What percentage of Covid tests are false negative?
COVID-19 Self-testing
Tests that Give Results at Home
Minnesota residents have many COVID-19 testing options. One option is a self-test which may also be called a home test, direct-to-consumer (DTC) test, or over-the-counter (OTC) test.
You can do a self-test anywhere, such as at home. Results are ready fast, often within 15 minutes. Vaccination status does not affect test results. If your test is positive, you should follow current recommendations for how long to stay home and when to wear a mask. Learn more at If You Are Sick or Test Positive.
The following information focuses on COVID-19 tests that are done entirely at home and are not sent to a laboratory.
Tips for doing COVID-19 self-tests
- Follow manufacturer instructions exactly to get an accurate result. Some self-tests are designed to be used in a series (also known as serial testing). The manufacturer’s instructions will tell you if you should repeat the test after a negative result, usually within 24-48 hours of the first test. Getting two or more negative results means that you can feel more sure that you are not infected with the virus that causes COVID-19.
- Go to CDC: Self-testing At Home or Anywhere for information about the type of sample you must collect, how to collect it, and tips on doing the test.
- Use self-tests that have been authorized by the Food and Drug Administration (FDA). Tests authorized for self-testing say «home» in the table column called «authorized setting(s)» at the links below.
- FDA: In Vitro Diagnostics EUAs — Antigen Diagnostic Tests for SARS-CoV-2
- FDA: In Vitro Diagnostics EUAs — Molecular Diagnostic Tests for SARS-CoV-2
- Order Your Free At-Home Rapid Tests
Every home in Minnesota is eligible to order at-home COVID-19 self-tests for free, while supplies last. - Tests for people who are blind or have low vision
New program from the federal government. Administration for Community Living (ACL), to expand the availability of testing options that are more accessible for people who are blind or have low vision. - Self-tests can be purchased at your local pharmacy or other retail stores. If you have health insurance, you may be able to get free tests or be reimbursed for costs of purchasing the tests. Visit CMS: How to get your At-Home Over-The-Counter COVID-19 Test for Free to learn more or ask your local pharmacy or contact your health insurance company for more information.
- Note: Some tests have been granted extensions on their expiration dates, so you may be able to use it for longer than it says on the box.
- You can look for the brand of test you have on the FDA At-Home OTC COVID-19 Diagnostic Tests web page to see if it has an extended expiration date. If it has an extended expiration date, there will be a link to a document showing Lot Numbers and expiration dates. Find the Lot Number printed on the test kit box and then look for that number on the list to see the new expiration date.
Accuracy of self-testing products
Two types of tests are available for self-testing: molecular tests (which detect the virus’ genetic material) and antigen tests (which look for proteins on the surface of the virus). Learn more at Types of COVID-19 Tests.
For both molecular or antigen tests, it is important to follow the manufacturer’s instructions exactly for the best results and the accuracy of the test.
Test manufacturers report that molecular self-tests that give results at home are sensitive and can detect small amounts of virus. A test’s sensitivity is the ability to correctly identify the virus. We will learn more about how well molecular self-tests work as more people use them.
Most self-tests are antigen tests. Antigen tests are not as sensitive as molecular tests, no matter where they are performed (such as home, clinic, pharmacy). Available data shows that if you have COVID-19 symptoms, antigen tests are about 70-80% sensitive. This means that you could have a negative test result even if you are infected with COVID-19. Available data also suggests that if you do not have symptoms, antigen tests are less sensitive. False-positive test results, or a result that says you have COVID-19 when you really do not have it, are rare.
To learn more about available tests, contact the manufacturer or review test information on the FDA websites linked above.
If your test is positive
A positive test means there is evidence of infection with SARS-CoV-2, the virus that causes COVID-19.
If your test is positive, you should:
- Stay home and away from others, even if you do not have symptoms. Follow recommendations at If You Are Sick or Test Positive.
- Tell your close contacts about your test result.
