Best procedure for skin scraping fungi.
A Two experts provided information about this question. Glenn D. Roberts, PhD, Division of Clinical Microbiology, Mayo Clinic, replied, "We have worked with the KOH preparation for over 30 years, using 10% KOH with 10% glycerin added to prevent dehydration. We have no experience with DMSO (dimethylsulfoxide), and I have never talked to anyone who uses it. We gener-ally allow the slide to sit for five minutes or more to hasten clearing of the clinical specimen. The former process was to gently pass the slide through a gas flame to speed things up, but we no longer do that. The addition of Calcofluor white to the KOH has markedly improved our ability to detect organisms in clinical specimens; however, the overall sensitivity for the method is not as good as we would like."
Deborah L. Grafelman, president, Delasco, Dermatologic Lab and Supply, answered, "The purpose of the KOH is to 'clear' (partially dissolve) the keratin. The fungal structures are resistant to the KOH and remain; but once the keratin is cleared, the fungal elements become visible and readily identifiable. KOH in water will do this, but it takes some time (15 to 20 minutes for clearing). This process can be speeded up by heating; however, there is a risk of overheating and drying the specimen, or boiling and splattering the caustic KOH. Additionally, the heating process takes a few seconds.
"The DMSO, in fact, does carry the KOH into the keratin much more rapidly; and, unless the specimen is quite thick or is, for example, a piece of nail plate (either of which may require five to 10 minutes of 'soaking'), the specimen can be examined immediately and is usually cleared much quicker than even heating KOH and water. The only disadvantage of which I am aware is the slight garlicodor of DMSO, but only one user has complained about it in over 20 years of providing the product. Another characteristic is the fact that these are saturated solutions; and, if there is any evaporation of the water component, the product may begin to separate, causing microscopic bubbles on the slide. The 'cure' for this is to add a few drops of water to the bottle, which reconstitutes the solution."
--Daniel M. Baer, MD
Daniel M. Baer, MD, is professor emeritus of laboratory medicine at Oregon Health and Science University in Portland, OR, and a member of MLO's editorial advisory board.
Edited by Daniel M. Baer, MD