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The
Woods lamp for fungal infections
Once upon a time and not so long ago the Wood's lamp was an important
part of any dermatology clinic. The Wood's lamp produces invisible
long-wave ultraviolet light (340-450 nm wavelength). This UV light
can help to detect medications that are taken systemically (tetracycline)
or that are applied to the skin as well as help diagnose skin disease
inducing infectious agents that have a characteristic fluorescence.
The test is simple, the UV light from the Wood's lamp is directed
over the area of the suspected infection and the dermatologist looks
to see if there is any fluorescence/visible light reflected back.
However, in recent times the Wood's lamp has become less helpful
as a diagnostic tool for fungal infections. Up until the 1960s the
typical fungal species encountered by a dermatologist readily fluoresced
under UV light. But today in the USA, and to a somewhat lesser extent
in Europe, the type of fungal species encountered in the dermatology
clinic have changed. The species that were common prior to 1970
and were fluorescent under UV light are now in the minority today.
The common fungal species that cause skin disease in the Western
hemisphere of the twenty first century are predominantly non-fluroescent.
Today the Wood's lamp is used for diagnosing a brown, scaly rash
in the scrotum or axilla: erythrasma, caused by the bacterium Corynebacterium
minutissimum, fluoresces a brilliant coral red, whereas tinea cruris
or cutaneous candidal infections do not fluoresce. UV light can
be used for diagnosing tinea (pityriasis) versicolor, which fluoresces
pale yellow to white. A minor percentage of tinea capitis cases
in North America and Europe caused by two zoophilic Microsporum
species that fluoresce blue-green can be identified with a Wood's
lamp. The Wood's lamp can also help diagnose Pseudomonas infections,
porphyrians, and pigmentary alterations.
The
potassium hydroxide test
The potassium hydroxide (KOH) test is used primarily as a method
to determine if there is a fungal infection of the skin, nails and/or
hair. If a person has flakes of dead skin (dandruff-like scaling)
on the hair; broken, crusted, or matted hair; redness, irritation
of the scalp or beard; swollen areas and blister-like bumps with
pus (kerions); or a patchy hair loss the KOH test may be done. It
does not define the specific fungal species involved in any infection,
but the process is quick and easy to perform so as a simple diagnostic
tool it is very useful.
The KOH test involves taking a sample of skin where the suspected
infection is. This can be done with a scalpel if the skin is particularly
crusty, but more usually a wet gauze or maybe a tooth brush is used
to collect some dead skin scales. With hair fiber infections collecting
a sample can be more difficult because hair infections weaken the
hair and cause it to break off at the root leaving very little of
the infectious agent behind. Normal length hair is probably not
infected. However, some damaged hair around the periphery of an
infected area is usually present and the dermatologist will take
around 10 broken, infected-looking hairs. With nails a sample of
the dead skin under the nail plate is used.
The hair or skin sample is placed on a slide with a little of a
10% to 20% concentration potassium hydroxide solution and gently
heated. This solution slowly dissolves the hair and skin cells but
not the fungus cells. The fungus cells can then be seen with a microscope.
Color stains may be used so that the fungus is easier to see. This
technique aids the visualization of the fungal hyphae (branching,
rod-shaped filaments of uniform width with lines of separation called
septa). In tinea capitis, the hair shaft may be uniformly coated
with minute dermatophyte spores.
Unfortunately the KOH test is not fool proof. It is possible to
get a negative KOH test result when a fungal agent is actually present.
In particular, a common fungal agent that causes ringworm called
Trichophyton tonsurans does not show up well in a KOH test. Thus,
even when a KOH test is negative, a fungal infection may still be
suspected. If so, a fungal culture must be done to confimr that
a fungal agent is involved.
The
fungal culture test
A more advanced and more sensitive test that can be used to determine
the particular nature of the fungal species involved in a skin,
nail, or hair infection is a fungal culture. Fungal cultures are
slow and expensive to perform. They require incubators, culture
mediums, and some expertise on the part of the technician looking
after the cultures. For this reason fungal culture tests are rarely
done in the average dermatology clinic. More usually, a sample is
sent away to a laboratory for testing.
The dermatologist will take skin or nail scrapings, or a hair sample.
The sample is put into a sterile container and sent to the laboratory.
At the laboratory, the container is opened in a sterile chamber
and the sample is inoculated on test medium - usually Sabouraud's
dextrose agar or something similar. If there are fungal spores present,
they will grow on the agar. The culture usually takes 7 to 14 days
to be declared positive. It must be held for 21 days without development
of fungal growth to be declared negative. If the culture is positive
the laboratory can take a sample of the fungal growth and examine
it to see which particular species is involved.
A dermatology clinic may conduct a simplified fungal culture test
that does not define the fungal species involved, but does provide
a more sensitive test than the KOH test. "Dermatophyte test
medium" is a commercially available, nutrient-rich culture
solution supplied in a form that is ready for direct inoculation
in the office. The dermatologist takes a sample of dead skin or
a hair sample and puts it in a small tube containing the culture
medium. The yellow medium has a phenol red indicator that turns
pink in the presence of the alkaline metabolic products produced
by fungal agents. It takes up to 7 days of culturing before a conclusion
can be made as to whether the test is positive or negative. The
medium must be discarded 14 days after opening the culture tube
because saprophytes will slowly produce a similar change in the
culture medium color. This is a false positive result.
This simplified fungal culture test is more complex to conduct
than the KOH test, but easier to conduct than a full scale laboratory
fungal culture. These basic fungal culture tests are most likely
to be done in mid sized, private dermatology clinics. Large hospital
based clinics will most likely have the facilities to do full scale
culture testing while small practices will probably run a KOH test
first and then if there is still some doubt they may collect a sample
and send it to a central test laboratory for culturing.
Hair
and scalp fungal infection test references
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The rise and fall of fluorescent tinea capitis. Pediatr Dermatol.
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