- If you are infected and have symptoms, you can spread COVID-19 to others starting two days before your symptoms start.
- If you are infected and do not have symptoms, you also can spread the disease starting two days before you took the test that showed you were positive for the disease.
- Anyone who was within 6 feet of you for a total of 15 minutes or more over a 24-hour period during that time should follow instructions found at Close Contact or Exposure to COVID-19.
If your test is negative
You can test negative for several reasons, including:
- You are not currently infected with COVID-19.
- You are infected with COVID-19, but the test returned a false negative result. Tests with lower sensitivity are more likely to lead to a false result.
- You took the test too early in your infection, and not enough virus was present for the test to detect it.
- The test was not performed according to instructions.
If you have a negative antigen test result, you should take another test. Multiple negative antigen tests mean you can be more sure that you are not infected with the virus that causes COVID-19.
- The CDC and FDA recommend repeat testing after a negative antigen result whether or not you have symptoms. Follow the manufacturer instructions in your self-test kit for when to take another test. For more details on repeat testing, refer to At-Home COVID-19 Antigen Tests-Take Steps to Reduce Your Risk of False Negative: FDA Safety Communication.
Talk to your health care provider if you have questions about what test to use and when to test. Refer to CDC’s guidance at COVID-19 Testing: What You Need to Know.
If your tests were negative and if you have symptoms and/or fever, you still should stay home until your symptoms are better and until at least 24 hours have passed with no fever, without the use of fever-reducing medication. If you have concerns about your symptoms, talk to your health care provider.
Seek medical care if your symptoms are not getting better or are getting worse.
People who test negative should continue to prevent the spread of COVID-19 by staying up to date on their vaccinations, wearing a high-quality and well-fitting mask when recommended, and following other MDH and CDC recommendations. Follow setting-specific guidance if you work or spend time in settings that are at high risk for COVID-19 spread (for example: health care, long-term care, corrections, shelters) found at Guidance Library: COVID-19.
For more information on testing, refer to CDC: COVID-19 Testing: What You Need to Know.
Reporting self-testing results
- If you spend time in facilities or settings that are at high risk for COVID-19 spread, such as long-term care; health care settings; correctional settings; or homeless shelters, you should report a positive self-test to the facility immediately. Facility management should have communicated to staff and clients the importance of reporting positive self-testing results to the facility or setting.
- If you are taking a self-test and are not connected to a setting such as those listed above, you do not need to report the results of the self-test to local or state public health. If your test result is positive, you should follow current public health recommendations on If You Are Sick or Test Positive.
More information
- COVID-19 Testing
- CDC: COVID-19 Testing: What You Need to Know
Study Raises Questions About False Negatives From Quick COVID-19 Test
People stand in line as they wait to get tested for COVID-19 at a just-opened testing center in the Harlem section of New York on Monday.
The fastest test being used to diagnose people infected with the coronavirus appears to be the least accurate test now in common use, according to new research obtained by NPR.
Researchers at the Cleveland Clinic tested 239 specimens known to contain the coronavirus using five of the most commonly used coronavirus tests, including the Abbott ID NOW. The ID NOW has generated widespread excitement because it can produce results in less than 15 minutes.
But the ID NOW only detected the virus in 85.2% of the samples, meaning it had a false-negative rate of 14.8 percent, according to Dr. Gary Procop, who heads COVID-19 testing at the Cleveland Clinic and led the study.
«So that means if you had 100 patients that were positive, 15% of those patients would be falsely called negative. They’d be told that they’re negative for COVID when they’re really positive,» Procop told NPR in an interview. «That’s not too good.»
Procop says a test should be at least 95% reliable.
Procop, who plans to publish the study soon, chairs the Commission of Science, Technology and Policy for the American Society for Clinical Pathology.
Although the paper has not yet gone through the traditional peer-review process, Procop says the findings have been carefully reviewed at his hospital and he is confident in the results.
In a statement, Abbott defended the test’s reliability.
