Table of COVID-19 Test Types (12/1/20)
(UPDATED) Antigen Test Results and Next Steps Handouts (2/22/21)
VDH Interim Guidance for COVID-19 Testing (12/14/20)
SARS-CoV-2 Testing Capabilities at Commercial and Private Labs ( no longer updated as of 1/11/21)
Visit the VDH FAQ Page and search for Healthcare Providers
Visit VDH FAQs and search Healthcare Providers: Treatment for COVID-19
On this page:
Who to test and how to prioritize if testing is limited
Where to test
Test selection and interpretation of results
Report all cases to VDH
Educate patients on when is it safe to be around others
- Billing and reimbursement
- Working with employers
- Emerging SARS-CoV-2 variants
- Epidemiology terminology for test performance
Interim COVID-19 Testing Guidance for Healthcare Providers
a) Test all patients with symptoms consistent with COVID-19.
Immediately test patients with COVID-19 symptoms. Testing informs patient management and is a critical first step in identifying and isolating people with COVID-19, quarantining their close contacts, and preventing community spread.
Children have similar symptoms as adults, but generally have milder illness. For more information on the clinical presentation and course among children, see Information for Pediatric Healthcare Providers. A new syndrome associated with COVID-19 has been described called multisystem inflammatory syndrome in children (MIS-C). Patients aged <21 years might have fever, laboratory evidence of inflammation, and evidence of clinically severe illness with multisystem organ involvement, requiring hospitalization. Healthcare providers should immediately report any patient who meets the MIS-C case definition to the local health department. With symptoms similar to MIS-C, multisystem inflammatory syndrome in adults (MIS-A) might also occur in adults aged 21-50 but is very rare. For a visual, refer to this algorithm ‘VDH Algorithm for Evaluating a Child with COVID-19 Symptoms or Exposure.’
Older adults and persons with other medical conditions might have delayed presentation of fever and respiratory symptoms, and older persons might have new confusion, delirium or falls.
Clinicians are encouraged to consider testing for other causes of respiratory illness, like influenza, in addition to testing for SARS-CoV-2, depending on patient age, clinical presentation, season, or clinical setting.
Counsel your patient on CDC’s ‘3 Key Steps to Take While Waiting for Your COVID-19 Test Result.’
b) Test people who have had close contact with someone with known COVID-19.
VDH recommends COVID-19 testing for people who have had close contact with someone known to have COVID-19. This recommendation applies regardless of whether the person with COVID-19 had symptoms or not.
- Being within 6 feet of a person who has COVID-19 for a total of 15 minutes or more over a 24-hour period, or
- Having direct exposure to respiratory secretions (e.g., being coughed or sneezed on, sharing a drinking glass or utensils; kissing), or
- Providing care for a person who has COVID-19, or
- Living with a person who has COVID-19.
Close contacts who have symptoms should be tested immediately. For close contacts without symptoms, it is best to test them five days or more after their last exposure.
c) Prioritize testing when personal protective equipment (PPE) and tests are in limited supply.
If testing is not readily available, prioritize testing of symptomatic people (including close contacts), those at increased risk for severe COVID-19, and select asymptomatic people listed below
- Medical purposes
- Women who are pregnant and present in labor; additional considerations here.
- Neonates born to women with confirmed or suspected COVID-19; additional considerations here.
- People undergoing aerosol-generating procedures, within 48 hours of procedure, with limited PPE at the facility; additional considerations here.
- People undergoing major time-sensitive surgeries, within 48 hours of procedure; additional considerations here.
- Immunocompromised people admitted to the hospital; additional considerations here.
- People undergoing immunosuppressive procedures; additional considerations here.
- Contact investigations
- People who have had close contact with a person with lab-confirmed COVID-19, including exposed healthcare personnel; additional considerations here.
- People who work or reside in a nursing home; additional VDH guidance here.
- People who work or reside in a congregate setting other than a nursing home that is experiencing an outbreak.
