In this paper, an easy-to-use and fast biosensor for phenylalanine quantification in patients affected by phenylketonuria is investigated. The phenylalanine concentration was indirectly estimated through the ammonia released as a by-product of an enzymatic reaction, which was then detected by exploiting an yttria-stabilized zirconia layer deposited over an interdigitated capacitive sensor. The latter was manufactured by rapid-prototyping technologies. A sensor limit of detection higher than 1.25 µM was estimated, along with an accuracy better than 18.31 µM.