The diagnostic accuracy of Rapid Antigen Tests (RAT) has been widely studied in various applications and in diverse populations.
Sensitivity, in the order of 75% in pooled estimates, is significantly influenced by the presence or absence of symptoms, viral load, and the timing of sampling relative to the onset of symptoms.
Specificity, in the order of 99% in pooled estimates, is consistently high across tests, populations, and sampling methods.
Post-test probability of being an infectious case following a positive test is highest in individuals with a high pre-test probability (population prevalence > 5%), such as those with COVID-19 symptoms, and those in settings with a high level of community transmission. Here, the positive predictive value is in the order of 95%. However, when used in settings with a lower pre-test probability (population prevalence < 0.5%), as in screening asymptomatic individuals, the positive predictive value is considerably reduced, as low as 25%.
Post-test probability of being an infectious case following a negative test is less than 1% (negative predictive value > 99%) in all settings except those with the highest levels of community transmission.
Badea, A; Reeder, B; Groot, G; Muhajarine, N; Minion, J; Miller, L; Howell-Spooner, B. In real world settings, what is the validity of Rapid Antigen Tests (RATs) in identifying SARS-CoV-2 and how well do they predict disease? 2022 Jan 12, Document no.: EOC211201 RR. In: COVID-19 Rapid Evidence Reviews [Internet]. SK: SK COVID Evidence Support Team, c2022. 15 p. (CEST rapid review report).
Although rapid antigen point-of-care tests (POCT) to detect SARS-CoV-2 (COVID-19) infection have the advantage of rapid result turn-around time compared to laboratory-based reverse-transcriptase polymerase chain reaction (RT-PCR) test, their sensitivity to correctly detect positive cases is lower (Larremore et al., 2020).
Increased frequency of testing compensates for lower test sensitivity of POCTs (See et al., 2021; Larremore et al., 2020). The majority of policy guidelines and public health directives recommend basing frequency of POCT on rates of community transmission or outbreak status of the setting (Public Health Canada, 2021; Ontario Ministry of Long-term Care [LTC], 2021; CDC, 2021; Arizona Department of Health Services, 2020).
Recommended POCT frequency for screening asymptomatic individuals is 3 times per week of staff, including designated support persons, and residents if the home is in an outbreak situation and once per week of staff and designated support persons in a non-outbreak situation (Ontario Ministry of LTC, 2021; CDC, 2021; Larremore et al., 2020).
Designated support persons (i.e. family caregivers) should be tested at the same frequency as LTC staff (Ontario Ministry of LTC, 2021; Micocci et al., 2020; Vilches et al., 2020; Tennessee Department of Health, 2020).
Recommendations are consistent regarding test interpretation and follow-up actions, with the majority of policies and directives recommending a high degree of caution and follow-up RT-PCR testing after a negative POCT if there is a high pre-test probability for COVID-19 infection (i.e. symptomatic, known contact exposure)(Public Health Canada, 2021; CDC, 2021). All reviewed guidelines recommend confirmatory RT-PCR test following a positive POCT if the individual is asymptomatic in order to avoid unnecessary isolation of residents and work restrictions of staff. Contrary to other guidelines, the Oregon Health Authority (2020) considers all positive antigen tests in a symptomatic individual as a positive test regardless of follow up testing.
Modelling studies consistently show that regular POCT screening of asymptomatic staff and residents in LTC during both outbreak and non-outbreak situations results in significant decreases in projected cases when combined with a multipronged approach to prevent transmission (Larremore et al., 2021; Holmdahl et al., 2020; See et al., 2021; Vilches et al., 2020).
Barriers to frequency of testing are availability of test kits, training of testers, human resources for testing, and a reporting strategy (Micocci et al., 2020).
Prioritization of testing should be given to symptomatic healthcare providers and residents first, then screening for residents and staff during outbreaks (See et al., 2020).
The Saskatchewan Health Authority (SHA) Point of Care COVID Testing: Long Term Care Algorithm contains most of the elements present in other algorithms. Additional information should be added on actions taken for presumptive positive or negative tests in different scenarios. Additional information should be provided on frequency of testing and the context for “high-risk contact”.
Ward, H; Tupper, S; Dalidowicz, M; Mueller, M. What are the efficacies and outcomes of Point-of-Care/Antigen testing in Long Term care? 2021 Feb 26; Document no.: LTC020201 RR. In: COVID-19 Rapid Evidence Reviews [Internet]. SK: SK COVID Evidence Support Team, c2020. 37 p. (CEST rapid review report)
· Studies suggest that antigen-based rapid diagnostic tests (Ag-RDTs) can be used in a population level with high prevalence of COVID-19 disease where health systems are overwhelmed or where nucleic acid amplification tests (NAATs) such as real time reverse transcription polymerase chain reaction (rRT-PCR) are not available.
