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France says virus PCR tests ‘reliable’ despite flaws

The French government has sought to reassure the public that coronavirus Covid-19 virological tests (PCR tests) are “reliable”, even as two apparent major flaws with the tests have emerged.

PCR tests (polymerase chain reaction) tests are generally undertaken using a cotton swab in the nose or throat. They test whether someone has the virus at the time of testing, using DNA samples.

This is as opposed to blood tests, which test for antibodies in a person’s blood.

Blood tests cannot tell if someone is ill at the time of testing, but only if they have been infected with the virus at some point previously, and therefore may have some level of immunity (although the exact level is disputed).

Read more: Millions of virus antibody blood tests set for France

Read more: New blood test by French lab detects Covid-19 immunity

At the end of April, Prime Minister Edouard Philippe said that the government was aiming for the country to perform “at least 700,000 PCR tests per week” from the start of deconfinement on May 11, as a major part of the strategy to continue fighting the virus.

Read more: Key questions as France aims for 700,000 Covid tests

Read more: France urges public to get coronavirus tests

Reliability issues

Jérôme Salomon, director general of health, has said that the PCR tests are “reliable”, but critics have said that there are two major flaws with the tests, which can make them less dependable.

The first flaw is that PCR tests have a 1% false positive rate, meaning that out of 100 people tested, one person will be told they are infected when they are not. This may be due to lab contamination.

The second is that PCR tests have a failure rate of 20%, meaning that of 100 people tested, 20 people will be incorrectly told they do not have the virus.

This may be caused by DNA degradation after the test, or because the viral load in that person is too low to be detected.

This may give the person a false sense of security, and lead them to take fewer precautions or not self-isolate, and unwittingly spread the virus to others.

The day on which the patient is tested also makes a difference.

The precision of the tests is improving, but a study published in mid-May, from scientific review Annals of Internal Medicine said that the rate of “false negatives” was still at 20% if the patient is tested seven days after infection.

The rate of false negatives increases the more days pass. Because of this, the government has said that anyone requiring a PCR test must be tested within seven days of suspecting that they may be infected.

However, symptoms can take four days to show up, meaning that in some cases, people may only have three days to get a test before the reliability rate drops sharply (after the full seven days).

These failure rates and flaws with the test have led one medical professor at prestigious Yale University in the US - Dr. Harlan M. Krumholz, writing in newspaper The New York Times - to warn: “If you have coronavirus symptoms, assume you have the illness, even if you test negative."

This means self-isolating if you have symptoms, maintaining social distancing, wearing a mask when outside of your home, and continuing precautions such as washing hands or using sanitiser gel.

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