And what the results mean
We know that the best thing to do to curb the spread of coronavirus is to continue testing to identify and isolate people in a timely way. This is how countries like South Korea and China got the virus under control -- and are now seeing some of the consistently lowest rates of new cases.
There is a lot of confusion and panic among us when it comes to testing and, even more so, what test results mean. All over the world, there have been reports of people getting different results from different labs for the Covid-19 test, or results that do not match the symptoms people are experiencing. All of this -- combined with how our everyday lives are already disrupted -- is undoubtedly overwhelming.
The science is not perfect
It is important to acknowledge and understand that the science of diagnostic tests is not perfect, especially for a new virus like SARS-CoV-2, the virus that causes Covid-19. No test is 100% accurate even at the best labs in the world, and like any other diagnostic test, the RT-PCR tests for Covid-19 may show some discrepancies in its results.
There is a chance that someone will have the virus but will test negative (false negative) or, less likely, will not have the virus but still test positive (false positive). As an immunologist with 30 years of experience working in laboratories, I understand the frustration that this can cause -- and I hope to share what I know and help clear up confusion that many Bangladeshis are feeling.
What counts as false-positive and false-negative?
Positive results mean that the laboratory machines detect genes of the coronavirus but this alone is not enough to determine whether a patient is infected. That’s because contracting the virus and having it cause Covid-19 disease depends on how much of the virus is in your body (called “viral load”) and an individual’s ability to clear the virus naturally. FDA guidelines state: “Laboratory test results should always be considered in the context of clinical observations and epidemiological data in making a final diagnosis and patient management decisions.”
Lab professionals have to look at the clinical correlation with patient history and other diagnostic information to determine the patient’s infection status. We do know, however, that there is a very small chance that a test can give a false-positive result -- so if you test positive, you should definitely treat it as so, and take the necessary precautions.
On the other hand, a false negative happens when a test says someone does not have a condition when they actually do. Negative results do not preclude SARS-CoV-2 infection, nor should they be used as the sole basis for patient management decisions. Negative results must be combined with clinical observations, patient history, and epidemiological information to determine the patient infection status. A review of global data regarding the accuracy of Covid-19 tests showed false-negative rates of between 2% and 29%. One report found that even a week after infection, one in five people who had the virus had a negative test result.
Why do false-positive and false-negative results happen?
Whether or not you could get a false negative or positive rate depends on many factors, including the time between exposure and getting tested.
For example, if you are tested too soon, your body may not have enough of the virus yet, which could cause a negative result one day, and a positive result the next. Studies estimate that it takes about five days for most people to develop symptoms once they’re exposed, and almost all people who develop symptoms do so within about 11 days.
Ideally, samples should be collected near the time that symptoms appear to achieve the highest likelihood that a true negative will be negative and true positive will be positive. Samples being stored too long could also give out false results since samples taken from patients have limited stability over time. If extended storage, transport, or handling causes a long delay before the sample is tested, it can be harder to detect the virus, leading to a false-negative result. Patients who are infected but not yet symptomatic may have false-negative test results, as may those whose symptoms are waning.
A false-positive test result could arise from RT-PCR test with low specificity. Specificity is based on the ability of the test to recognize the unique genetic sequence of SARS-CoV-2 (the virus that causes Covid-19) and not recognize other virus sequences. A test that is 100% specific means all healthy individuals are correctly identified as healthy.
A recent article in BMJ calculates that a test specificity of 95% could generate a 1% chance of a false positive result. Most tests used these days, however, have specificities of 98%-100% and chances of false positives are very unlikely.
Because many people carrying Covid-19 are asymptomatic, in many cases, when they test positive, they may doubt the result; but because the false positive rate is quite low, this result should be taken seriously and discussed with your doctor. It is also possible to test positive one day and negative the next day -- this does not necessarily mean the first test was a false positive; it might mean that the virus has passed through your body already.
How am I supposed to take correct action if I don’t know whether my test is a false negative or false positive?
A single negative test should not be used to rule out infection; it is a piece of the puzzle that doctors use to help you make decisions about your health. For example, if you have symptoms consistent with Covid-19 or recently travelled to a Covid hotspot, a doctor may still recommend isolating or taking certain protective measures -- even if you test negative.
The chances of a false positive are very low -- and if your swab test comes back positive, then your doctor will work with you to determine the best next steps based on your medical history, travel history, and symptoms.
While not a silver bullet, the CDC and WHO and global health community strongly advise that diagnostic testing is still one of the most important ways we can prevent the spread of Covid-19. The best way to manage your symptoms or prevent further spreading the virus is for you to get tested and then talk with your doctor about what the results mean for you.
For various reasons, there has always been rampant distrust in the Bangladeshi health care system, and now in the face of Covid-19, it continues to expand exponentially. To ensure that labs are providing the highest standards of care and accuracy of their lab results that patients can trust, they should participate in External Quality Assessment (EQA).
It is an essential aspect of any laboratory operation and is designed to monitor laboratory performance. Ideally, all labs should go through the process of getting international accreditation, including from the College of American Pathologists -- the leading international organization advancing excellence in laboratory medicine.
Praava is going through the process of accreditation from the College of American Pathologists and has been participating in Randox International Quality Assessment Scheme, or RIQAS, the world’s largest external quality assessment (EQA) scheme since November 2018 with an average accuracy score of 99.9%.
We strongly encourage all other labs to do this as well to hold all of us accountable and ensure all Bangladeshis have access to the highest standard of world-class health care.
Since the first case of Covid-19 in December 2019 in Wuhan, China, the novel coronavirus has rapidly spread across the world and has been continuing to gravely impact our lives. Globally, medical and scientific communities are learning new things about this virus almost daily. And the brightest minds are at work researching new medicines, treatment plans, and researching vaccines, to help fight this new virus.
As we continue to learn more about the virus and develop new tools to detect, treat, and prevent it, we have to accept this new normal and continue seeking out testing -- as well as social distancing as much as possible and taking all precautionary measures, to ensure that we are not being the cause behind further transmission of this novel coronavirus.
Zaheed Husain is Senior Lab Director, Praava Health, Retired Faculty, Department of Medicine, Harvard Medical School, Retired Senior Scientist, Division of Medicine, Beth Israel Hospital, Boston.