Two investigations account for most of the diagnoses that turn out to explain adult eczema that will not respond, and neither requires a specialist referral or a laboratory that most clinics lack.
The first is a skin scraping with potassium hydroxide. Chronic dermatophyte infection, particularly when it has been treated with a topical steroid or a steroid-antifungal combination, loses its annular edge and central clearing and presents as a poorly demarcated, itchy, scaly eruption indistinguishable from eczema on inspection. Steroid-modified tinea is common wherever these combination creams are sold over the counter, which in India is everywhere. Scrape the advancing edge, not the centre.
The second is a product history that goes beyond 'do you use anything on it'. Ask about every topical applied to the affected area in the past three months, including the emollients you prescribed, hair dye, cosmetics, footwear, occupational contacts and traditional or herbal preparations. Allergic contact dermatitis superimposed on atopic dermatitis is common, and the sensitiser is frequently something the patient does not consider a treatment or does not think worth mentioning. Where the history raises a suspicion, patch testing is the definitive step.
The reason to do both before escalating is asymmetry of consequence. Missing a diagnosis costs the patient months of an ineffective and expensive drug. Doing a scraping costs five minutes.
- Scrape the advancing edge for potassium hydroxide microscopy before starting any systemic agent.
- Suspect steroid-modified tinea in any itchy scaly eruption that lost its typical morphology after topical treatment.
- Take a full three-month product history including emollients, cosmetics, footwear and herbal preparations.
- Refer for patch testing where the history suggests contact sensitisation, including to prescribed emollients.
- Re-examine the whole skin surface, including web spaces and genitalia, for scabies.
The statistics, in plain English
Potassium hydroxide microscopy has moderate sensitivity — it misses a meaningful proportion of true fungal infections, particularly after partial treatment — so a negative scraping reduces the probability of tinea without excluding it. That asymmetry is why a negative result should not end the question in a patient whose history strongly suggests steroid-modified infection. Its specificity is good, so a positive result is informative and should change management immediately.
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