«ID NOW performs as expected and we have confidence in the performance of the test,» the statement said.
Abbott said any problems with the test could stem from samples being stored in a special solution known as viral transport media before being tested, instead of being inserted directly into the ID NOW testing machine. As a result, the company recently instructed all users to only test samples put directly into the machines.
«When a direct swab is used, the test is performing as expected,» according to the company statement.
Abbott did not provide a scientist to discuss Procop’s study, but instead referred NPR to a doctor in private practice using the machines.
«We feel that the test is very accurate because we’re using it the way it’s supposed to be used» by placing samples directly into the machine, said Dr. Warren Wollin, senior medical director at Physicians Immediate Care, which operates urgent care clinics in Illinois.
«So many patients come in and have so much anxiety because they have minor symptoms. And to be able to tell them right away gives them such relief,» Wollin said.
The Cleveland Clinic’s Procop acknowledged that all of the samples in his study were stored in viral transport media before being tested. But Procop argues the company should back up its claim with data that this could affect the test’s accuracy, especially if the test is being used in settings where people may not be aware of possible false negatives.
«They need to prove to us that they can actually deliver a sensitivity greater than 95% [if samples are placed directly in the machine], or else we’ll be putting citizens at risk,» Procop says.
Based on his study, Procop said his hospital has stopped using the test to screen patients being admitted for care. The hospital also stopped using another test, called the DiaSorin Simplexa, because it only detected 89.3% of infections in his study, Procop said.
«If you have patients coming into hospital and you’re going to put them into what has been determined to be a COVID-free ward, you have to have the most sensitive test available,» Procop says. «Because once you put somebody with COVID into a COVID-free ward, it’s no longer a COVID-free [ward] any more. It’s your new COVID ward.»
The Cleveland Clinic plans to rely on other tests that performed better in the study, Procop said. That includes the test developed by the Centers for Disease Control and Prevention, which detected 100% of positive samples. Another test, made by Roche, detected 96.5% of positive samples. The fifth test in the study, made by Cepheid, detected 98.2% of infected samples, Procop said. The Cepheid test produces results in less than an hour.
Abbott is shipping 50,000 coronavirus tests every day for use on 18,000 ID NOW testing machines in doctors’ offices, clinics and hospitals around the country. The company hopes to boost that to at least 2 million every month by June.
Still, Procop, like Wollin, believes the Abbott test can be useful in many settings, especially when results are needed urgently and as long as doctors advise patients about the potential false negatives. And patients, indeed everyone, still need to be counseled to continue taking precautions to avoid infecting other people, he says.
«It is a risk that if you tell somebody they’re negative and they’re truly positive that they will relax social distancing, not wear a face mask, etc., and could transmit the disease,» Procop says.
«If you get back a negative test and think, ‘I don’t have it and I can go to a mixer,’ » Procop says. «And now everybody at the mixer has now been exposed to you.»
«A high rate of false negatives would definitely be cause for concern,» says Dr. Thomas Inglesby, who runs the Center for Health Security at the Johns Hopkins School of Public Health.
«If that happens, then they wouldn’t be told to isolate themselves, public health wouldn’t seek out or quarantine their close contacts, doctors wouldn’t know what illness they have. Their family members wouldn’t know to protect themselves,» Inglesby wrote in an email.
Other researchers noted that all tests can produce false-negative results, especially if the samples are not collected properly or at a time when a patient doesn’t have readily detectable levels of virus in their bodies.
«All of the tests do miss a number of patients — anywhere from 5 to 30%,» says Dr. Alan Wells, a professor of pathology at the University of Pittsburgh. «So this is not unique to ID NOW.»
And just because someone tests negative one day, doesn’t mean they might not get infected the next day.
«So regardless of whether it’s ID NOW or other tests, we recognize that we cannot say with any assuredness that you are safe or not,» Wells says.
But Wells added that «the ID NOW test seems not to be as effective as» other tests, based on his own research.
