Should You Trust a Rapid Diagnostic Test?
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You wake up one morning to find that your throat feels like it’s on fire and swallowing is excruciating. Forty years ago, your doctor might have strongly suspected strep throat, but she would have had to send your tonsil swab off to a lab. And you would have had to wait several days for those results.
Nowadays, with a rapid diagnostic test (RDT), she can swab you, get a preliminary diagnosis, and send you on your way with an antibiotic prescription and a treatment plan.
RDTs, also called point-of-care or near-patient tests, are easy-to-do diagnostic tests that use just a small sample of, say, saliva, blood, mucus, or urine and give your doctor results very quickly, no laboratory required.
“The advantage of an RDT is that I’ll have a result in minutes and can start an appropriate treatment while the patient is still in the office,” says Jesse Hackell, M.D., practicing pediatrician at Pomona Pediatrics in Pomona, N.Y., and member of the American Academy of Pediatrics’ committee on practice and ambulatory medicine.
How common are they? A 2019 Roche Molecular Systems–sponsored study, published in BMC Infectious Diseases, found that of 18.8 million strep “events” in the U.S. between 2011 and 2015, some 43 percent were diagnosed by rapid diagnostic tests alone. Worldwide, the market for all RDTs is projected to expand from last year's $28.5 billion to $46.7 billion by 2024, according to the research firm MarketsandMarkets.
At your doctor’s office, you’re most likely to encounter RDTs for strep, influenza, respiratory syncytial virus (RSV), and mononucleosis, but they’re also available for illnesses like pneumonia and malaria. (Some do-it-yourself tests like—home pregnancy kits—are technically RDTs, but home tests are not usually for diagnosing infectious illnesses). Unless an RDT is very new to the market, it's usually covered by insurance.
But are rapid diagnostic tests as reliable as laboratory cultures, the gold standard for diagnosis, which can take up to five to seven days to yield results? Here’s what you need to know.
How Do They Work?
For many RDTs, your doctor only has to put together a couple of reagents—substances designed to cause a chemical reaction—add your sample, and read one line on a dipstick for negative and two for positive. It’s "pretty straightforward,” Hackell says.
Some newer tests employ a technology called PCR. This uses heat to quickly grow the amount of a disease-causing pathogen's DNA in a sample. The machines for doing this cost thousands of dollars so they’re less commonly used than the simple dipstick RDTs.
When Are They Useful?
In developing countries or remote areas, RDTs may be used to diagnose potentially life-threatening diseases like tuberculosis or malaria when no laboratories are available.
But in the U.S., the tests are usually used to give doctors a probable—but not definitive—diagnosis of an infectious illness that might call for swift medical treatment or steps to keep your sickness from spreading to others.
For instance, rapid diagnostic tests for flu may be used in a hospital, where it's easy for a patient with the virus to spread it to others who may be at risk for flu complications like pneumonia. Strep throat not only spreads easily, but if left undiagnosed and untreated, can seem to subside but then develop into serious illnesses like scarlet or rheumatic fever.
How Accurate Are They?
Many types of rapid tests achieve a reasonable rate of accuracy but they aren’t always as on-target as lab cultures, which approach 100 percent.
For instance, the authors of a 2015 French review of 98 studies determined that “we would expect that amongst 100 children with strep throat, 86 would be correctly detected with the rapid test."
PCRs may be more accurate than the simpler rapid tests. In one study, a PCR was able to correctly identify strep between 93 and 99 percent of the time—and to rule it out 90 to 96 percent of the time.
As with any medical tests, RDTs need to be performed properly for the best results, experts say. Food and Drug Administration regulations require that detailed instructions be provided with each test. These include guidance on how to read test results, says Carmen L. Wiley, Ph.D., president of the American Association for Clinical Chemistry. "And they will advise the doctors as to whether doing a culture in the laboratory is recommended or not.”
Usually, a lab culture is also advised, to confirm an RDT’s results. So, when your doctor takes a sample for your RDT, she’ll typically take a second sample to send to a laboratory for a firm diagnosis.
In some cases, when exactly in the course of an illness an RDT is given can also have an impact on its accuracy. For instance, an antibody test taken too early may give incorrect results if you haven't yet developed antibodies (proteins your body creates to combat a pathogen) to a particular condition, according to Hackell.
But in general, “the technology is becoming more accurate and more precise,” says Hackell. RDTs are also becoming easier for doctors to conduct and read. When "we started out with some RDTs for strep something like 30 years ago, they required a fair amount of judgment,” he notes.
Sometimes they still do, say experts, and your healthcare provider may consider more than just the test results. “If you do a flu test in the middle of the summer, when nobody else in the community has the flu, and you get a positive, chances are pretty good it’s a false positive,” says Hackell. “In the middle of flu season, chances are good that it’s a true positive.”
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