Clostridium difficile infection (CDI) results in excess of 14,000 deaths and over $1 billion in excess healthcare costs annually. Early and reliable diagnosis is key for both improving treatment outcomes, and instituting precautions to prevent transmission. Antibiotic therapy can actually increase the odds of coming down with a hospital-acquired infection, especially when the cause is a bacterium named Clostridium difficile. Currently there are no serologic assays for C. difficile toxins A and B. Such an assay would be advantageous for defining the epidemiology of CDI and selecting patients in whom to target future vaccines. The ideal serologic assay for CDI would measure circulating IgG and IgA antibodies to C. difficile toxin A and B as well as detect early and specific immunologic responses to these toxins. Agave BioSystems proposed to develop microsphere-based diagnostic assays to meet these objectives and to further develop the microsphere-based assay such that a single blood sample in a single well can detect toxin antibodies and early immunogenic responses to these toxins. These C. difficile microsphere assays will be developed sufficiently enough to differentiate patients that are susceptible for primary infection (asymptomatic), positive for colonization only and those that are likely to have CDI recurrence.