National laboratory testing indication guidance for COVID-19
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- Consideration for testing of individuals who are asymptomatic
- Choice of testing modality
- Forward planning
The first version of the National laboratory testing indication guidance for COVID-19 document was developed at the beginning of the COVID-19 outbreak. It was based on the scientific knowledge and testing resources available at that time, and reflects that testing was limited to molecular polymerase chain reaction (PCR). The original guidance document was finalized and approved by the Special Advisory Committee on April 16, 2020.
Since then, significant developments in four areas have prompted the need to review and update the guidance. Specific developments include:
- Expanded laboratory resources;
- Viral transmission from asymptomatic individuals or individuals in the pre-symptomatic phase;
- Outbreaks in congregate living and work settings; and
- New testing modalities (molecular Point of Care and serological tests).
This document was updated in May 2020 to reflect these developments. It will be updated continuously as new testing technologies become available, epidemiology changes and more is learned.
To facilitate a consistent national approach to testing, acknowledging regional variations in COVID-19 epidemiology and that testing requirements are expected to vary over time.
- To outline the elements of a national approach to laboratory testing for COVID-19 for consideration by Canadian provinces and territories (PTs).
- To support identification of triggers for public health actions.
It is recognized that there will be variation in how PTs implement this guidance, depending on resources, local epidemiology, and other considerations.
Social, demographic and economic factors are important to the epidemiology of the epidemic.
The guidance below relates to molecular detection tests; the current role of serological testing is briefly addressed.
The overall performance of COVID-19 molecular tests in distinguishing between individuals who do and do not have SARS-CoV-2 infection depends on both the test attributes and the epidemiological considerations.
Test attributes include the sensitivity (the ability of the test to correctly identify those who truly are infected with SARS-CoV-2) and the specificity (the ability of the test to correctly identify those who truly are not infected with SARS-CoV-2) at the time the clinical specimen was collected for laboratory analysis. Since the sensitivity and specificity of molecular testing for COVID-19 is linked to viral load, test performance varies during the course of illness. False negative results will occur if the patient is not shedding sufficient virus at the time or at the site of specimen collection and when the specimen is not collected properly. Molecular tests are also vulnerable to non-specific PCR reactions, which can lead to false positive results.
Epidemiological considerations include the pre-test likelihood of a positive and negative test. The proportion of false positive tests increases as the prevelance of SARS-CoV-2 in the population decreases. A positive test in an asymptomatic person with no known exposure to SARS-CoV-2 is particularly challenging to interpret; the person could be infectious, could no longer be infectious, or not be infected (i.e. false positive).
A positive test should lead to the appropriate public health actions being taken. If the test is thought to be a false positive, it should be repeated. A negative test is not interpretable for several reasons (e.g. an infected individual may have been tested early in their incubation period) and actions should not be made solely on the basis of a negative test result. False reassurance from a negative test is a concern. Where the clinical index of suspicion is high, a negative test does not rule out disease.
Overall, the recommended approach to testing is to focus on symptomatic individuals (including those with mild symptoms) and prioritize those with symptoms for testing. This approach is consistent with the Infectious Diseases Society of America testing guidelines Footnote 1. Due to the non-specific nature of many COVID-19 symptoms, symptoms used to determine the indication for testing will need to be considered in relation to testing capacity. Considerations for testing individual patients include clinical judgement, local context for laboratory testing, and guidance from regional public health authorities. The overall effectiveness of testing to identify COVID-19 infection is dependent on individuals likely to have been exposed to SARS-CoV-2 coming forward and being able to access the appropriately offered testing.
Rationales and objectives for testing all symptomatic individuals are listed in Table 1. In addition, specific groups are identified for whom special considerations and/or additional public health implications may apply. The groupings are not presented in any particular order. Should testing resources become constrained, consideration for matching testing resources to public health objectives may become necessary.
|Group||Sub-group descriptions||Rationale/Objective for testing|
|All children and youth||N/A||
Member of, or high degree of interaction with, populations who are at high risk, vulnerable and/or who live in congregate settings
Symptomatic health care workers (regardless of care delivery setting) and staff who work in health care facilities.
Symptomatic residents and staff in long-term care facilities, prisons, homeless shelters, other temporary shelters, single-room occupancy residences and work camps.
|Hospitalized patients with no alternative laboratory-based diagnosis.||
|Members of remote, isolated, rural and/or Indigenous communities who are symptomatic.||
|Travellers identified at a port of entry to Canada who are symptomatic.||
Risk represented by travel: At this time, there is community transmission occurring in a number of Canadian jurisdictions. As such, the role of travel in determining indication for testing should be considered in the context of the jurisdiction’s epidemiology.