- People who have taken part in activities that put them at higher risk for COVID-19 because they cannot physically distance as needed, such as travel, participating in close contact sports, attending large social or mass gatherings, or being in crowded indoor settings.
- People who have been asked or referred to get testing by their healthcare provider or local health department.
Print and display ‘VDH COVID-19 Testing Algorithm’ in your healthcare facility or office.
Print and display VDH’s ‘COVID-19 Testing Site Signs’ (see Testing and Laboratory Resources here).
Verify with your diagnostic laboratory which specimens are appropriate for the tests they offer. Follow the directions on the laboratory’s test menu for specific specimen collection instructions, submission forms, and shipping requirements.
For initial diagnostic testing for SARS-CoV-2, collecting and testing one of the following upper respiratory specimens is typically recommended (see table below). To conserve PPE, consider diagnostic tests that use specimens that can be collected using minimal PPE. These include a patient self-collected nasal swab. VDH developed a brief video: Patient Self-Collecting Nasal Swab for COVID-19. This VDH infographic is a resource for patients about what to expect when they have a specimen taken with a nasopharyngeal (NP) swab as part of a COVID-19 test. The New England Journal of Medicine also has a brief video: How to Obtain a Nasopharyngeal Swab Specimen.
The Virginia Emergency Support Team (VEST) has coordinated with Amazon to provide a portal for healthcare professionals to purchase PPE in varied amounts and types at a reasonable price.
Table 1. General guidance for the preferred types of specimen collection*
|Nasopharyngeal (NP) collected by healthcare professional (HCP)||N95 or higher-level respirator (or face mask if a respirator is not available), eye protection, gloves, and a gown||Flocked, synthetic fiber mini-tip swabs with plastic or wire shafts||Tilt the patient's head back slightly so that nasal passages are more accessible. Insert minitip swab through the nostril parallel to the palate (not upwards) until resistance is encountered or the distance is equivalent to that from the ear to the nostril of the patient, indicating contact with the nasopharynx. Swab should reach depth equal to distance from nostrils to outer opening of the ear. Gently rub and roll the swab. Leave swab in place for several seconds to absorb secretions. Slowly remove swab while rotating it.|
|Nasal mid-turbinate swab (Deep Nasal Swab) collected by HCP||N95 or higher-level respirator (or face mask if a respirator is not available), eye protection, gloves, and a gown||Flocked tapered swab||Tilt patient’s head back 70 degrees. While gently rotating the swab, insert swab less than one inch (about 2 cm) into nostril (until resistance is met at turbinates). Rotate the swab several times against nasal wall and repeat in other nostril using the same swab.|
|Nasal mid-turbinate swab (Deep Nasal Swab) collected by patient while medically-supervised with HCP located >6 feet away||Standard precautions, gloves, universal source control (e.g., facemask or cloth mask) at all times in healthcare facilities||Flocked tapered swab||Patient should tilt head back 70 degrees. While gently rotating the swab, insert swab less than one inch (about 2 cm) into nostril (until resistance is met at turbinates). Rotate the swab several times against nasal wall and repeat in other nostril using the same swab.|
|Nasal swab (anterior nares) collected by HCP||N95 or higher-level respirator (or face mask if a respirator is not available), eye protection, gloves, and a gown||Use flocked or spun polyester swab||Insert the swab at least 1 cm (0.5 inch) inside the nostril (naris) and firmly sample the nasal membrane by rotating the swab and leaving in place for 10 to 15 seconds. Sample both nostrils with the same swab.|
|Nasal swab (anterior nares) collected by patient while medically-supervised with HCP located >6 feet away||Standard precautions, gloves, universal source control (e.g., facemask or cloth mask) at all times in healthcare facilities||Use flocked or spun polyester swab||Patient should insert the swab at least 1 cm (0.5 inch) inside the nostril (naris) and firmly sample the nasal membrane by rotating the swab and leaving in place for 10 to 15 seconds. Sample both nostrils with the same swab.|
