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Calcofluor white microscopy for dermatophytes performs very well

July 28 2020

The laboratory diagnosis of dermatophyte infection of skin, hair and nails relies on both microscopy and culture. Neither are perfectly sensitive. Microscopy takes considerable time using the standard potassium hydroxide (KOH) staining technique, but both sensitivity and technical time spent examining samples can be improved with fluorescent brighteners such as Calcofluor white.  Dr Marjan Motamedi and colleagues from Shiraz University in Iran compared Calcofluor white with culture and PCR on 307 clinical samples.

 Calcofluor white 0.01% performed well – in 241 skin scale, 62 hair and 4 nail samples, its sensitivity and specificity was 99% compared with an in house pan-dermatophyte PCR assay (95 % and 96% respectively. There was excellent concordance between KOH 20% and Calcofluor white (only 3 mismatches amongst 190 positive samples). Culture was negative in 18%. PCR was negative in 10 KOH positive samples (5%) and an additional 5 samples (3%) were positive by PCR that were negative by KOH.

 Fluorescent brighteners are used in washing powder to whiten already white clothes. They bind to 1-3 beta and 1-4 beta polysaccharides on both chitin and cellulose in the cell walls of fungi, as well as algae and plants.

 Older studies pre-affirm these findings, including papers by Hageage (1984), Gip (1986), Honig (1996), Kwon (1998), Haldane (1990), Weinberg (1993), Hamer (2006), Haghani (2011), Bonifaz (2013), Yadav (2013) and Yue (2018). There were very few nail specimens in the recent study from Motamedi et al, but other studies show improved sensitivity of Calcofluor over KOH microscopy.  A fluorescent microscope is required, but time of sample analysis can be greatly reduced with a fluorescent brightener – with one new product (fluorescent brightener 85) recently assessed (Yue et al, 2018).