Consideration for testing of individuals who are asymptomatic
Currently the role for testing people who are asymptomatic in certain settings is unknown. The science and protocols are evolving as more is learned. There are several factors to consider in testing individuals who are asymptomatic, including:
- The performance of the test in individuals who are infected but asymptomatic is unknown. Since the viral load may be low at the time of sampling, there is an increased likelihood of a falsely negative test result for those tested early in their incubation period;
- The progressive decreasing predictive value for positive test results as the prevalence of SARS-COV-2 infection decreases in targeted populations of individuals who are asymptomatic, particularly with no known exposure;
- The persistence of RNA beyond the presence of infectious virus in individuals who are tested, leading to unnecessary implementation of public health measures; and
- The opportunity costs associated with the deployment of resources to test individuals who are asymptomatic and unlikely to be infected with SARS-COV-2, based on local epidemiology.
Certain populations and settings are disproportionally vulnerable to further transmission of SARS-COV-2 and/or subsequent poor outcomes from COVID-19 infection and may, in some contexts, be considered for testing in the absence of symptoms. Testing for individuals who are asymptomatic should generally be considered only when testing for individuals who are symptomatic is routinely available.
Testing individuals who are asymptomatic with known exposure to a confirmed case
Individuals without symptoms who may be considered for testing for reasons of contact tracing or outbreak management include:
- Close contacts of a case in the community;
- Health care workers and staff who work in health care facilities for outbreak management;
- Residents and workers in high-risk congregate living settings (e.g. long-term care facilities, prisons, homeless shelters, other temporary shelters, single-room occupancy residences and work camps) for outbreak management;
- Work settings where physical distancing cannot be maintained (e.g. meat and poultry-processing facilities) for outbreak management; and
- Other outbreak or cluster investigations.
Testing of individuals who are asymptomatic with no known exposure to a case
There is limited evidence regarding the utility of testing individuals who are asymptomatic with no known exposure to a case and at this time is not generally recommended. Presently, testing of individuals who are asymptomatic with no known exposure to a case is best done in the context of pilot or systematic surveillance activities to generate the knowledge needed to make future evidence-informed decisions. The need for such activities will be driven by local epidemiology. Such situations may include:
- Individuals upon admission to a health care facility;
- Individuals within 24h of a medical or surgical procedure;
- Health care workers and staff who work in health care facilities;
- Residents and workers in high-risk congregate living settings (e.g. long-term care facilities, prisons, homeless shelters, other temporary shelters, single-room occupancy residences and work camps);
- Essential workers;
- Other work settings;
- Daycare and school attendees and staff; and
- General population.
There are reports of sudden deaths and deaths occurring at home in individuals who have not sought care. Consideration should be given to testing deceased individuals with no known cause of death.
As always, local context and judgement by regional public health authorities should be exercised. As scientific knowledge and experience continue to grow, firmer evidence-based recommendations may become available.
Choice of testing modality
Most molecular testing needs can be efficiently and effectively delivered by high throughput lab capacity, resulting in a turn-around time that enables timely public health action. While these laboratories are able to support remote, rural, isolated and/or Indigenous communities, the delays introduced secondary to sample shipment and transit times can increase turnaround time significantly. Consideration for supporting the deployment of Point of Care (PoC) molecular testing in these settings is recommended. PoC testing may also be implemented in situations where testing needs to be on-site and rapid, such as before admission to a high-risk congregate living setting. The allocation of limited PoC resources to locations where such devices will have the most beneficial impact is recommended.
While serological testing does not currently have a role in COVID-19 infection diagnosis, it may be employed in the future in targeted sampling studies to model the spread of the virus and the immune response dynamics to inform the risk of further epidemic waves. It may also be used for retrospective case identification, diagnosing post-infectious complications, and to more accurately determine the prevalence of COVID-19 infection. With regard to the latter, the federal government has convened the COVID-19 Immunity Task Force to establish priorities and oversee the coordination of a series of country-wide sero-surveys that will tell us how widely the virus has spread in Canada and provide reliable estimates of potential immunity and vulnerabilities in Canadian populations.
This guidance will be reviewed and updated as necessary.
- Footnote 1
Hanson KE, Caliendo AM, Arias CA, et al. Infectious Diseases Society of America Guidelines on the Diagnosis of COVID-19. IDSA 2020. Accessed 13 May 2020. Available at: https://www.idsociety.org/practice-guideline/covid-19-guideline-diagnostics/.
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