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Performance characteristics of SARS-CoV-2 nucleic acid detection assays are understudied within contexts of low pre-test probability, including screening asymptomatic persons without epidemiological links to confirmed cases, or asymptomatic surveillance testing. SARS-CoV-2 detection without symptoms may represent presymptomatic or asymptomatic infection, resolved infection with persistent RNA shedding, or a false-positive test. This study assessed the positive predictive value of SARS-CoV-2 real-time reverse transcription polymerase chain reaction (rRT-PCR) assays by retesting positive specimens from 5 pre-test probability groups ranging from high to low with an alternate assay.
In total, 122 rRT-PCR positive specimens collected from unique patients between March and July 2020 were retested using a laboratory-developed nested RT-PCR assay targeting the RNA-dependent RNA polymerase (RdRp) gene followed by Sanger sequencing.
Significantly fewer (15.6%) positive results in the lowest pre-test probability group (facilities with institution-wide screening having ≤3 positive asymptomatic cases) were reproduced with the nested RdRp gene RT-PCR assay than in each of the 4 groups with higher pre-test probability (individual group range, 50.0%–85.0%).
Large-scale SARS-CoV-2 screening testing initiatives among low pre-test probability populations should be evaluated thoroughly prior to implementation given the risk of false-positive results and consequent potential for harm at the individual and population level.
Clostridium difficile spores play an important role in transmission and can survive in the environment for several months. Optimal methods for measuring environmental C. difficile are unknown. We sought to determine whether increased sample surface area improved detection of C. difficile from environmental samples.
Samples were collected from 12 patient rooms in a tertiary-care hospital in Toronto, Canada.
Samples represented small surface-area and large surface-area floor and bedrail pairs from single-bed rooms of patients with low (without prior antibiotics), medium (with prior antibiotics), and high (C. difficile infected) shedding risk. Presence of C. difficile in samples was measured using quantitative polymerase chain reaction (qPCR) with targets on the 16S rRNA and toxin B genes and using enrichment culture.
Of the 48 samples, 64·6% were positive by 16S qPCR (geometric mean, 13·8 spores); 39·6% were positive by toxin B qPCR (geometric mean, 1·9 spores); and 43·8% were positive by enrichment culture. By 16S qPCR, each 10-fold increase in sample surface area yielded 6·6 times (95% CI, 3·2–13) more spores. Floor surfaces yielded 27 times (95% CI, 4·9–181) more spores than bedrails, and rooms of C. difficile–positive patients yielded 11 times (95% CI, 0·55–164) more spores than those of patients without prior antibiotics. Toxin B qPCR and enrichment culture returned analogous findings.
Clostridium difficile spores were identified in most floor and bedrail samples, and increased surface area improved detection. Future research aiming to understand the role of environmental C. difficile in transmission should prefer samples with large surface areas.
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