Diagnóstico laboratorial da onicomicose dos pés causada por dermatófitos e fungos filamentosos não dermatófitos: estudo comparativo entre o exame micológico do raspado subungueal e a análise histopatológica da queratina ungueal distal (clipping ungueal)

Abstract

Mycological examination has been considered the gold standard for the diagnosis of onychomycoses. However, it has been suggested that the histopathological analysis of the distal nail plate (nail clipping) may be a more sensitive, although less specific technique, and valuable as an additional diagnostic tool in cases of diagnostic difficulty. To evaluate and compare the performance mycological and histopathological examinations in onychomycoses caused by dermatophyte and non-dermatophyte moulds (NDM). Patients with lateral/distal subungual onychomycosis in at least one hallux were enrolled in a protocol for a cross-sectional, open-label, prospective, nonrandomized and uncontrolled study. All of the patients were assessed via mycological and histopathological tests. The isolation of filamentous fungi was considered the gold standard. Test performance was evaluated through sensitivity, specificity and positive and negative predictive values. The test performance was evaluated through sensitivity, specificity and positive and negative predictive values (PPV and NPV). A total of 212 patients were enrolled in the study; the majority were white and female, with a mean age of 58.8 years. Direct microscopy (DM) was positive in 57.5% patients, and culture was positive in 34.4%. Among these patients, 23.3% were positive for dermatophytes, and 86.3% were positive for NDM. Trichophyton rubrum was the most frequently dermatophyte isolated, with predominance of N. dimidiatum among NDM. The histopathological examination was positive in 41.0% samples. There was no significant difference between the special stains, PAS and Grocott s methenamine silver. Histopathological sections stained with HE showed a significant correlation between neutrophil accumulation or its association with paraceratosis and a positive fungal elements, as septate hyphae and/or arthroconidia (p-value=0.034). There was no significant difference on accuracy between nail clipping and DM. Direct microscopy was the most sensitive test (p-value =0.000), and nail clipping was the most specific (p-value=0.066). The PPV of the DM showed greater overall efficiency for NDM (p-value=0.001) and the sensitivity was the same for both (100%). The NPV was statistically higher for dermatophytes (p-value=0.001). Direct microscopy and histopathological examination presented better performance for the detection of really sick individuals when the etiological agents of onychomycosis were NDM, p-value=0.001 and p-value=0.006, respectively. Direct microscopy was the best test to rule out onychomycosis. The histopathological examination showed higher accuracy for dermatophytes and the direct microscopy for NDM. The combination of at least two diagnostic tests may increase the likelihood of reaching a correct diagnosis.