Comparison between PAS and GMS stains for the diagnosis of onychomycosis

Onychomycosis is the most common nail disease in adults. Periodic acid-Schiff (PAS) stain has been considered the most sensitive method for diagnosis. By comparing originally stained PAS slides (i.e. without repeating PAS) with newly stained Grocott methenamine-silver (GMS) slide, D'Hue et al. concluded that GMS is superior to PAS. We conducted a similar study with the addition of repeating PAS stain and compared the newly stained PAS and GMS slides in tandem.

We performed both PAS and GMS stains on 30 clinically suspected onychomycosis cases. The presence or absence of fungal organisms was evaluated. The results were also compared with the original PAS stain.

Twenty-two cases were positive with PAS, staining three cases more than GMS, which stained only 19 cases. The difference between the two stains is statistically insignificant. Repetition of GMS stain on these three cases (cutting in a deeper level in the paraffin block) resulted in positive results. Four out of 30 cases were negative with original PAS stain, but stained positive both with new PAS and GMS.

PAS and GMS stains are quantitatively similar. Sensitivity of both methods can be increased by preparing two slides from different levels in the paraffin block.