In difficult cases, what is your approach to find a causative agent of fixed drug or fixed food eruption?

I don't think there is any answer other than advising the patient of the natural course/behavior of the eruption and having them keep a strict journal of all intake until the culprit is determined. Data on testing has been mixed at best (Jayasundera & Watts, PMID 38336399) -- though there may be a r...