Coronavirus: what does a 'presumptive positive' PCR test result mean?

People who get a presumptive positive result have at least one respiratory specimen of the virus that causes Covid-19

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The polymerase chain reaction, or PCR, test is considered to be the gold standard for a Covid-19 diagnosis.

A positive result means you probably have Covid-19, whereas a negative test means you more than likely did not have the virus at the time of testing.

But there is a third possible result: PP, or presumptive positive. But what does that mean?

The National explains.

What is a PCR test?

It seeks to detect the presence of the Sars-CoV-2 virus, which causes Covid-19.

Sample collection is done using a swab on a long stick to collect respiratory material found inside the nose.

The sample is then sealed in a tube and sent off for analysis.

What happens next?

The lab performs a procedure called extraction, where they isolate genetic material from the sample to check if any virus is present.

Special chemicals are then used to create a reaction that makes millions of copies of a portion of the Sars-Cov-2 virus’s genetic material. One chemical creates a reaction that lights up in a fluorescent colour if the virus is detected.

What does a PP, or presumptive positive, result mean?

The result is inconclusive.

"It might be positive but it needs to be repeated again in three days," said Dr Engy Iskandar, a specialist in internal medicine at Dubai's International Modern Hospital.

"In Covid-19 tests, they look for several genes. If they find only one it means it is presumptive positive and it is not confirmed yet."

Doctors said all PCR testing looks for genes specific to Covid-19, but tests could pick up those in general coronaviruses too.

"A presumptive positive result indicates that not all genes were detected," said Dr Heba Sami, head of laboratory and consultant clinical pathologist at Saudi German Hospital, Ajman.

Once the presumptive positive result is obtained, the patient should self-quarantine and repeat the test within 72 hours

Perhaps genes specific to general coronaviruses were detected only. But it could alternatively indicate a low viral load, doctors said.

“As per National Crisis and Emergency Management Authority (NCEMA) guidelines provided by Dubai Health Authority, presumptive positive means only one of the multiple genes is isolated, or a low viral load is possible,” said Dr Surbhi Gupta, a specialist in clinical pathology at NMC Royal Hospital in Dubai.

At any event, anyone who receives this result should immediately begin quarantining and repeat the test in three days.

“Once the presumptive positive result is obtained, the patient should self-quarantine and repeat the test within 72 hours," Dr Gupta said.

Do all laboratories have the capacity to detect this result?

Yes, all testing facilities must be able to detect this.

“All testing laboratories have to provide it. It is part of NCEMA guidelines,” Dr Gupta said.

Is it common?

No. Laboratories say the result only accounts for a tiny percentage of samples.

“Based on the testing in our lab, the presumptive positive rate is 0.24 per cent,” Dr Gupta said.

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