Standardized cotton swab sampling with nested quantitative polymerase chain reaction is effective for diagnosing ordinary scabies

Low sensitivity of the PCR assay for diagnosing scabies has been noted because of the difficulty in obtaining tissue containing Sarcoptes scabiei DNA. To evaluate nested real-time quantitative PCR (nRT-qPCR) with nonexpert-dependent standardized cotton swab sampling (CSW) as a tool for diagnosing scabies. All patients underwent dermoscopic and microscopic examination (MS) with scraped sampling (Sc). Patient samples were acquired with a single, dry swab rubbed across the flexor areas of both wrists as well as the eight interdigital spaces and on any suspected scabies lesions. nRT-qPCRs were performed with Sc and CSW samples. Out of 125 patients with suspected scabies, 120 patients were sampled, and 57 were positive (positive with: MS n = 53; nRT-qPCR with Sc n = 52; nRT-qPCR with CSW n = 46) and 63 were negative for scabies. The sensitivities of these tests were 93.0%, 91.2% and 80.7%, respectively, which were not different statistically (P > 0.05). However, upon subsequent monitoring after treatment, the sensitivity of nRT-qPCR with CSW was only 36.6%, which was significantly lower than 83.0% for MS and 92.7% for nRT-qPCR with Sc (P < 0.001). The obtained sequences showed 97%-100% homology with scabies sequences deposited in GenBank. CSW with nRT-qPCR shows sensitivity close to MS with scraping performed by experts for diagnosing scabies in an outpatient setting, but not for post-treatment monitoring. CSW with nRT-qPCR may be useful for physicians unfamiliar with a traditional diagnostic method, and for screening an outbreak in community facilities.