Study Question: Which test is the most accurate in the diagnosis of onychomycosis?
Setting: Outpatient (specialty)
Study Design: Diagnostic test evaluation
Synopsis: Treatment of onychomycosis requires the use of long-term, expensive medication with potential side effects. Because only 50 percent of dystrophic nails have a fungal cause, it is important to diagnose the infection correctly. In this study, 105 patients with suspected onychomycosis were evaluated using four diagnostic methods: potassium hydroxide (KOH) preparation, fungal culture for four weeks, nail plate biopsy evaluated with periodic acid–Schiff stain, and calcofluor white stain.
Nail specimens for periodic acid–Schiff stain evaluation were obtained using standard nail clippers along the distal free edge of the nail plate, including any attached subungual debris, just distal to the nail attachment to the nail bed. Clippings were placed in 10 percent formalin solution and evaluated by a pathologist. The authors do not state whether individual test results were evaluated independently by personnel blinded to other test results. Because of its reported high sensitivity (92 percent) and specificity (95 percent), calcofluor white stain was chosen as the gold standard for statistical analysis.
Seventy-six (72 percent) of the nails were positive for onychomycosis by calcofluor white stain. The sensitivities and specificities of each technique are as follows: KOH, 80 and 72 percent; periodic acid–Schiff stain, 92 and 72 percent; and fungal culture, 59 and 82 percent. Thus, given a 50 percent prevalence of onychomycosis in patients presenting with dystrophic nails, the positive and negative predictive values of each technique are as follows: KOH, 74 and 78 percent; fungal culture, 76 and 67 percent; and periodic acid–Schiff stain, 77 and 90 percent. All tests, therefore, had a similar false-positive rate, but periodic acid–Schiff stain had the lowest false-negative rate. Interestingly, the fungal culture had the highest false-negative rate.
Bottom Line: Nail plate biopsy followed by periodic acid–Schiff stain has a false-positive rate equal to KOH preparation and fungal culture, but a lower false-negative rate because of a superior sensitivity in accurately detecting onychomycosis. Thus, periodic acid–Schiff stain is indicated if the results of other methods are negative and clinical suspicion is high. Because all of the tests—periodic acid–Schiff stain, KOH, and fungal culture—are equally likely to be accurate when the result is positive, it makes sense to start with the easiest, most readily available, and cheapest. (Level of Evidence: 2b–)