Onychomycosis is common enough that a dystrophic nail gets treated before it gets diagnosed. When several nails are involved, and particularly when the toenails are, that is usually reasonable. When a single nail is affected - especially a single fingernail, and above all a thumb or index finger in an older man - the prior probability shifts, and treating empirically buys months of delay for the diagnoses that matter.
So the habit is to confirm before treating a solitary nail: send a clipping with subungual debris for periodic acid-Schiff staining as well as culture, because culture alone misses a substantial proportion. If the stain is negative, or if the nail has not responded after an adequate course of an agent it should have responded to, the next step is not a second course. It is examination of the nail fold and matrix, dermoscopy, and a low threshold for biopsy.
The differentials worth holding are squamous cell carcinoma, subungual melanoma, and inflammatory nail disease. All three are treated as fungus at some point in their history, and all three do better the earlier they are named.
- Confirm a solitary dystrophic nail before treating: clipping for PAS staining as well as fungal culture.
- Do not prescribe a second antifungal course for a nail that failed an adequate first one - re-examine instead.
- Examine the proximal nail fold and matrix, and use dermoscopy, before concluding.
- Keep squamous cell carcinoma, subungual melanoma and inflammatory nail disease on the list for any single persistent nail.
- Photograph the nail at first presentation; change over months is the most useful single piece of evidence.
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