|Nasopharyngeal wash/aspirate or nasal wash/aspirate (NW) collected by HCP||Use catheter attached to suction apparatus||Have the patient sit with head tilted slightly backward. Instill 1 mL-1.5 mL of non-bacteriostatic saline (pH 7.0) into one nostril. Insert the tubing into the nostril parallel to the palate (not upwards). Catheter should reach depth equal to distance from nostrils to outer opening of ear. Begin gentle suction/aspiration and remove catheter while rotating it gently. Place specimen in a sterile viral transport media.|
|Saliva collected by HCP||N95 or higher-level respirator (or facemask if a respirator is not available), eye protection, gloves, and a gown||Leak-proof screw cap container||Collect 1-5 mL of saliva in a sterile, leak-proof screw cap container.|
|Saliva collected by patient while medically-supervised with HCP located >6 feet away||Standard precautions, gloves, universal source control (e.g., facemask or cloth mask) at all times in healthcare facilities||Leak-proof screw cap container||Collect 1-5 mL of saliva in a sterile, leak-proof screw cap container.|
*Oropharyngeal (OP) swabs should only be collected if nasal or NP is not available. HCP collecting OP swabs should wear the same PPE as used when collecting NP swabs. Testing lower respiratory tract specimens is also an option. For patients who develop a productive cough, sputum can be collected and tested when available for SARS-CoV-2. However, the induction of sputum is not recommended. Under certain clinical circumstances (e.g., those receiving invasive mechanical ventilation), a lower respiratory tract aspirate or bronchoalveolar lavage specimen should be collected and tested as a lower respiratory tract specimen.
For more information, read CDC Guidelines for Clinical Specimens.
In general, healthcare providers should send specimens for COVID-19 testing to commercial, private, or hospital laboratories. VDH recommends healthcare providers work with laboratories that use viral tests granted an Emergency Use Authorization (EUA) by FDA, or use viral tests offered under the policies in FDA’s Policy for COVID-19 Tests. Healthcare providers should use caution when working with high complexity laboratories that are performing tests that have not received an EUA. In this instance, it is suggested that the healthcare provider obtain performance data from the manufacturer or laboratory.
Many community sites are conducting specimen collection and either performing point-of-care (POC) tests or sending specimens to a private or reference laboratory for testing. To find a site for your patient, visit Virginia COVID-19 Testing Sites. Each site has different policies and procedures for testing and billing.
At-home testing can also be considered if testing at a local healthcare facility or community testing event is not available, or for homebound people. At-home collection kits can be used by people to collect a specimen (nasal swab, saliva) by themselves at home. Home collection kits typically come with printed instructions on specimen collection, or the instruction material may be available online. The specimen can either be sent to a testing facility or tested at home. As of this writing, all home collection and/or testing kits are prescription only items, except one. Some tests may require a health assessment. For more information on at-home testing visit the website CDC At-Home Testing.
For certain situations that meet VDH prioritization criteria, send specimens to Virginia’s public health laboratories. Clinicians may request public health testing for these scenarios by contacting their local health department. Submitters using public health testing must be willing and capable of collecting and packaging specimens, and either delivering them to a Division of Consolidated Laboratory Services (DCLS) courier site or shipping them via commercial courier (e.g., FedEx).
Viral tests (molecular-based test or antigen test) are recommended to diagnose acute infection. These tests check samples from the respiratory system or saliva to determine whether COVID-19 is present.
Molecular-based tests, such as PCR tests, detect the virus’s genetic material.
