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Message: <blockquote data-quote="imhotep" data-source="post: 27338846" data-attributes="member: 562115">Recently there were some posts on RAT accuracy etc and hence this post. RAT kits have a come a long way since it's introduction during the worst pandemic times. At that time some test kits were notoriously error prone with false negatives.Now the technology has evolved and we do have quite accurate tests and many countries have got rid of the mandatory PCR testing and rely mostly on the cheaper RAT alternative. Not all the RATs are the same and as with anything else, who produces it matters.There are three main things that matters in any diagnostic test. - Sensitivity, specificity and pre-test probability. In this discussion we will assume the infection is Covid.Sensitivity - How well the diagnostic picks up the disease - When a RAT kits says 85% sensitivity, it implies that if you test 100 people who definitely has Covid, you will get 85 positives and 15 false negatives.Specificity - How well the diagnostic correctly identifies the absence of a disease. So when a RAT says 98% specificity, it means that if you test 100 people who are definitely free of COVID, 98 would test negative – and two people would have a “false positive”.Pre-Test Probability - This is the disease prevalence...the chance a person has COVID infection based on their clinical symptoms, specific exposure history or even based on the current infection rate in the community.Above all there's another very important factor. - How well the sample is taken, &  the timing of the sampleThere are several videos on the YouTube showing you how to obtain a Nasal or a throat swab. Please refer to these.When it comes to timing - RATs are relatively insensitive early or late in infection and the evidence particular brands have differing ability to detect the Omicron variantDo NOT perform a RAT within 48 hours of exposure to COVID and preferably wait or repeat between 5–7 days post exposure.If your test result is a surprise based on your assessment of pre-test probability - then repeat the test 24–48 hours later or have a PCR test done.Someone was commenting about RAT and the Ct values of a PCR test. Studies done on EU made RAT kits mid 2021 has shown that in direct comparison with the RT-PCR, the RAT kits showed a sensitivity of 100.0% for Ct values < 25 and 91.8% for Ct values < 30.There are only a very few test kits that can specifically detect Omicron. It mainly depends on how the RAT kits targets the parts of the virus. The Indians (ICMR) recently came up with OmiSure - an Omicron specific RAT which targets both the SGTF & SGMA (S-gene target failure  &  S-gene mutation amplification)The generally available kits does show less sensitivity with Omicron. That's a fact but manufacturers dodge the issue by saying both Yes & No... There can be several reasons for this - proabably the mutations of Omicron itself and/or with the fact that there could be lower viral load on the swab.</blockquote>

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