Diagnosis of onychomycosis using the periodic acid–Schiff (PAS) test for sensitive identification of hyphae and fungal culture for identification of species has become the mainstay for many clinical practices. With the advent of polymerase chain reaction (PCR) testing, physicians can identify a fungal toenail infection quickly with the added benefit of species identification. We compared PAS testing with multiplex PCR testing from a clinical perspective.
A total of 209 patients with clinically diagnosed onychomycosis were recruited. A high-resolution picture was taken of the affected hallux nail, and the nail was graded using the Onychomycosis Severity Index. A proximal sample of the affected toenail and subungual debris were obtained and split into two equal samples. One sample was sent for multiplex PCR testing and the other for PAS testing. The results were analyzed and compared.
Six patients were excluded due to insufficient sample size for PCR testing. Of the remaining 203 patients, 109 (53.7%) tested positive with PAS, 77 (37.9%) tested positive with PCR. Forty-one patients tested positive with PAS but negative with PCR, and nine tested positive with PCR but negative with PAS.
Physicians should continue the practice of using PAS biopsy staining for confirmation of a fungal toenail infection before using oral antifungal therapy. Because multiplex PCR allows species identification, some physicians may elect to perform both tests.