A retrospective review of diagnostic testing for onychomycosis of the foot

Background: Onychomycosis is a fungal infection of the nail that can be caused by dermatophytes, yeasts, or nondermatophyte molds. To diagnose onychomycosis, a clinician must use the patient’s history, physical findings, and diagnostic testing, which can include calcofluor white/potassium hydroxide (KOH) mount, fungal culture (FC), and periodic acid–Schiff (PAS) stain. Some insurance companies require authorization for antifungal medication and request laboratory results to confirm infection. We sought to compare the reliability of KOH, PAS, and FC diagnostic results for confirmation of fungal disease, to determine the sensitivity and specificity of each test, and to investigate the cost of each test. In addition, we statistically observed the relationship between the test results and demographic variables. Methods: Toenail clippings were obtained from 108 patients clinically diagnosed as having onychomycosis. Diagnostic tests were then performed on each sample; the results were obtained from medical records. Results: For PAS, KOH, and FC, 60.2%, 43.5%, and 39.8% of results, respectively, were positive. Agreement for each pair of tests was slightly higher for FC and KOH. Sensitivities for KOH and PAS were 0.64 and 0.79, respectively. Specificity was 0.79 for KOH and 0.54 for PAS. Both PAS and KOH had a higher percentage of positive test results for men than for women. Conclusions: Of the three tests evaluated, PAS gives the most consistent positive results and has the highest sensitivity. Therefore, PAS should be considered as the best test to verify clinically significant onychomycosis.