Can my Covid-19 result change if I immediately test again?

False negatives and false positives after a Covid-19 test are common. 

If the test gives a negative result in someone who is infected that is called a false negative. A person who does not have the infection but whose test gives a positive result is a false positive. 

According to the National Institute for Communicable Diseases (NICD), a division of the national health laboratory service, a negative test does not exclude Covid-19 infection as the test result depends on when a sample was collected and the quality of the sample collected.

“The amount of virus in your body (called the viral load) changes during the course of an infection, starting from low levels which increase over a few days and then start to decrease towards the later stages of infection.

“How long you have the virus (called viral shedding) depends on many factors including: your age, whether you have an underlying condition and how severe your disease is,” said the NICD.

Viral shedding can be detectable around two to three days before the onset of symptoms and tends to decrease after about seven days from the onset of symptoms, said the institution. 

“Although the PCR test is a very sensitive test, there is no test that is 100% sensitive so if you are tested very early in your infection, and the viral loads in your body are low, the test may not detect the virus,” said NICD.  

“However, if tested later in infection, when viral loads are higher, your test may change to positive. Similarly, if you are tested towards the end of the infection your test result may change to negative.”

The NICD cautions that the test result is also affected by the type and quality of the sample collected, meaning if a poorly collected sample is tested, it is possible not enough clinical material was collected from you and your test result will be negative when you are infected. 

“If your Covid-19 test result changes on repeat samples, it does not mean a laboratory error was made but could reflect that the viral load has gone down or could be due to poor sample collection,” it said. 

In July, Dr Marlin McKay, who runs his own medical practice in Roodepoort, advised against getting an antibody test.

He told TimesLIVE the test has no value in terms of diagnosis because it doesn’t determine when or where someone contacted Covid-19.

“It won’t tell you whether your antibodies will finish in the next week or month so it’s a bit of a waste,” he said.

“It can also give you a false sense of security, making you feel like you don’t have to stick to wearing a mask.“

McKay said antibody tests should not be used for diagnosis or to give people a sense of protection.