Performance of the Xpert™ Mpox PCR assay with oropharyngeal, anorectal, and cutaneous lesion swab specimens

Abstract

Anorectal and oropharyngeal exposures are implicated in sexual transmission of mpox, but authorized assays in the United States are only validated with cutaneous lesion swabs. Diagnostic assays for anorectal and oropharyngeal swabs are needed to address potential future outbreaks. The Cepheid Xpert® Mpox is the first point-of-care assay to receive FDA emergency use authorization in the United States and would be a valuable tool for evaluating these sample types. Our exploratory study demonstrates 100 % positive agreement with our in-house PCR assay for natural positive anorectal and oropharyngeal specimens and 92 % sensitivity with low-positive spiked specimens. The Xpert® assay detected viral DNA in specimens not detected by our reference PCR assay from four participants with mpox DNA at other sites, suggesting it may be more sensitive at low viral loads. In conclusion, the validation of the Xpert® for oropharyngeal and anorectal sample types can be rapidly achieved if clinical need returns and prospective samples become available.

Keywords: Anorectal swab; Diagnostic; Mpox; Oropharyngeal swab; Point-of-care; Sexually transmitted infections.

Figure 1.. The Xpert^® Mpox assay demonstrates 100% PPA and likely has a lower LOD compared to the NVAR reference assay.

Concentric circles indicate the results of the Xpert Mpox assay (smaller circle) and the reference assay (larger circle). Gray shading represents negative results. Green shading represent positive natural specimens while blue shading represents positive contrived specimens.

Figure 2.. Lower Ct values were observed with Xpert^® OPX compared to the NVAR reference assay and lower MPX Ct values compared to OPX Ct values were observed within the Xpert^® Mpox assay.

Correlation of Ct values between the Xpert^® OPX target and NVAR qPCR reference assay for (A) oropharyngeal, (B) anorectal, and (C) lesion swab specimens showed generally lower Ct values produced by the Xpert^® in the oropharyngeal and cutaneous lesions specimens. This was not as apparent in the anorectal swab specimens, but there are few data points and most have a low Ct value. Several specimens that were not detected by the NVAR assay produced positive results with high Ct values on the Xpert^® assay. (D) Within the Xpert^® results, MPX targets produced consistently lower Ct values than OPX targets.MPX, Xpert MPXV clade-II target; OPX, Xpert non-variola Orthopoxvirus target; NVAR, non-variola Orthopoxvirus reference assay; Ct, cycle threshold.