In Wells’ study involving specimens from 50 patients, the ID NOW missed more infections than three other tests, especially in samples containing the lowest levels of virus, Wells says.
«The ID NOW missed roughly about half those patients,» Wells told NPR in an interview.
«We do not use ID NOW to rule out a diagnosis of COVID disease, based on the limits of it not picking up as many patients as other platforms that we have,» Wells said. «Our use of ID NOW is put on hold.»
Wells also doubts the false-negatives are due to the specimens being diluted by being stored in fluid first.
«It’s not due to the dilution,» Wells said. «Our evidence is that this is not the sole cause of the lower effectiveness of the ID NOW.»
PCR, antigen and antibody: Five things to know about coronavirus tests
To diagnose and contain the spread of coronavirus, testing is critical. There are two types of Covid-19 tests — those that are designed to detect whether you have the infection now, or those crafted to check whether you have been previously infected by the virus — SARS-CoV-2 — that causes the disease. Like any other product these tests have varying degrees of accuracy and reliability, and can be used to achieve different aims.
18 December 2020
By Natalie GroverThere is a trade-off between speed and reliability when it comes to tests for diagnosing coronavirus. Image credit — Statutární město Hradec Králové/Wikimedia, licensed under CC BY 3.0
We want technologies that are fast, accurate, have high capacity, that don’t require expensive, complex laboratory equipment or the expertise of highly trained people, but there’s nothing which fills all of those criteria at the moment, says Professor Jon Deeks, a biostatistician and testing expert at the University of Birmingham, UK. ‘We haven’t got any perfect test like that, but there are some which are sort of good in some aspects, but not in others.’
Here are five things to know about coronavirus tests:
- PCR and antigen tests are the most common but they work differently
While antigen tests look for proteins on the surface of the virus to ascertain the presence of the pathogen, PCR (polymerase chain reaction) tests are engineered to seek genetic material called RNA that instructs the virus to make these proteins.
Both tests also require a swab from the back of your nose or throat as a sample and cannot determine whether you are contagious if positive but that’s where the similarities end.
In the case of PCR, the sample is sent to a lab where it is heated and cooled using special reagents to convert the virus’s RNA into DNA, and then make millions of copies of the DNA, which allows for the identification of the organism. This process can take hours, requires sophisticated lab equipment and technicians, and is typically done one sample at a time, although there are machines that can process multiple samples. Although the sample needs to be sent to a lab, the time consuming process delivers as the results are almost 100% accurate in spotting infected people when there is virus on the swab.
In contrast, antigen tests — often referred to as rapid tests — work by mixing the sample with a solution that unleashes specific viral proteins. That combination is then applied to a paper strip that contains a bespoke antibody optimised to bind these proteins if they are present. Like a home pregnancy test the result is reflected as a band on the paper strip.
The process doesn’t require a lab, and can be done in up to 30 minutes, but that speed comes at the cost of sensitivity. Although these tests are reliable when an individual has a high viral load, they are far more prone to false-negative results if a person has low amounts of the virus in their body.
- Sensitivity and specificity are measures of a test’s usefulness
These two measures are used to determine the credibility of a test: ‘How well does it detect disease, and how well does it detect the absence of disease,’ explained Prof. Deeks.
Sensitivity is defined as the proportion of patients with Covid-19 who correctly get a positive result, while specificity is the proportion of patients without an infection that the test correctly identifies as negative.
In general, a highly sensitive test has a low false negative rate but runs a risk of false positives if its specificity is not up to scratch. On the other hand, a highly specific test runs a risk of false negatives if the test’s sensitivity is poor, but will generally have a low false positive rate. PCR tests are considered the gold standard because they are generally highly sensitive and highly specific.
- When it comes to rapid tests, the person who administers the test can be crucial
In the UK, an antigen test called the Innova lateral flow test was piloted in the city of Liverpool as part of the government’s plans to carry out mass vaccinations in the country. The aim was to allow workers back into offices and allow families to hug their loved ones again in care homes, Prof. Deeks notes.
