Scabies polymerase chain reaction with standardized dry swab sampling: an easy tool for cluster diagnosis of human scabies

Abstract

Background: Expert visualization of Sarcoptes scabiei remains essential for diagnosing human scabies, but access to said experts can be difficult. Polymerase chain reaction (PCR) is a specific tool for the detection and confirmation of S. scabiei but has poor sensitivity. Objectives: To evaluate PCR as a diagnostic method for scabies using nonexpert-dependent standardized sampling. Methods: The dry swab was systematically rubbed across the front of both wrists, the eight interdigital spaces and on any suspected scabies lesions in all patients referred for scabies. A new PCR-based diagnostic test was run on the samples. All patients underwent clinical and dermoscopic examination. Scabies diagnosis was confirmed when dermoscopic examination was positive or the patient had typical clinical signs of scabies. Results: Of 183 suspected cases of scabies, 164 patients were sampled, 87 had confirmed scabies (dermoscopy positive n = 87, typical clinical signs n = 1) and 77 did not. Of the 87 patients with proved scabies, 33 patients had positive scabies PCR, resulting in a 37·9% [95% confidence interval (CI) 28·4–48·4%] sensitivity and a 61·7% (95% CI 52·4–72·7%) negative predictive value. None of the 77 patients ruled out for scabies had a positive PCR result. Conclusions: This method is nontraumatic, repeatable and non-expert-dependent. It shows sensitivity similar to previous studies involving expert skin scraping. However, this method facilitates the multiplication of sampling, which increased the sensitivity for cluster scabies diagnosis. This method may be suitable as a first-line diagnosis tool where a large cluster scabies outbreak is suspected. What's already known about this topic?. Scabies diagnosis requires expertise. Scabies polymerase chain reaction (PCR) is specific but has poor sensitivity. Poor sensitivity is the consequence of the low efficiency of sampling methods. What does this study add?. This PCR-based diagnostic method based on nontraumatic standardized skin sampling is not expert-dependent and is reproducible. This diagnostic method may be relevant as a non-expert sentinel diagnosis tool in large clusters where a scabies outbreak is suspected.