![]() ![]() Although he was negative on repeat swabs on the same day and two days later, he was positive three months earlier and was antibody positive. The other individual was asymptomatic at the time of swabbing and tested positive. Although he initially tested negative on admission, this was likely a false negative. ![]() However, on admission he had CXR changes and lymphopenia. He was asymptomatic at the time of swabbing and tested positive. †One was most likely a true positive case. One had a previous positive result and the result likely reflected residual genetic material. *The range of Ct values in symptomatic patients 36–43, mean 39.5. Ct values vary according to the assay being used but in clinical practice Ct35 as weak positive result.įurther information is also presented to demonstrate the likely false positive results (including repeat swab results where available, absence of symptoms and evolution following the result where available).įlow chart summarising the key findings of likely false positive and true positive results. Low cycle thresholds indicate presence of greater amounts of the target sequence in the sample, and higher viral loads. The positive result can be semi-quantified by the number of cycles required to reach the threshold: the ‘cycle threshold’ (Ct). A positive result is generated when the level of fluorescence crosses a predetermined threshold. PCR involves repeat cycles of amplification of an RNA target, detected in real time by use of fluorescent dyes. ![]() Data were analysed in Excel.įor the purpose of this paper, a result was considered as false positive if it was positive in only a single gene with a Ct value >35. These platforms use a combination of targets including the N and E gene (Cepheid and Luminex) and the N, E and RdRp gene (Seegene). Tests were carried out on three platforms: Seegene, Cepheid and Luminex. We collected data from the ICNET platform of patients that tested positive by SARS-CoV-2 RT-PCR including inpatient and outpatient testing within the Swansea Bay University Health Board from 20 June 2020 to 21 July 2020 during a period of very low prevalence in this area. ![]()
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