DermNet NZ

Facts about the skin from DermNet New Zealand Trust. Topic index: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Laboratory tests for fungal infection

To establish or confirm the diagnosis of a fungal infection, skin, hair and nail tissue is collected for microscopy and culture (mycology).

Long wavelength ultraviolet radiation (Wood lamp) can help identify some fungal infections of hair (tinea capitis) because the infected hair fluoresces green.

Specimen collection

Specimens for fungal microscopy and culture may be:

They are transported in a sterile container or a black paper envelope.

Direct microscopy

The material is examined by microscopy by one or more of these methods:

Microscopy can identify a dermatophyte by the presence of:

Fungal elements are sometimes difficult to find, especially if the tissue is very inflamed, so a negative result does not rule out fungal infection.

A yeast infection can be identified by the presence of:

PAS stain of aspergillus seen in a skin biopsy
PAS stain of
aspergillus seen
in a skin biopsy
KOH of M. canis
Potassium hydroxide
(KOH) preparation
of microsporum canis
showing hyphae
KOH of candida
KOH preparation
of candida
showing pseudohyphae


Culture identifies which organism is responsible for the infection:

Growing the fungus in culture may take several weeks, incubated at 25-30ºC. The specimen is inoculated into a medium such as Sabouraud's dextrose agar containing cycloheximide and chloramphenicol. The cycloheximide is left out if a mould requires identification.

A negative culture may arise because:

Culture of yeasts and moulds may be due to harmless colonisation rather than infection. The infection may be secondary to an underlying skin disesase such as psoriasis.

Blood tests

Blood tests are not useful for the diagnosis of superficial fungal infections. But in subcutaneous and systemic infection, several tests may be useful.

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