The invention is directed to droplet actuator devices and assay methods for multiplexed newborn testing for metabolic disorders. The methods include, among other things, droplet-based enzymatic assays and immunoassays for testing for metabolic disorders. The invention includes methods and devices for conducting multiple assays for different metabolic disorders on a single droplet actuator, as well as multiple assays for the same metabolic disorder using samples from different subjects and/or multiple samples from the same subject on a single droplet actuator. In various embodiments, the invention includes methods for conducting enzymatic activity assays and/or immunoassays in a single fresh blood and/or plasma samples and dried blood and/or plasma samples.