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Comparison of Diagnostic Methods in the Evaluation of Onychomycosis

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Comparison of Diagnostic Methods in the Evaluation of Onychomycosis
Jeffrey M. Weinberg MD, Evelyn K. Koestenblatt MS MT (ASCP)
Dermatology Online Journal 7(1): 23G

Department of Dermatology, St. Luke's-Roosevelt Hospital Center, New York, NY.

We evaluated 105 patients with suspected onychomycosis using four diagnostic methods: potassium hydroxide (KOH) preparation, culture, biopsy with PAS stain, and calcofluor white stain. Calcofluor white (CW) stain binds to cellulose and chitin and fluoresces when exposed to UV radiation. It is a highly sensitive and specific technique for the detection of dermatophytes. Ninety-three of the patients had at least one positive sample. To determine the clinical usefulness and performance characteristics of each test, CW was chosen as the gold standard for statistical analysis.

The following were calculated for each test: sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV). The sensitivities of each of the techniques were as follows: KOH 80%; biopsy with PAS 92%; and culture 59%. Both KOH and biopsy methods were more sensitive than culture (p=0.00002). Biopsy was also more sensitive than KOH (p=0.03). The specificities were as follows: KOH 72%; biopsy with PAS 72%; and culture 82%. The PPV calculated for the different techniques were KOH 88%; biopsy with PAS 89.7%; and culture 90%. In terms of negative predictive value, the results were KOH 58%; biopsy with PAS 77%; and culture 43%.

In conclusion, biopsy with PAS is the most sensitive method for the diagnosis of onychomycosis. It is also superior in its negative predictive value. It is indicated if other methods are negative and clinical suspicion is high, and potentially is the single method of choice for the evaluation of onychomycosis.

© 2001 Dermatology Online Journal