practical notes on clincal dermatology3

What laboratory tests are useful for diagnosing tinea capitis?
Fluorescing the affected area with a Wood’s light is the quickest technique. If the hair fluoresces a yellow-green, then a fungal infection is likely. However, the lack of fluorescence does not exclude tinea capitis, because Trichophyton tonsurans accounts for 75-90% of scalp ringworm infections, and it dose not fluoresce by wood’s light.
Examination of KOH-treated infected hair is more sensitive and rapidly performed. The best results are obtained when broken-off hairs are examined since these are the ones infected by hyphae and arthrospores. Infection can be immediately apparent on KOH examination if the fungus grows outside the hair shaft (ectothrix). Some dermatophytes, such as T. tonsurans, grow within the hair shaft (endothrix), and a few minutes are required to let the KOH break down the hair shaft so that the arthrospores can be more easily visualized.
The diagnosis can also be proved by fungal cultures (the commonly used culture media are dermatophyte test media and Sabouraud`s dextrose agar with or without antibiotics and cyclohexamide that added to suppress bacterial contaminants and yeasts). The easily broken, infected hairs are embedded in the culture media which needed more than two weeks to grow.