COVID-19 screening of asymptomatic patients admitted through emergency departments in Alberta: a prospective quality-improvement study

AB-EL-COVID-19 Screening Asymptomatic

Alberta (ED)

Background: The prevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection among asymptomatic patients admitted to hospital has implications for personal protective equipment use, testing strategy and confidence in the safety of acute care services. Our aim was to estimate the positivity rate of reverse transcription polymerase chain reaction (RT-PCR) testing among people admitted to hospital without symptoms of coronavirus disease 2019 (COVID-19) in Alberta, Canada.


Methods: Between Apr. 9 and May 24, 2020, we screened for COVID-19 symptoms and tested for SARS-CoV-2 infection in all consecutive adult patients (≥ 18 yr) admitted via emergency department to 3 Alberta hospitals. We summarized the parameters of the epidemic curve and assessed the performance of symptom screening versus RT-PCR results on nasopharyngeal or oropharyngeal swab samples.


Results: The study period encompassed Alberta’s initial epidemic curve, with peak active cases per 100 000 of 71.4 (0.07%) on Apr. 30, 2020, and 14.7 and 14.6 at the beginning (Apr. 9, 2020) and end (May 24, 2020), respectively. Testing for SARS-CoV-2 infection (64.9% throat and 35.1% nasopharyngeal swabs) was done on 3375 adults (mean age 51, standard deviation 21, yr; 51.5% men). None of the asymptomatic patients (n = 1814) tested positive, and 71 of those with symptoms tested positive (n = 1561; 4.5%, 95% confidence interval [CI] 3.6%–5.7%). Sensitivity of symptom screening (v. RT-PCR) was 100% (95% CI 95%–100%), and specificity was 55% (95% CI 53%–57%). Posttest probabilities for prevalence of SARS-CoV-2 infection ranging from 1.5 to 14 times the peak prevalence of active cases during the study did not change when we assumed lower sensitivity (92%).

Interpretation: In a region with low disease prevalence where protocolized symptom assessment was in place during the admission process, we did not identify people admitted to hospital without COVID-19 symptoms who were RT-PCR positive. There may not be additive benefit to universal testing of asymptomatic patients on hospital admission in a setting of low pretest probability and strong public health containment.

Compared with other highly pathogenic human coronaviruses (Middle East respiratory syndrome coronavirus and severe acute respiratory syndrome coronavirus), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus responsible for coronavirus disease 2019 (COVID-19), has a lower case-fatality rate but spreads more efficiently. SARS-CoV-2 mostly spreads by respiratory droplets among people who are in close contact. Aerosol transmission can occur in some settings, especially in indoor, crowded and inadequately ventilated spaces where people stay for long periods. Studies are underway to examine the conditions under which aerosol transmission occurs outside of medical facilities where aerosol-generating procedures are conducted. Contact spread (direct or via contaminated articles or surfaces) can also occur.

Transmission of SARS-CoV-2 is possible from people without classic respiratory symptoms (e.g., asymptomatic, presymptomatic and paucisymptomatic), although this has mostly been documented in close quarters (e.g., within households and cruise ships). Studies in various settings have shown that 15%–50% of people with positive results on reverse transcription polymerase chain reaction (RT-PCR) testing were asymptomatic at testing. Although presymptomatic spread has been described, the contribution of truly asymptomatic transmission remains unclear. If people who are unknowingly positive for SARS-CoV-2 infection are admitted to hospital, they can infect health care workers or other patients.

The prevalence of asymptomatic carriers of SARS-CoV-2 has been shown to depend on how widespread SARS-CoV-2 infection is in a population, with estimates ranging from 0.34% in Iceland (where 0.8% of the population was positive for SARS-CoV-2) to 10% on the Diamond Princess cruise ship (where 20% of passengers were positive).

Given the frequency of close human interaction in hospital settings, the risk of SARS-CoV-2 transmission from asymptomatic patients could be higher than in settings of community transmission, if appropriate standard precautions are neglected. According to a report during the first explosive outbreak in New York, 15% of currently asymptomatic women admitted for delivery had positive SARS-CoV-2 testing, further underscoring the importance of local epidemiology in guiding protective measures. Swab results alone without a 4-week symptom history, however, may overestimate the risk of asymptomatic transmission, as RT-PCR can remain positive after COVID-19 recovery owing to the detection of nonviable virus. According to a recent report from the United Kingdom, 40% of asymptomatic health care workers who tested positive had symptoms more than 1 week before testing.

To inform the appropriate use of personal protective equipment (PPE) and other in-hospital precautions, including isolation requirements, room assignments and follow-up strategies for contact tracing, we screened for symptoms of COVID-19 and tested for SARS-CoV-2-infection in all patients admitted to 3 tertiary care hospitals via an emergency department during the peak of the epidemic curve in Alberta, Canada. Our aim was to estimate the positivity rate of RT-PCR testing among people admitted to hospital without COVID-19 symptoms.


Authors: Pietro Ravani, Lynora Saxinger, Uma Chandran, Kevin Fonseca, Stephanie Murphy, Eddy Lang, Laura McDougall, Braden Manns

Eddy Lang - eddy.lang@ahs.ca

Preliminary data gathering/ baseline