· The Canadian COVID-19 Testing and Screening Expert Advisory Panel recommends the use of frequent screening with rapid diagnostic tests in selected groups to limit outbreaks.
· WHO and European Center for Disease Prevention and Control recommend using Ag-RDTs with high sensitivity and specificity when NAATs are not available or turnaround time negatively affects NAATs’ clinical utility. For example, COVID-19 Ag-RDTs can be used to surveil health care workers or residents of congregate dwellings during outbreaks or when community transmission rates are high, to screen at-risk individuals to support outbreak investigations, or to screen suspected COVID-19 outbreaks in early stages in settings where NAATs are not available.
· WHO does not recommend Ag-RDTs usage when expected prevalence is low (e.g., screening at points of entry) unless an Ag-RDT’s specificity is high (>99%).
· Studies have shown that Panbio™ COVID-19 Ag Test (Abbott) can have overall sensitivity of 72.6% to 95.2% and specificity of 98.0% to 100% and suggest that this test is appropriate for contagious case identification and asymptomatic case screening, especially in high prevalence (>5%) settings.
· WHO recommends that iterative Ag-RDT testing or confirmatory rRT-PCR testing be done in symptomatic patients or asymptomatic contacts of COVID-19 cases since a negative Ag-RDT result cannot completely exclude an active COVID-19 infection.
· Challenges of population level testing (whether they succeed or fail) such as required logistics and resources (e.g., immunizers, access to Ag-RDTs and equipment), performance accuracy of Ag-RDTs (e.g., false positive or negative rates in real world settings), and public trust and engagement in testing and future measures (e.g., vaccine uptake) are yet to be considered.
Generally speaking, data indicate that adult cancer patients and those who have recently received or are receiving anti-cancer therapy are at a higher risk of severe outcomes and death resulting from COVID-19 compared to those without cancer. However, more data are beginning to elucidate the nuances of these risks depending on patient specific factors.
Limited data indicate that pediatric cancer patients are not at a high level of risk of severe outcomes from COVID-19.
Limited evidence indicates some differences in the course and severity of SARS-CoV-2 infection depending on the type of immunosuppressive therapy a patient receives.
Generally speaking, data indicate that adult cancer patients and those who have recently received or are receiving anti-cancer therapy are at a higher risk of severe outcomes and death resulting from COVID-19 compared to those without cancer.
Pediatric cancer populations may not be at the same level of risk as adult populations.
There is not enough evidence at this time to determine if there are differences in the course of SARS-CoV-2 infection in patients receiving chemotherapy vs. those who are not aside from outcomes and severity.
Vanstone, J; Groot, G; Miller, L; Mueller, M. What are the differences in the clinical course of COVID-19 between patients undergoing chemotherapy and otherwise healthy individuals? 2021 Jan 22; Document no.: EOC062201v2 RR. In: COVID-19 Rapid Evidence Reviews [Internet]. SK: SK COVID Evidence Support Team, c2020. 5 p. (CEST rapid review report)
· Diagnostic accuracy of tests for SARS CoV-2 varies based on the type of test, target antigen, type of sample and time of testing.
· There is heterogeneity in clinical performance of rapid antigen tests; however, clinical sensitivity is lower than amplification-based assays.
· Information is limited about test strategies that combine multiple approaches (e.g. molecular and serological methods) but may add value by increasing sensitivity and specificity.
Williams-Roberts, H; Waldner, C; Dalidowicz, M; Howell-Spooner, B. What is the accuracy of diagnostic tests for the detection of SARS-CoV-2? 2020 Jul 23; Document no.: EPM072101 RR. In: COVID-19 Rapid Evidence Reviews [Internet]. SK: SK COVID Evidence Support Team, c2020. 13 p. (CEST rapid review report)
· A wide range of tests are available for detection of viral RNA as well as serological and immunoassays for antibodies developed due to exposure to SARS CoV-2 in infected persons. Despite the emerging research, information about clinical validity of tests is limited.
· Multiple factors affect test performance including the nature of the specific test, type of specimen and its quality, severity and duration of illness at the time of testing. These individual variations hamper assessment of diagnostic accuracy and suggest that a combination of tests on multiple types of specimens at serial time points might be needed to confirm a COVID 19 diagnosis.
· Point of care tests are desirable and needed to scale up testing in low resource settings; however, tests are of variable quality and more research is needed before they can be relied on for clinical decision making.
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Williams-Roberts, H; Waldener, C. What is the accuracy of diagnostic tests for COVID-19 detection? 2020 May 12; Document no.: EPM051201 RR. In: COVID-19 Rapid Evidence Reviews [Internet]. SK: SK COVID Evidence Support Team, c2020. (CEST rapid review report)