Table 2. Molecular-based test (e.g., PCR) result interpretation for symptomatic and asymptomatic persons
|Test Result||Person being Tested|
(test as close to symptom onset as possible and as recommended by manufacturer)
|Asymptomatic Person with Close Contact‡ to a known COVID-19 case||Asymptomatic Person without Close Contact‡ to a known COVID-19 case|
§ If the person had a known exposure to someone with COVID-19, the person should continue to quarantine until 14 days after the last known exposure. VDH and CDC continue to recommend a 14-day quarantine period as the safest option. If the person cannot stay home for the full 14 days after exposure and does not have symptoms, the person may end quarantine earlier. Counting the date of last exposure as Day 0, the person may leave home after Day 10 without testing, or after Day 7 with a negative PCR or antigen test performed on or after Day 5. If the person does not stay home for the recommended 14 days, the person should continue monitoring for symptoms and follow all other recommendations (e.g., wear a mask, watch their distance, and wash hands often) for the full 14-day period after the last exposure. If the person did not have a known exposure to someone with COVID-19, the person should isolate until at least 24 hours after symptoms resolve (if symptomatic), or follow protective measures in place in the community (if asymptomatic).
‡ Close contact
⥉ A negative molecular test result for SARS-CoV-2 means that at the time of collection, RNA from this virus was not present in the specimen at or above the limit of detection.
- A positive molecular test result indicates current infection.
- Caveats: A positive molecular test does not necessarily mean viable virus is present (i.e., that the person is infectious). Viral RNA can persist in the respiratory tract. Immune competent persons with a previously positive molecular test who have a repeat positive molecular test within 90 days are not subject to re-isolation provided they meet the symptom-based or time-based criteria. Persons who are severely immunocompromised should consult with their healthcare provider in addition to meeting the test-based criteria. These persons might actually have prolonged viral shedding and be infectious longer than 10 days after onset (or specimen collection date). For more information from CDC about persistently positive molecular results, see here.
- A negative molecular test result indicates RNA was not detected at the time of testing.
- Caveats: Virus might not have been detected because the person is not currently infected, the virus caused the illness but is no longer detectable or present in the upper respiratory tract, or an inadequate specimen was collected. For symptomatic people with a negative molecular test, clinician judgment is also important. If the clinician feels COVID-19 is the most likely diagnosis, the patient should be treated accordingly. While uncommon, false negative molecular tests occur.
- For at-home testing, an invalid result or a test error indicates that the test did not work properly. Please refer to the instructions in the package insert and contact the manufacturer, if needed, for assistance.
An antigen test is another type of viral test. It detects the presence of SARS-CoV-2 nucleocapsid protein antigen. Antigen is generally detectable in upper respiratory specimens during the acute phase of infection. Antigen tests are generally less expensive than molecular tests and can be used at the point of care; however, they are generally less sensitive than molecular tests, and molecular tests remain the “gold standard” for detection of SARS-CoV-2.
If using an antigen test, select one that has been authorized by the FDA under an EUA and ensure proper interpretation of results for accurate clinical management of patients with suspected COVID-19, or for identification of potentially infected persons when used for screening. As of this writing, 13 antigen tests have received FDA EUAs. Nine of these are prescription items that have a point-of-care test option, and are CLIA waived. One antigen test is available as an over-the-counter product without a prescription. The remaining three tests are meant to be done in a medium or high-complexity lab. All antigen tests have language in their EUAs stating the test is meant for use in symptomatic individuals who are suspected of having COVID-19.
Laboratory and testing professionals who conduct diagnostic or screening testing for SARS-CoV-2 with antigen tests must comply with Clinical Laboratory Improvement Amendments (CLIA) regulations. Any laboratory or testing site that intends to report patient-specific test results must first obtain a CLIA certificate and meet all requirements to perform that test. For more information, see the Centers for Medicare & Medicaid Services’ (CMS) summary of the CLIA regulations. For more information from CDC about antigen testing, see here. For more information on antigen testing in Virginia, view VDH’s Antigen Testing Recommendations.
