Between a lack of access, insufficient supplies, and inconsistent or inaccurate results, testing for SARS-CoV-2, the virus that causes COVID-19, remains a very controversial and confusing issue. Here’s a breakdown of the key concepts based on the evidence we have today.
Terminology (Mahutte, 2019)
There are several terms that you should understand that help describe the accuracy of a test, or its ability to measure what it was designed to measure.
- Sensitivity is the probability that an individual with the disease will test positive; high sensitivity will not miss patients who have the disease and decreases the risk of false negative results.
- Specificity is the probability that an individual without the disease will test negative; high specificity will infrequently provide a positive result in patients who do not have the disease and decreases the risk of false positive results.
An ideal test will be both highly sensitive and specific, yielding very few false positive and false negative results. In addition, the predictive value of a test is the proportion of positive and negative results that can be expected depending on the prevalence of a disease within a population. For example, a patient with a positive test is more likely to truly have the disease if the patient lives in an area, or belongs to a population, with a high prevalence of the disease.
A diagnostic test will indicate whether an individual has a current
, active infection with the SARS-CoV-2 virus. There are two types available – molecular and antigen testing.
Molecular Test (U.S. Food and Drug Administration [FDA], 2020a)
- Molecular tests detect the presence of genetic material of the virus. Examples include:
- Nucleic acid amplification test (NAAT)
- Reverse transcription polymerase chain reaction (RT-PCR) test
- Specimen sources:
- Nasopharyngeal swab (preferred)
- Oropharyngeal swab
- Blood sample
- Stool sample
- The test is reasonably sensitive and specific, but laboratory analysis takes time.
- Positive results are typically verified with a confirmatory test.
- Negative results may be unreliable due to several factors:
- When the sample was obtained; peak viral shedding is unknown and the virus may not start replicating for a few days after the initial exposure, thus a sample collected too soon may not detect the virus (false negative result).
- Poor sampling procedure.
- Incorrect supplies; the swab should be made of Dacron or polyester material and placed in a viral transport medium.
- Delays in delivering the specimen to the lab.
- Molecular testing of respiratory tract samples is the recommended method to identify and confirm COVID-19 cases (World Health Organization [WHO], 2020).
Antigen Test (FDA, 2020a)
- Antigen tests detect the presence of viral proteins and are used in acute or early infection as the antigen is detected when the virus is actively replicating.
- Known as rapid diagnostic tests (RDT), these tests are not as sensitive as molecular tests.
- Positive results are highly accurate.
- Negative results do not rule out infection and may necessitate confirmation with a molecular test prior to making treatment decisions.
- Based on similar RDTs for influenza, the sensitivity may vary from 34% to 80%.
- Over 50% of patients with COVID-19 may be missed
- False positive results may occur if the tests detect other coronaviruses such as those that cause the common cold.
- Factors affecting test reliability include (WHO, 2020):
- Time from onset of illness
- Concentration of virus in the specimen
- Quality of the specimen collected
- How the specimen is processed
- Accuracy of the formulation of reagents in the test kits
- Rapid results within minutes; if ample concentration of the virus is detectable, it will bind to antibodies on the test strip and change color within 30 minutes.
- Can be manufactured at a lower cost than PCR tests.
- May be scaled to test millions of individuals per day due to simple design.
- Specimen sources:
- Nasopharyngeal swab
- Nasal swab
- While the FDA recently issued an emergency use authorization (EUA) for a COVID-19 antigen test (FDA, 2020b), the WHO (2020) does not currently recommend the use of antigen-detecting RDTs for patient care at this time. Based on current evidence, the WHO recommends these tests in research settings only.
An antibody test will indicate whether an individual had a previous
infection even if the individual was asymptomatic.
- Does not indicate if an individual has a current infection and shouldn’t be used to diagnose COVID-19 infection.
- Detects antibodies called immunoglobulins:
- IgM – the body produces these initially during the acute phase of infection; these antibodies last for approximately 2 weeks before levels drop.
- IgG – the body produces these more slowly, within 2-3 weeks after initial infection, but can last for months or years.
- Neutralizing antibodies – inhibit viral replication; their presence correlates with immunity.
- Antibody response will depend on several factors (WHO, 2020):
- Nutritional status
- Severity of disease
- Immunosuppressing medications
- Immunocompromising conditions such as HIV
- Immunity based on the detection of SARS-CoV-2 antibodies is uncertain at this time, however these tests offer some public health advantages:
- Help determine the scope of infection within a population, potential herd immunity, and fatality rate (Centers for Disease Control and Prevention (CDC, 2020a).
- Assist in identifying individuals who had been infected with SARS-CoV-2 and now may qualify to donate convalescent plasma (CDC, 2020a).
- May support the development of vaccines (WHO, 2020).
- Varying false positive and false negative rates.
- The CDC (2020a) recently stated that serologic tests may be wrong “up to half the time” and should not be used to make policy decisions regarding the return to school or the workplace.
Binding Antibody Detection (CDC, 2020a)
Neutralizing Antibody Detection
- Enzyme-Linked Immunosorbent Assay (ELISA)
- Performed on blood sample and sent to a laboratory for analysis
- Detects IgG, IgM separately or combined as total antibody
- Results are more accurate, but require trained laboratory personnel and specialized instruments.
- Point-of-Care RDTs (WHO, 2020):
- Detect IgG or IgG and IgM, or total antibody
- Results are determined within a few minutes.
- Many of these tests have not been validated. The WHO (2020) does not recommend the use of antibody-detecting RDTs for patient care at this time, but supports their use in disease surveillance and epidemiologic research.
- Performed on the following samples:
- Serum, plasma, and whole blood which can be obtained by fingerstick instead of venipuncture
- Saliva, sputum or throat swab
- Determine the ability of antibodies to prevent infection of viruses in vitro.
- Testing outside of research settings is NOT authorized by the FDA at this time.
Who should be tested?
The CDC (2020b) outlined which individuals should be offered priority testing.
- Hospitalized patients with symptoms
- Healthcare facility workers, workers in congregate living settings, and first responders with symptoms
- Residents in long-term care facilities or other congregate living settings, including prisons and shelters, with symptoms
- Individuals with symptoms of potential COVID-19 infection, including fever, cough, shortness of breath, chills, muscle pain, new loss of taste or smell, vomiting or diarrhea, and/or sore throat.
- Persons without symptoms who are prioritized by health departments or clinicians, for any reason, including but not limited to: public health monitoring, sentinel surveillance, or screening of other asymptomatic individuals according to state and local plans.
“These recommendations have been made in the context of a shortage of tests. In an ideal world, repeat testing of all people who reside or work in nursing homes would be a high priority” - Betsy Todd, MPH, RN
How to Improve Testing Reliability (CDC, 2020a)
To minimize false positive results and optimize positive predictive value (95%), clinicians and health care organizations should consider the following:
- Choose tests with high specificity (99.5% or greater)
- Test populations with high prevalence or high pre-test probability of having antibodies (i.e. individuals with a history of COVID-19 symptoms or who are exposed to outbreak areas).
- Utilize a testing algorithm in which individuals who initially test positive are tested with a second, different test.
In an interview with Betsy Todd, MPH, RN, nurse epidemiologist and clinical editor for the American Journal of Nursing,
Ms. Todd recommended seeking tests offered in a hospital-based or public health setting as larger institutions are more likely to have conducted proper vetting to select the best tests. For more information on COVID-19 testing, listen to the full podcast
with Ms. Todd.
What has your experience been with COVID-19 testing? Please share in the comments below.