The objective of this study was to develop rapid, inexpensive, and easily applied in vivo phenotyping strategies for characterizing drug-metabolizing phenotypes with reference to the cytochrome P450 (CYP) enzymes in biological fluids. Therefore, the accurate detection of low concentration of theophylline, which can be used as a probe for cytochrome P450 (CYP450) enzymes (e.g. CYP1A2) activity, could benefit drug-metabolizing studies. In this study, a portable, specific, and sensitive functionalized surface plasmon resonance (SPR) sensor using polyacrylamide molecularly imprinted polymers (MIPs) as the highly specific selector is developed for the detection of low concentration theophylline in the presence of other confounding components, such as, caffeine which has a very similar chemical structure.