Table 3. Antigen test result interpretation for symptomatic and asymptomatic persons
|Test Result||Person being Tested|
(test as close to symptom onset as possible and as recommended by manufacturer)
|Asymptomatic Person with Close Contact# to a known COVID-19 case||Asymptomatic Person without Close Contact# to a known COVID-19 case|
§ If the person had a known exposure to someone with COVID-19, the person should continue to quarantine until 14 days after the last known exposure. VDH and CDC continue to recommend a 14-day quarantine period as the safest option. If the person cannot stay home for the full 14 days after exposure and does not have symptoms, the person may end quarantine earlier. Counting the date of last exposure as Day 0, the person may leave home after Day 10 without testing or after Day 7 with a negative PCR or antigen test performed on or after Day 5. If the person does not stay home for the recommended 14 days, the person should continue monitoring for symptoms and follow all other recommendations (e.g., wear a mask, watch their distance, and wash hands often) for the full 14-day period after the last exposure.
# Close contact
* A positive antigen test result in an asymptomatic, unexposed individual should be immediately followed by a PCR test run in a high-complexity CLIA-certified laboratory to verify the positive result. This follow-up specimen should be collected within 24 hours of the original test, if possible, and no more than 48 hours after the antigen test. Specimens collected more than 48 hours after the initial test may lead to discordant results. If the confirmatory PCR is negative on an appropriate specimen collected in the proper time frame, and the individual has remained asymptomatic, the antigen test would be considered a false positive and the individual not counted as a COVID-19 case.
+ While multiple specimen types may be acceptable, if possible, confirmatory tests should be performed using specimens with evidence of the most sensitivity, such as nasopharyngeal or mid-turbinate swabs.
- A positive antigen result indicates current infection or presumptive current infection.
- Caveats: False positives can occur, especially in areas of low COVID-19 prevalence. Results should be interpreted in the context of disease prevalence, the test’s performance characteristics and instructions for use, and the patient’s clinical signs, symptoms, and history.
- A negative antigen result indicates no antigens were detected at the time of testing.
- Caveats: Studies have shown that antigen levels in some patients who have been symptomatic for more than five days might drop below the limit of detection of the test. This might result in a negative test result, while a more sensitive test, such as a molecular test, might return a positive result. It might be necessary to confirm a rapid antigen test result with a molecular test, especially if the result of the antigen test is inconsistent with the clinical context. When confirming an antigen test result with a molecular test, it is important that the time interval between the two sample collections is less than two days, and there have not been any opportunities for new exposures between the two tests. If more than two days separates the two tests, or there have been opportunities for new exposures between the two tests, the molecular test should be considered a separate test – not a confirmatory test.
For 1-page patient handouts about what antigen test results and next steps, see the ‘VDH Antigen Testing Results and Next Steps Handout.’
Cycle Threshold (Ct) Values for PCR Tests
Ct stands for “Cycle Threshold.” Ct is a number generated during a PCR test. The Ct number refers to the number of cycles needed for a sample to grow and cross a cutoff point (threshold) where it changes from negative (not detectable) to positive (detectable).
Ct values and cutoffs differ by test and cannot be compared from one test to another. Some PCR tests also do not use Ct values but use a different value instead to report the test being positive or negative.
PCR positive results and the Ct values cannot determine infectiousness. More research is needed to determine the amount of virus necessary to infect someone else. Viral load changes drastically over the course of infection.
For more information on Ct values, refer to the CDC Lab FAQs for Interpreting Results of Diagnostic Tests and the Association of Public Health Laboratories (APHL) resource: ‘Ct Values: What They Are and How They Can be Used.’
Serology, or antibody tests, are NOT recommended for making a diagnosis of acute COVID-19 infection. Serology tests identify the body’s immune response to exposure to SARS-CoV-2. These tests could be useful to determine how widespread infection has been in different populations, to identify people with antibodies who can donate plasma, for serosurveys in populations to determine true infection rates, and to learn more about the antibody response to SARS-CoV-2 and its relationship with symptom history. Confirmed and suspected cases of reinfection of the virus that causes COVID-19 have been reported, but remain very rare. For more information, see Infectious Diseases Society of America Guidelines on the Diagnosis of COVID-19: Serologic Testing.
