Cookie Notice

This site uses cookies. By continuing to browse this site, you are agreeing to our use of cookies. Review our cookies information for more details.



A 61 year-old male with HIV has a history of a papular reaction on his arm that developed approximately 24 hours after the second dose of trimethoprim-sulfamethoxazole (Bactrim). The lesions in 24 hours after discontinuation of the medication. This reaction occurred 15 years ago and since then patient has avoided trimethoprim-sulfamethoxazole.

A review of AAAAI 'Drug Allergy: An Updated Practice Parameter' highlighted the drug challenge protocols that have been developed and successful. The two protocols that were discussed in the practice parameters was the six hour challenge and the ten day challenge but it did not elaborate on when and for what type of patient reactions were each of these challenges used.

We are leaning towards the six hour challenge protocol but were wondering if you could provide some additional insight about factors to consider when deciding between the two protocols.


Thank you for your question. I was not able to find a clear cut rationale for one challenge protocol over the other. We did ask Dr Macy for his thoughts: "A challenge with a single co-trimoxazole single-strength tablet and 5 days of follow-up is adequate to confirm current tolerance and the absence of serious T-cell mediated hypersensitivity. This has been the standard practice in our group for several years. Expect about 10% of challenged individuals to have a recurrent rash, similar to the index event. It is not possible to desensitize individuals with a clinically significant T-cell mediated hypersensitivity, but rechallenges are often uneventful. Multiple dose challenges add nothing diagnostically. "

I hope this is helpful.

Andrew Murphy, MD, FAAAAI


Close-up of pine tree branches in Winter Close-up of pine tree branches in Winter