What laboratory tests are useful for diagnosing tinea capitis?

Testing for fluorescence in the affected area using a Wood’s light is the quickest technique. If the hair fluoresces yellow-green, then a fungal infection is likely. However, lack of fluorescence does not exclude tinea capitis, because Trichophyton tonsurans accounts for 80% to 95% of scalp ringworm infections in the United States, and it does not fluoresce. Therefore, examination of KOH-treated infected hair is more sensitive and can also be rapidly performed. The best results are obtained when broken-off hairs are examined, because these are the ones infected by hyphae  and arthrospores. Most dermatophytes, such as T. tonsurans, grow within the hair shaft (endothrix), and a few minutes are required to let KOH break down the hair shaft and visualize the infection. Finally, the diagnosis can also be proved by fungal cultures. The easily broken, infected hairs are embedded in the media. The specimen can be obtained using a no. 15 blade, curette, or hemostat.

Lucky AW: Epidemiology, diagnosis, and management of tinea capitis in the 1980s, Pediatr Dermatol 2:226–228, 1985.