymes or the cells is presented. In this study, SU-8 is investigated as a suitable substrate for a simple enzyme immobilization. Two SU-8 microreactors are designed inside a microfluidic cartridge and functionalized with different enzymes. The implemented microreactors are used for detection of two metabolites simultaneously in a few microliters of a sample extracted from the cell-culture medium. Sub-micromolar concentrations are detectable using this device. The results of measuring variations in glucose and lactate concentration inside a cell culture, before and after exposing the cells to three different toxicants, are presented. In order to eliminate the enzymes disruption by the toxicants present inside the medium, a protocol for a toxicant-free sampling is investigated.