NO TEST IS 100% ACCURATE
The COVID 19 RT-PCR (Real time reverse transcription polymerase chain reaction)test:
This(more simply referred to as PCR) test detects SARS-CoV-2 genetic matter(RNA) in the upper and lower respiratory tract specimens. The genetic material is first extracted then larger copies are made making it easier to quantify-therefore even when there is a low viral load the chances of detecting the virus are high.
The specimens are obtained by taking a swab from the back of the nose and throat.
In more complex situations fluid from lung tubes can also be sent for testing.
The test can be done at a designated centre or a test kit sent to you to take the swabs at home then send back to the testing laboratory. Which method is used will depend on the facilities available in your local area.
The PCR test is considered the most accurate .
It takes a minimum of 24 hours to get a result.
The test may give false positive results(people who do not have COVID 19 but test positive) ie it is a very sensitive test and can detect dead virus.
The test may give a false negative result(People who have COVID 19 but test negative) up to 30% meaning it may not give the complete all clear-It may fail to detect some cases.This could be due to poor sampling technique.
You should isolate in the meantime while you wait for your result.
If the test result is positive then you should continue to isolate for at least 10 days from the start of your symptoms, or if you are continuing to have fever until 48hours after the end of the fever(T37.8C and above). If your fever is persisting beyond 14 days it is advisable to seek medical advice.
ACOVID19 PCR test may remain positive for up to 12 weeks . If you are well there is no need to be alarmed or perform another test. Only in a few cases may be due to re-infection and if you develop new symptoms or remain unwell you should seek medical advice.
Lateral Flow COVID 19 Tests
A sample from the back of the nose is placed on an absorbent pad similar to the home pregnancy test.
The sample moves by capillary attraction towards the antibody line fixed on the pad.
The antibodies bind to the SARS-Cov-2 proteins in the sample and show as a coloured line. Indicating a positive lateral flow test result.
The benefit of the lateral flow test is that it can be done at home and results obtained within 15-30 minutes.
The lateral flow test unlike the PCR relies on adequate amounts of virus particles to bind to the antibody on the test pad so can be easily missed when there is a low viral load .
The test is less sensitive compared to the PCR test so if you have symptoms a negative result should be confirmed with a PCR test .Likewise where a negative test is expected eg where there are no symptoms or recent contact a positive test should be confirmed by doing a PCR test.
References :
FDA.GOV summary COVID19 RT-PCR Test
BMJ2021:372;n287 How the UK is using Lateral flow tests
Prolonged SARS-C-CoV-2 RNA shedding Not a rare Phenomenon. Na Li,Xao Wang,Tangfeng Lv First published 29April 2020