A long-sought milestone in the defense against bioterrorism is the development of rapid, simple, and near-patient assays for diagnostic and theranostic purposes. Here, we present a powerful test based on a host response to a biological weapon agent, namely the ricin toxin. A signature for exposure to ricin was extracted and characterized in mice and then integrated into a plastic microfluidic cartridge. This enabled early diagnosis of exposure to ricin in mice using a drop of whole blood in less than 1 h and 30 min. The cartridge stores the reagents and implements all of the steps of the analysis, including mRNA extraction from a drop of blood, followed by tens of parallel RT-qPCR reactions. The simple and low-cost microfluidic cartridge developed here may find other applications in point-of-care diagnostics.