Article Figures & Tables
Figures
- Figure 1:
Daily incidence of testing (dashed lines) and positive cases (solid lines) across Alberta by zone using moving average methodology (with a width of 7 days for the rolling window).
- Figure 2:
Estimates of the effective reproductive number (Rt, solid line) and 95% credible interval (grey area) during the coronavirus disease 2019 outbreak in Alberta.
- Figure 3:
Distribution of daily hospital admissions via emergency department by hospital during the study period in each study site (Calgary, Peter Lougheed Centre; Central Zone, Red Deer Regional Hospital Centre; and Edmonton, Royal Alexandra Hospital).
Tables
1. Do you have the symptoms below? Please circle • Fever (> 38°C) Yes No • Cough Yes No • Shortness of breath Yes No • Difficulty breathing Yes No • Sore throat Yes No Note: COVID-19 = coronavirus disease 2019.
↵* Used to determine the need for testing (outside the context of this study) during the study period.
- Table 2:
Alberta Health Services COVID-19 risk assessment screening questions*
1. Have you returned to Canada from outside the country (including the US) in the past 14 days? Yes No 2. Do you live with or have had close contact† (within 2 m/6 ft) with a person with an influenza-like illness who has travelled outside of Canada within the last 14 days before their illness? Yes No 3. Do you live with or have had close contact† (within 2 m/6 ft) with someone who is ill with fever and/or cough and influenza-like illness symptoms? Yes No 4. Have you had close contact† (within 2 m/6 ft) with a confirmed or probable case of COVID-19? Yes No If the answer is “No” to all of the above, the patient is considered low risk. Note: COVID-19 = coronavirus disease 2019, PPE = personal protective equipment.
↵* Questionnaire used during the study period to determine the need for isolation, but not specifically to guide testing.
↵† A close contact is an individual who provided care for the person who was ill, including health care workers, family members or other caregivers, or who had other similar close physical contact without consistent and appropriate use of PPE, or lived with or otherwise had close prolonged contact (within 2 m/6 ft) with a person while they were ill, or had direct contact with infectious bodily fluids of a person (e.g., was coughed or sneezed on) while not wearing recommended PPE.
Study participants Asymptomatic Symptomatic No. (%) 3375 (100) 1814 (53.7) 1561 (46.3) Age, yr, mean ± standard deviation 51 ± 21 55 ± 22 47 ± 19 SARS-CoV-2 positive SARS-CoV-2 negative Total Screening positive 71 1490 1561 Screening negative 0 1814 1814 Total 71 3304 3375 Point estimates (95% CI) Apparent prevalence 0.46 (0.45–0.48) True prevalence 0.02 (0.02–0.03) Accuracy 0.56 (0.54–0.58) Sensitivity 1.00 (0.95–1.00) Specificity 0.55 (0.53–0.57) Positive predictive value 0.04 (0.04–0.06) Negative predictive value 1.00 (0.998–1.00) Positive likelihood ratio* 2.19 (2.09–2.29) Negative likelihood ratio* 0.02 (0.004–0.18) Odds ratio* 87.65 (12.16–631.50) Note: CI = confidence interval, SARS-CoV-2 = severe acute respiratory syndrome coronavirus 2.
↵* To estimate positive likelihood ratio, negative likelihood ratio and diagnostic odds ratio, we added 1 to each cell.
- Table 5:
Performance of the symptom screening tool (sensitivity analysis)*
SARS-CoV-2 positive SARS-CoV-2 negative Total Screening positive 71 1490 1561 Screening negative 6 1808 1814 Total 77 3298 3375 Point estimates (95% CI) Apparent prevalence 0.46 (0.45–0.48) True prevalence 0.02 (0.02–0.03) Accuracy 0.56 (0.54–0.57) Sensitivity 0.92 (0.84–0.97) Specificity 0.55 (0.53–0.56) Positive predictive value 0.04 (0.04–0.06) Negative predictive value 0.997 (0.993–0.998) Positive likelihood ratio 2.04 (1.89–2.20) Negative likelihood ratio 0.14 (0.07–0.31) Odds ratio 14.36 (6.22–33.13) Note: CI = confidence interval, SARS-CoV-2 = severe acute respiratory syndrome coronavirus 2.
↵* Assuming asymptomatic prevalence = 0.0035 (6 people with SARS-CoV-2 infection among 1814 without symptoms).
- Table 6:
Probabilities of SARS-CoV-2 infection before and after symptom screening*
Pretest probability Posttest probability | T+ Posttest probability | T− SE = 0.986
SP = 0.549 (Table 4)0.001 0.0022 0 0.005 0.01 0.0001 0.010 0.02 0.0003 SE = 0.922
SP = 0.548 (Table 5)0.001 0.002 0.0001 0.005 0.01 0.0007 0.010 0.02 0.0014 Note: SARS-CoV-2 = severe acute respiratory syndrome coronavirus 2, SE = sensitivity, SP = specificity.
↵* Pretest probability indicates the prevalence of SARS-CoV-2 infection in the population; posttest probability | T+ indicates the probability of having SARS-CoV-2 infection given a positive symptom screening; posttest probability | T− indicates the probability of having SARS-CoV-2 infection given a negative symptom screening. Test performance measures are from main results (Table 4) and from sensitivity analysis (Table 5). Of note, a pretest probability of 0.005 is about 7 times the prevalence of active cases in Alberta during the study period (0.07%).
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