Although botulism can be diagnosed based on clinical symptoms, distinguishing it from other diseases is often difficult, especially in the absence of other known persons affected by the condition. Common misdiagnoses include Guillain-Barré syndrome and myasthenia gravis, which also manifest flaccid paralysis. Sometimes misdiagnosis may be due to not performing a thorough neurological examination in the emergency room—an examination that should uncover the cranial nerve palsies of botulism. In some cases, the neurological exam may be deferred because of rapidly progressing or advanced respiratory symptoms—in those cases, the need for respiratory support becomes the focus of clinicians.
Once suspected, the most direct and effective way to confirm the diagnosis of botulism in the laboratory is testing for the presence of the botulinum toxin in the serum, stool, or gastric secretions of the patient. The food consumed by the patient can also be tested for the presence of toxins. While an explosive growth in the therapeutic and cosmetic use of botulinum toxins has taken place over the past two decades, diagnosis and treatment of botulism retain elements of the 1930s. Although the CDC and state laboratories have now developed polymerase chain reaction (PCR) testing to supplement the diagnosis of botulism, the only currently validated laboratory diagnostic test is the time-consuming mouse bioassay. The mouse bioassay involves injecting serum into mice and looking for signs of botulism. This test typically takes 48 hours, while the direct culturing of specimens takes 5-7 days.