While molecular testing with PCR is now widely available, testing resource constraints continue, especially related to swabs, reagents, and media. For this reason, it is important to continue to use an evidence-based approach when making COVID-19 testing priorities. LAC DPH priorities for COVID-19 diagnostic testing plus rationale are outlined below.
Laboratories Providing COVID-19 PCR Molecular Testing (7-27-20) Reference Guide LA County Department of Health Services (LAC DHS) has created this guide to help organizations identify a laboratory provider that best fits their needs. This document provides an overview of some of the laboratories offering testing to LA County providers and residents.
LA County Public Health Laboratory (PHL): Diagnostic testing for SARS-CoV-2 infection is available from PHL for high priority patients and settings. View the PHL COVID-19 Testing page for PHL testing criteria and specimen instructions.
All laboratories (regardless of their location) that test Los Angeles County residents must register with the California Department of Public health (CDPH).
Only tests with emergency use authorizations (EUA) from the FDA should be used for patient care. A wide variety of molecular tests (nucleic acid detection or PCR), antigen tests, and SARS-CoV-2 serologic tests now have EUA.
Molecular Tests (also known as reverse-transcriptase polymerase chain reaction [PCR] tests or nucleic acid amplification tests [NAATs]) detect the presence of viral RNA. These tests are the recommended method for diagnosing current infection. The test is typically performed on nasopharyngeal swabs but can also be performed on other respiratory tract specimens (e.g., oropharyngeal swabs, nasal swabs, lower respiratory tract samples). A positive molecular test for SARS-CoV-2 generally confirms the diagnosis of COVID-19; however, tests may remain positive long after a patient is no longer infectious due to prolonged detection of RNA. False-negative tests have been well documented (see Variation in False-Negative Rate of RT-PCR–Based SARS-CoV-2 Tests by Time Since Exposure). Molecular test diagnostic accuracy is dependent on adequate specimen collection, clinical probability of disease, and the specific test characteristics.
Antigen Tests detect the presence of viral proteins that are part of the SARS-CoV-2 virus. These tests are often faster and simpler tests to run. The FDA has now issued EUAs for antigen tests. The Association of Public Health Laboratories published Considerations for Implementation of SARS-CoV-2 Rapid Antigen Testing (6-23-20) which provides an overview of the currently available antigen tests for SARS-CoV-2 including these key points:
APHL also provides scenarios where SARS-CoV-2 Antigen tests may be considered for use and when they are not recommended, see APHL guidance.
Serology Tests detect waning or past SARS-CoV-2 virus infection indirectly, by measuring the antibody response to the virus. Serology assays should not replace direct viral detection methods for diagnosing an active SARS-CoV-2 infection. At this point in time, until the presence, durability, and duration of immunity is established, serology tests they should not be used to determine immune status. Healthcare providers and systems considering using serologic tests for patient care should adopt strategies to minimize false positive results as outlined in the recently released CDC antibody testing guidance and summarized briefly below.
A list of EUA authorized serology tests plus performance information can be found here.
Minimizing False Positive Serology Results
The CDC emphasizes the importance of minimizing false positive test results, particularly when the results will be returned to individuals. The CDC expects that in most parts of the U.S. at this point in time, including areas that have been heavily impacted, the prevalence of SARS-CoV-2 antibody is expected to be low, ranging from <5% to 25%, so that testing will result in relatively more false positive results. As recent local community-wide seroprevalence studies indicate that the prevalence of SARS CoV-2 antibodies in the LA County adult population is <5%, false positive results could be very common unless testing is done strategically.
Suggested strategies to improve the positive predictive value include using a serological assay with high specificity (99.5%) and by testing populations and individuals with an elevated likelihood of previous exposure to SARS-CoV-2 (such as persons with a history of COVID-19-like illness or in outbreak settings). An additional approach when a high positive predictive value (e.g., 95%) cannot be assured with a single serology test, is to use an orthogonal testing algorithm where a first positive serology is tested with a second test with a different design. See CDC Table 1 for examples of using one or two tests in populations with various prevalences of SARS-CoV-2 antibodies. The FDA is providing a calculator that will allow users to see the estimated performance of a single test or two independent tests based on their performance characteristics and the estimated prevalence of SARS-CoV-2 antibodies in the target population. For more information, see the CDC guidelines section on Testing Strategies.Use of Serologic Tests
CDC Recommendations for Use of Serologic Tests include:Diagnostic uses:
When SARS-CoV-2 serology tests should not be used:
Healthcare providers and systems are encouraged to expand SARS CoV-2 diagnostic testing to more patients with symptoms of COVID-19. Testing asymptomatic persons may be recommended if they are part of an outbreak or case investigation, or if they are staff or residents in a high-risk congregate living setting, or if they are a close contact to a confirmed case.
Healthcare providers and systems should be able to provide their patients with timely access to SARS-CoV-2 diagnostic testing when indicated. A Reference Guide to Laboratories Providing COVID-19 PCR Molecular Testing is now available to help healthcare organizations and other entities identify a laboratory provider that best fits their needs.
Rationale includes, ensuring optimal care options for all hospitalized patients, lessening the risk of nosocomial infections, and detecting and controlling outbreaks in acute- and subacute-care health facilities and high-risk congregate living settings.Testing is Strongly Encouraged
Rationale includes, ensuring that those who are at highest risk of complications are rapidly identified.Recommend Testing assuming sufficient resources
Rationale is to detect new cases to decrease community spread.
Routine testing of asymptomatic persons in the general population is not recommended in the absence of a known exposure.
Note: if facilities elect to conduct pre-admission or pre-procedure screening of patients for COVID-19, it is important to continue to practice COVID-19 infection prevention precautions on patients testing negative due to the risk of false negatives.