- A positive antibody result indicates the person might have been infected with the virus at some point in the past.
- Recommendation: Test with PCR or antigen test if clinically indicated.
- Caveat: Detection of antibodies does not mean the person is infectious or immune to SARS-CoV-2. A false positive is possible if the test cross-reacts with commonly circulating coronaviruses.
- A negative antibody result means that antibodies to SARS-CoV-2 were not detected in serum.
- Recommendation: Retest if antibody status is needed, or test with PCR or antigen if clinically indicated.
- Caveat: This could indicate that the person has not been infected, that antibodies had not developed at the time of specimen collection, antibody levels are too low for the test to detect, or the immune system has not mounted a response to the exposure (e.g., because of immunosuppression).
Refer to the ‘VDH COVID-19 Testing Algorithm’ for a visual overview of molecular and antigen testing. Get helpful advice on informing your patients of COVID-19 test results. Provide additional guidance and education to patients with confirmed or suspected COVID-19.
Refer to the ‘VDH Table of COVID-19 Test Types’ for a comparison chart of frequently asked questions about PCR, antigen, and antibody tests.
Clinicians should utilize the Confidential Morbidity Portal to report suspected and confirmed COVID-19 cases as soon as they are identified. Please include symptoms and other critical epidemiologic fields, including race and ethnicity, in your report.
VDH has developed a reporting portal specifically for point-of-care (POC) COVID-19 test results. This portal will assist testing sites in meeting the requirement of the CARES Act to report every diagnostic and screening test performed to detect SARS-CoV-2 or to diagnose a possible case of COVID-19 (e.g., molecular, antigen, antibody). This portal allows the rapid entry of person-level test results for positive and negative results, and provides the ability to enter aggregate negative results as necessary for high-volume testing sites. All COVID-19 test results should be reported to VDH within 24 hours. Testing sites should go here to register for the POC Reporting Portal.
All COVID-19 reports from clinicians or facilities based on point-of-care tests should be reported using the VDH Point-of-Care (POC) Portal, not the Confidential Morbidity Portal. For other reports not involving POC testing, healthcare providers should continue to report patients with suspected or confirmed COVID-19 through the Confidential Morbidity Portal.
Ending isolation and quarantine: when is it safe for my patients to be around others?
People with a positive COVID-19 viral test and are symptomatic may be around others after:
- At least 10 days have passed since symptoms first appeared, and
- At least 24 hours with no fever without fever-reducing medication, and
- Other symptoms have improved.
People with a positive COVID-19 viral test but who never had any symptoms may be around others after 10 days have passed since the first positive diagnostic test.
In general, retesting for COVID-19 is not recommended for 90 days after the first positive viral test. For people who are severely immunocompromised, a test-based strategy could be considered in consultation with an infectious diseases expert.
Close contacts who do not have symptoms and who test negative for COVID-19 on a viral test (PCR or antigen test) should still complete the full 14-day quarantine. VDH and CDC continue to recommend a 14-day quarantine period as the safest option.
If the person cannot stay home for the full 14 days after exposure and does not have symptoms, the person may end quarantine earlier. Counting the date of last exposure as Day 0, the person may leave home after Day 10 without testing or after Day 7 with a negative PCR or antigen test performed on or after Day 5. If the person does not stay home for the recommended 14 days, the person should continue monitoring for symptoms and follow all other recommendations for the full 14-day period after the last exposure.
In general, retesting asymptomatic people within 90 days of their initial COVID-19 infection by a viral test is not recommended. People with COVID-19 can have persistently positive PCR tests for many weeks after their infection, even though they are not infectious. Although there have been limited reports of people with confirmed COVID-19 reinfection, the data collected so far indicate that reinfection within 90 days is not likely to occur. For people who are severely immunocompromised, a test-based strategy could be considered in consultation with an infectious diseases expert.
