Modified PAS stain: A new diagnostic method for onychomycosis

Onychomycosis is the most common nail disease and represents around 50% of nail disorders. Accurate diagnosis with adequate evidence is ideal before starting any treatment. Current diagnostic methods offer low specificity and sensitivity.

To create a new method for the diagnosis of onychomycosis, and to compare its sensitivity and specificity with the existing methods.

One hundred and ninety-two samples with clinical suspicion of onychomycosis were included and underwent modified PAS stain (M-PAS), KOH/chlorazol black (KOH/CB) and culture testing. Sensitivity, specificity, positive and negative predictive values were calculated.

In 152 out of 192 samples (79.2%) fungi structures were found in at least one of the three tests performed, and the patients were diagnosed with onychomycosis; 40 samples out of 192 (20.8%) were negative. Using M-PAS, filaments and/or spores were seen in 143 samples from the 152 positive (94%); 39 of them were negative to KOH/CB and positive to M-PAS (25.6%). With KOH/CB, filaments and/or spores were seen in 113 cases from the 152 positive samples (73.8% of the onychomycosis cases). Thirty-five cultures were positive, of which 77% were identified as Trichophyton rubrum; 117 onychomycosis cases were diagnosed despite the negative culture (76.9%). M-PAS showed 92.5% sensitivity and 55.55% specificity, a 67.5% positive predictive value and a 81.6% negative productive value.

This procedure, a combination of the existing methods to diagnose onychomycosis, KOH/CB together with a nail clipping biopsy, proved to have high sensitivity, as well as being rapid, easy, inexpensive and readily available in most hospital settings. M-PAS allowed us to diagnose 39 cases (25.6% of the cases of onychomycosis) that were false negative using only KOH/CB and culture.