Specimens processed with an extraction solution of the Dermatophyte Test Strip can be used for direct microscopy

Diagnosis of Onychomycosis: Utility of an Immunochromatography Strip Test Compared with Conventional Culture

BACKGROUND

Ringworm is highly prevalent in our setting and is frequently observed in our routine clinical practice. Diagnostic confirmation depends on techniques that are not always accessible (PCR), with highly variable sensitivity depending on the observer (direct microscopy) or delayed results (culture, histopathology). Recently, an immunochromatography-based rapid test (Diafactory®) for the antigenic detection of dermatophytes has been developed. This diagnostic tool can help diagnose ringworm, allowing early initiation of treatment and fewer consultation visits.

OBJECTIVE

To determine the sensitivity and specificity of the rapid antigen detection test compared to conventional culture.

MATERIAL AND METHODS

For a full year, 333 nail samples were collected from patients with suspected onychomycosis. The rapid test and the conventional culture were simultaneously performed on each sample. Those with a positive antigenic test result began treatment early. The remaining patients had appointments for serial cultures and subsequent medical consultation to evaluate the results.

RESULTS

Compared to conventional culture, the sensitivity and specificity rates of the rapid antigen detection test are 97.2% and 80.7%, respectively.

CONCLUSION

The effectiveness of the rapid antigen detection test is similar to that of conventional culture for the detection of dermatophytes in nail samples. It is a quick and simple diagnostic technique that reduces the number of patient visits to the hospital, and allows early treatment start.

Dermatophytic onychia: Effectiveness of rapid immunochromatographic diagnostic testing directly on samples compared to culture

BACKGROUND

Until now, definite diagnosis of dermatophytic onychia has been made by taking a nail sample and placing it in culture. The result is usually obtained only after 2 to 3weeks. More recently, diagnosis within a few minutes after sampling has become possible thanks to an immunochromatography technique developed in Japan and now available in France: the Diafactory Tinea Unguium® test strip (Biosynex, France).

METHODS

Over a 12-month period, 80nail samples from 80patients giving rise to a positive fungal culture were included in the study. For each patient, part of the removed nail was stored at room temperature and an immunochromatographic test was retrospectively performed according to the supplier's instructions. A small fragment of nail (≥ 1mg) was mixed with a few drops of reagent in a tube for 1min and the test strip was then placed in the tube with the result being visible to the naked eye (control strip, positivity strip) after incubation for a few minutes.

RESULTS

Compared with the culture method used for 51 isolated dermatophytes (42 Trichophyton rubrum, 9 Trichophyton interdigitale), the sensitivity of the rapid test was 96.07% (49/51). For the 29other fungal cultures (10Fusarium sp., 3Scytalidium sp., 3Scopulariopsis brevicaulis,3Aspergillus sp., 1Alternaria sp., 3Candida albicans, 1Candida parapsilosis, 1Trichosporons sp., 1Rhodotorula sp., and 3Corynebacterium sp.), the specificity was 75.86% (22/29). False positives were mainly due to the genera Fusarium and Scopulariopsis (6 of 7false positives), which were the likely cause of onychomycosis.

DISCUSSION

This rapid test could be useful in limiting excessive clinical diagnosis of dermatophyte onychomycosis. The rapid test has several advantages: ease of application, speed of results, and good performance, which, together, could improve diagnostic certainty during the actual consultation, thus limiting prolonged unnecessary prescriptions of antifungal treatments, while waiting for the laboratory culture results (3weeks for a negative result).