The symptoms you've described may be due to either the underlying infection or the drug. A serum sickness reaction can produce fever and arthralgia, but there's no test for this type of reaction. The reaction in these cases is unlikely to be IgE-dependent, as it doesn't involve a rash. Skin testing to confirm an IgE-dependent mechanism isn't routinely performed for sulfas, since they usually produce rashes through lymphocyte-dependent mechanisms, which can't be detected in this way. An in vitro test does exist that can determine if a reaction is lymphocyte-dependent, but it's most likely to be positive if both rash and fever are present. The positive predictive value hasn't been established for this test, which isn't routinely available; its use is currently limited to research purposes. If a patient had the reaction you described, I'd recommend an alternative antibiotic rather than proceeding with any tests. PK