Failure of a single day metronidazole desensitization protocol, and success of a modified two-day protocol in an outpatient setting

BACKGROUND: True allergy to metronidazole, a common anti-infective in clinical practice, is rarely reported in the literature. In the case of Trichomonas, there are few alternatives to the nitrimidazole class of drugs, and the alternatives that do exist are associated with worse clinical outcomes. Accordingly, for the rare patients with Type 1 hypersensitivity reactions to metronidazole but compelling need, desensitization protocols have been adapted previously. Reactions during these protocols appear common. Patients in previous regimens have required higher level care for observation, which is costly and resource-intensive. CASE PRESENTATION: We report here on a successful outpatient two-day regimen for metronidazole desensitization. Our patient had compelling indication for metronidazole, but reacted after receiving the very first dose of a previously described desensitization protocol. Accordingly, the protocol was adapted further. Despite this, she went on to develop objective hives prior to reaching the full intended dose. With appropriate symptom management and pre-medication on the second day in clinic, she was successfully desensitized and able to complete a week of full-dose metronidazole. No acute care resources were needed. CONCLUSION: We propose this two-day desensitization regimen for patients who react during the previously described desensitization protocols. This regimen was effective and safe, and did not necessitate the use of acute-care resources. Two-day desensitization protocols while relatively uncommon, can be successful.