What is the best method of diagnosing onychomycosis?

Journal of Family Practice, Dec, 2003 by Kathy Rea, Mark H. Greenawald

Weinberg JM, Koestenblatt EK, Tutrone WD, Tishler HR, Najarian L. Comparison of diagnostic methods in the evaluation of onychomycosis. J Am Acad Dermatol 2003; 49:193-197.

* PRACTICE RECOMMENDATIONS

Nail plate biopsy followed by periodic acid-Schiff staining is the most accurate method for diagnosing onychomycosis. The positive predictive value (PV ) of periodic acid-Schiff staining was equal to both potassium hydroxide (KOH) preparation and fungal culture, with a greater negative predictive value (PV-) due to superior sensitivity.

However, availability of periodic acid-Schiff staining may vary geographically, and the cost of the diagnostic tests is not addressed in this study. Thus, it makes clinical sense to start with the most accessible test, using periodic acid-Schiff staining if other methods are negative and clinical suspicion is high.

* BACKGROUND

Treatment of onychomycosis requires long-term and expensive therapy with potential negative side effects. Diagnosis based on clinical findings tends to overestimate fungal disease, as only 50% of dystrophic nails have a fungal cause. Traditional diagnostic methods have shown inconsistent sensitivity.

* POPULATION STUDIED

The study evaluated 105 patients with suspected onychomycosis presenting to an outpatient dermatology clinic. No information was provided on patient demographics or exclusion criteria. Seventy-two percent of patients had onychomycosis.

* STUDY DESIGN AND VALIDITY

The investigators evaluated 105 patients with suspected onychomycosis via 4 diagnostic methods. For periodic acid-Schiff staining, nails were clipped with standard nail clippers at the distal free edge of the nail plate, including any attached subungnal debris, and placed in a 10% formalin solution for evaluation by a pathologist. Subungnal curettage was then used to obtain material that was evenly divided for K0H preparation, fungal culture, and calcofluor white staining.

The KOH preparations used 20% KOH with the specimen placed on a slide and heated briefly prior to microscopic examination for fungal elements. Fungal cultures used Sabouraud's dextrose agar and mycosel agar, and were checked periodically over 4 weeks. Specimens stained with calcofluor white were examined using fluorescent microscopy. The authors chose calcofluor white as their reference standard for statistical analysis because of its published sensitivity and specificity of up to 95%.

The authors do not provide information on how participants were recruited into the study. Additionally, they do not state if those interpreting the test results were blinded to the results of the other tests. The KOH and fungal culture preparations are readily available for most family physicians. The availability of periodic acid-Schiff staining may vary geographically.

* OUTCOMES MEASURED

The authors calculated the sensitivity, specificity, PV , and PV-of K0H preparation, fungal culture, and periodic acid-Schiff staining using calcofluor white as the reference standard.

* RESULTS

Overall, 93/105 patients tested positive with at least 1 of the diagnostic methods. A total of 76 (72%) samples were positive by calcofluor white, the reference standard. The sensitivities of the other techniques were as follows: periodic acid-Schiff staining, 92%; KOH preparation, 80%; fungal culture, 59%. Periodic acid-Schiff staining was significantly more sensitive than KOH preparation (P=.03) and both periodic acid-Schiff staining and KOH preparation were significantly more sensitive than culture (P=.00002). The specificity of fungal culture was 82% vs 72% for both periodic acid-Schiff staining and KOH preparation. This difference was not statistically significant.

Using a typical prevalence of 50% in primary care, the positive predictive values were similar among the 3 diagnostic methods: periodic acid-Schiff staining (77%), KOH (74%), and fungal culture (76%). However, fungal culture had the highest false-negative rates (PV for periodic acid-Schiff staining=90%; KOH=78%; culture=67%).

Cathy Rea, MD, and Mark H. Greenawald,MD, Department of Family Practice, Carilion Health Ssytem, Roanoke, Va. E-mail: crea@arilion.com