Diagnosing Onychomycosis in the Office

Summarized by Robert W. Griffith, MD
March 21, 2003

Introduction

The availability of effective oral antimycotic medications means that most cases can be treated without referral to a dermatologist. However, although the history and clinical appearance make the diagnosis fairly certain, confirmation is desirable before starting an expensive course of treatment. The differential diagnosis includes psoriasis, nail trauma, contact irritants, lichen planus, neoplasms, and bacterial infection (Pseudomonas or Proteus). Showing the presence of a dermatophyte, without elaborate laboratory work-up to identify the genus and species, is sufficient to justify prescription of an oral antifungal.

The present study was done to test the usefulness of a rapid, easily performed, low-cost confirmatory test for onychomycosis; this is the in-office dermatophyte test medium (DTM) culture. Growth of the organism is indicated by a change in the culture medium from yellow to red in response to alkaline metabolites from the dermatophyte; this usually occurs within 3 to 7 days.

Method

Primary care physicians and podiatrists throughout the USA enrolled 670 patients with the signs and symptoms of onychomycosis. At each patient's first visit a specimen was taken from the toenail bed, which was divided, one part being cultured in the office using DTM, and the other being submitted to a central reference laboratory (the University of Texas Fungus Testing Laboratory, San Antonio), for KOH evaluation and Sabouraud dextrose agar culture. The physicians were given a videotape showing how to obtain a nail-bed sample and how to inoculate the DTM tube. The tube was kept at room temperature for up to 2 weeks, with the tube color checked daily.

If the KOH test at the central lab was positive but the culture was negative, the physician was asked to submit a second sample from the patient for repeat culture (provided the patient was not being treated with an antifungal medication).

To analyze the usefulness of the office-based system, positive DTM results were correlated with cultures growing a dermatophyte at the central lab.

Results

Eighty podiatrists and 69 primary care physicians entered a total of 670 patients into the study. Complete comparative data were available for 617 patients (92%). Sexes were equally distributed. 45% of the subjects were over 65, and 17% were diabetic; 60% had involvement of both feet. 10% had fingernail onychomycosis, and 29% had symptoms of tinea pedis - these were not confirmed by DTM or fungal culture, however.

The central lab cultures were positive in 297 (44%), and in-office DTM cultures positive in 345 (51%) patients. The DTM and laboratory culture results were in agreement (both positive in 206, both negative in 214) in 68% of the 617 patients where paired results were available. This provided a K (kappa) coefficient of 0.37, which indicates a 'fair degree of agreement' between multiple tests, 'beyond that which would occur by chance'.

Of the 342 patients with negative cultures, re-testing results were available for 105; re-test cultures were positive in 23 (22%) of these. About 10% of specimens that were DTM negative were positive by the central lab culture technique.

The tests also confirmed that dermatophytes were the primary pathogen in onychomycosis, accounting for 93% of infections. Men were more likely to have positive results by both culture methods (64% and 59%, respectively).

Comment

Ideally, as the investigators point out, treatment for onychomycosis should be based on positive KOH microscopy and a positive fungal culture, together with a clinical diagnosis. Their study was planned to determine if DTM in-office culture represents a satisfactory substitute for KOH testing plus culture in a central lab setting.

The study confirmed that dermatophytes are the primary cause of onychomycotic symptoms in family practice. Fungal cultures were positive in 44% of patients using the central lab method, and in 51% using the in-office DTM method. The approximate cost of the latter is quoted as $1 per test, as opposed to $25 for fungal culture at a central lab. It's also evident that results are obtained much more quickly with DTM - less than 2 weeks compared with 4-6 weeks for the central lab.

The data from the study support the concept of primary care physicians and podiatrists setting up an in-office DTM culture and starting antimycotic therapy in patients with clinical signs and symptoms of onychomycosis. A negative read-out at 2 weeks would necessitate re-evaluation of the patient - has treatment proved effective, is the diagnosis correct, etc; nothing has been lost by not requesting a KOH test and central lab culture at the first visit.

Source

• Office practice-based confirmation of onychomycosis. BE. Elewski, J. Leyden, MG. Rinaldi,  et al., Arch Intern Med, 2002, vol. 162, pp. 2133--2138

Related Links
Online Atlas of Dermatology: Onychomycosis
Disease Digest: Psoriasis

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