What's a PCR test cycle threshold and why it matters - Full Fact

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It's been falsely claimed the WHO said PCR tests are flawed. It said "weak positive" cases with high "cycle thresholds" may need to be retested.
What's a PCR test cycle threshold and why it matters

8 April 2021

What was claimed The World Health Organisation confirmed that the PCR test is flawed and all tests with a high cycle threshold (Ct) are invalid. Our verdict False. It reiterated guidance that tests with a high Ct may need to be retested. There are lots of different reasons for high Ct value results. Interpreting them requires clinical context.

A post on Instagram claims that the World Health Organisation (WHO) has pronounced PCR tests for Covid-19 as flawed, and all positive tests in the past 14 months 'invalid' if a cycle threshold of 35 or above has been used.

A PCR test is performed by repeatedly replicating target viral material in the sample to the point that it becomes detectable. The number of cycles before the virus is detectable is known as the cycle threshold (Ct). We have written previously about how PCR tests work, and how effective they are.

A positive test with a high Ct value may indicate a test from someone who had a very small amount of detectable viral RNA on their initial swab, and may not be infectious or have ongoing active infection.

However, there are other clinical scenarios that can result in a positive test with high Ct value in someone who may still be infectious or who may soon become infectious. The claims in the Instagram post misinterpret clarifications released by the WHO.

What are cycle thresholds?

During a PCR test for SARS-CoV-2 (the virus that causes Covid-19), viral genetic material on the swab sample is purified, stabilised and concentrated.

If the genetic material of interest is present in the sample, it is then copied again and again by heating and cooling the material in the presence of various substances. Each iteration of this is called a thermal cycle.

As genetic material is amplified (with real time PCR) fluorescence is produced; how this happens exactly varies by PCR method, but basically…
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