For people who have recovered from COVID-19 but subsequently develop new COVID-19 symptoms within 90 days of their initial infection, retesting can be considered if there is no alternate diagnosis. For additional information, see CDC’s recommendations here.
Payment is available to healthcare providers to counsel patients, at the time of COVID-19 testing, about the importance of self-isolation after they are tested and prior to the onset of symptoms. Reimbursement for providers who are eligible to bill CMS for counseling services will be handled using existing evaluation and management payment codes.
The COVID-19 Uninsured Program Portal was launched by the federal government to allow healthcare providers who have conducted COVID-19 testing or provided COVID-19 treatment for uninsured persons on or after February 4, 2020 to submit claims for reimbursement. Providers can access the portal at COVIDUninsuredClaim.HRSA.gov.
VDH does not recommend that employers require test results or a healthcare provider’s note to excuse them from work, qualify for sick leave, or allow return to work. For more information see, the VDH Algorithm for Evaluating Non-Critical Infrastructure Workers with COVID-19 Symptoms or Exposures here and VDH Graphic on Ending Isolation in Non-Healthcare Settings here.
Multiple SARS-CoV-2 variants that cause COVID-19 have been documented during this pandemic. Currently, several variants are circulating globally. These new variants seem to spread more easily and quickly than non-variants. Research is ongoing to assess other variant characteristics, such as disease severity, the impact on vaccine-induced immunity, and the impact on diagnostic tests.Most currently available PCR tests will still detect the virus, even if there is a mutation.
The variant that first emerged in the United Kingdom in 2020 (B.1.1.7) has been identified in many U.S. states, including Virginia. At least one preliminary report suggests the possibility of increased disease severity with this variant, but additional studies are ongoing. The variant that first emerged in South Africa in 2020 (B. 1.351) has also been identified in several U.S. states, including Virginia. VDH continues to work with laboratory partners to study new COVID-19 variants to track their spread and ensure these are detected with the available viral tests.
Please ask patients suspected of having COVID-19 about recent travel to and from areas outside the United States with known circulating variant strains. If you diagnose COVID-19 by nucleic acid or antigen test in a person who traveled to one of these areas in the 14 days before symptom onset (or specimen collection, if asymptomatic), please contact your local health department.
- Sensitivity: measures the ability of a test to correctly identify persons who have COVID-19. Calculated by: [a/a+c]*100
- Specificity: measures the ability of the test to correctly identify persons who do not have COVID-19. Calculated by: [d/b+d]*100
- PCR – reagents used for SARS-CoV-2 test react with non-SARS-CoV-2 organisms by either actual lab testing or by theoretical sequence analysis
- Antigen – reagents react with related pathogens, high prevalence organisms or normal flora likely to be encountered in a clinical specimen
- Serology - detection of other related antibodies (e.g., produced by past infection with other coronaviruses) which would produce a false positive result. Researchers (Gorse et. al) estimate that 90% of persons age 50 or older have antibodies to common non-SARS-CoV-2 human coronaviruses HKU1, NL63, OC43, or 229E.
- Pretest probability: Probability of a patient having an infection before the test result is known; based on the proportion of people in a community with the disease at a given time (prevalence) and the clinical presentation of the patient.
- Prevalence: proportion of population that have been infected with SARS-CoV-2.
- Positive predictive value (PPV): probability a person with a positive SARS-CoV-2 test result actually has SARS-CoV-2 and was infected. Calculated by: [a/a+b]*100; PPV goes up as prevalence increases and down as prevalence decreases
- Negative predictive value (NPV): probability a person with a negative SARS-CoV-2 test result does not actually have SARS-CoV-2 and was not infected. Calculated by: [d/c+d]*100; As prevalence increases, NPV decreases, and as prevalence decreases, NPV increases
|Disease +||Disease -|
|Test +||a=true positive||b=false positive|
|Test -||c=false negative||d =true negative|
Page Last Updated: February 5, 2021