But this ‘test to enable’ strategy backfired when scientists found that in a population of mostly people with symptoms the test’s sensitivity fell to about 58% when administered by self-trained staff, versus 73% when tested by skilled research nurses and 79% when tested by laboratory scientists. In a study that looked at people without symptoms, the sensitivity fell to roughly 49% versus PCR tests.
‘So, there’s this gradient of saying, the more experienced people are at doing the test, then less cases will be missed,’ he said. There are some stages that have to be followed very carefully, he says, like reading it accurately. ‘Sometimes it’s hard to tell whether that’s a line or whether it’s a smudge,’ said Prof. Deeks.
PCR tests are processed in the lab, so the potential for error is much lower, he adds.
Test makers are also trying to develop at-home tests but given the lessons we’ve already learned about how rapid test accuracy depends on who administers it, that’s a problem, Prof. Deeks suggests.
‘If people can do tests more easily, then more people will be tested… but I don’t think we’ve got the test to do it yet,’ he said. He adds that there aren’t any good studies looking at what benefit this extra testing would have, for instance, what impact repeated false-negative results could have on behaviour.
In a proposal for common rules on rapid antigen tests published on 18 December, the European Commission said that rapid antigen testing should be carried out by trained healthcare personnel or other trained operators.
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‘If people can do tests more easily, then more people will be tested… but I don’t think we’ve got the test to do it yet.’
Prof. Jon Deeks, University of Birmingham, UK
- Until rapid tests are made more accurate, negative results should not be used to encourage risky activities
If a test, like the Innova test, is missing up to half the cases then nobody can really be considered free of the risk of having or transmitting infection, Prof. Deeks notes.
‘You’re always going have a small percentage of those people who are missed by all tests,’ said Gary Keating, chief technology officer of HiberGene, an Ireland-based company that has a Covid-19 test. The test utilises LAMP technology, which is a low-cost alternative to PCR technology.
‘I think it’s always dangerous to taking a single diagnostic test in isolation, and use that as the basis to go and make a very significant medical or lifestyle decision,’ said Keating.
Used on a large scale, results could lead to false sense of security, according to Prof. Deeks.
Governments are keen to use rapid tests as they are cheaper and faster to deploy for mass vaccination campaigns, but because they have limitations in terms of accuracy then it is key to not use negative results to enable more risky activities such as meeting your elderly or vulnerable loved ones, he says.
Some countries, like the United States, advise getting a PCR test if individuals with symptoms test negative with a rapid antigen test to confirm the result.
Although rapid tests are good at catching people who have high viral loads, it isn’t yet clear what the viral threshold must be to thwart contagiousness. With Covid-19, those that are infected see a peak in viral load in the early phase of the infection, but viral RNA can stick around for weeks or even months.
- Antibody tests could be useful in measuring the durability of vaccine responses
Antibodies are soldiers deployed by the immune system in response to a foreign invader — in this case SARS-CoV-2. ‘Originally, there was hope that antibody tests might enable us to diagnose disease quickly and easily. But it turns out, the tests don’t turn positive for two to four weeks (post infection),’ said Prof. Deeks.
And the plot thickens, because even if you test positive for antibodies — that information doesn’t tell you much, apart from the high likelihood that you had contracted Covid-19 in the past.
‘We don’t really know what levels of antibodies leads to protection (from disease) and indeed, which kind of antibody is the most important — I’m not sure whether there’s consensus on that either,’ he said.
It is also unclear how long Covid-19 antibodies persist in the body, or that anybody who has tested positive for antibodies won’t catch the virus again.
Where these tests could come handy is in estimating the spread of Covid-19 at a population level — for instance, what percentage of the population and what ethnic groups have contracted Covid-19, as well as in measuring the durability of vaccine responses, Prof. Deeks adds.
HiberGene’s coronavirus test was one of the initial 18 emergency projects funded by the EU. If you liked this article, please consider sharing it on social media.
