Testing and Diagnosis

1. How to test for COVID-19?                                                                                                        

2. Who to test for COVID-19?                                                                                                        

3. Where to test for COVID-19?                                                                                                     


How to Test for COVID-19?                                                                                                                              

  • The decision to test for COVID-19 is based on both clinical and epidemiological factors and an assessment of the likelihood of infection [1]

  • COVID-19 Specific Tests:

    • RT-PCR

    • Chest X-ray & CT scans

    • Lung ultrasonography

  • RT-PCR:

    • Specimens to be collected from symptomatic patients and contacts [1]:

  • RT-PCR (cont'd)

    • An analysis of serial RT-PCR assays and CT scans demonstrated that 60-93% of patients had initial positive chest CT consistent with COVID-19 before the initial positive RT-PCR results. Further, 42% of patients showed improvement of follow-up chest CT scans beforeRT-PCR results turned negative [4]

    • Thus, a single negative RT-PCR does not exclude COVID-19, especially If obtained via NP or early on in disease course, and the sensitivity may not be as good as CT scans for COVID-19 [6]

    • COVID-19 RT-PCR is believed to have high specificity, but sensitivity may be as low as 60-70%; thus patients with pneumonia due to COVID-19 may have lung abnormalities on chest CT but a negative initial RT-PCR [5,7]

  • Chest X-ray (CXR) and CT scans

    • Sensitivity of CXR (59%) is lower than CT scan (86%) [8]

    • CT scanning on 15 healthcare workers exposed to COVID-19 before they became symptomatic revealed ground-glass opacification on 14/15 healthcare workers [9]

    • CT scan abnormalities may appear even before symptoms [6]

  • Lung ultrasonography 

    • Can be used as a supplementary method to CT, but not replace CT [10]:

    • Using ultrasound, COVID-19 foci are mainly observed in the posterior fields in both lungs, especially the posterior and inferior fields

    • Ultrasound is superior to CT in showing smaller peripulmonary lesions and pleural and peripulmonary effusion; CT shows more clear and complete intrapulmonary and apical lesions than ultrasound

Who to Test for COVID-19?

  • This will vary from jurisdiction to jurisdiction based on public health recommendations. Notably, detection of other respiratory viruses does not rule out COVID-19; about 5% of patients with COVID-19 may be co-infected with other pathogens [3]

    • Canada

    • United Kingdom

      • Priorities for testing:

        • Prioritization​

          • Group 1 (test first): patient requiring critical care for the management of pneumonia, acute respiratory distress syndrome (ARDS) or influenza-like illness (ILI)*, or an alternative indication of severe illness has been provided

          • Group 2: all other patients requiring admission to hospital** for management of pneumonia, ARDS, or ILI 

          • Group 3: clusters of disease in residential or care settings (e.g. long-term care facilities and prisons)

            • * ILI: fever 37.8°C 

            • **admission to hospital: a hospital practitioner has decided that admission to hospital is required with an expectation that the patient will need to stay at least one night.

      • Repeat Testing

        • If patient is producing sputum or is intubated, obtain samples from both lower respiratory tract and upper respiratory tract

        • If a suitable lower respiratory tract sample is negative, repeat tests are not needed

      • Step down testing

        • Priority testing for viral clearance should focus on:​

          • Severely immunocompromised patients being stepped down to a setting where isolation is not possible 
            (repeated, for example, at fortnightly intervals)

          • Patients being discharged to a residential or care facility

          • Patients who require multiple repeat healthcare admissions and attendances (such as those with ongoing cancer treatment)

      • ​Sources and useful links

    • United States

      • CDC recommendation for priorities for testing [2]:

        • Patients with severe symptoms such as difficulty breathing should seek care immediately

        • Older adults and those with severe underlying chronic medical conditions (e.g. heart disease, lung disease, diabetes, etc.) are at higher risk for developing serious complications from COVID-19, should contact their physician early even if mild illness

        • Hospitalized patients who have signs and symptoms

        • Any individuals, including healthcare personnel, who have had close contact with a suspect or laboratory-confirmed COVID-19 patient within 14 days of symptom onset, or have a history of travel from affected geographic areas within 14 days of symptom onset

        • Mildly ill patients should be encouraged to stay at home and contact their healthcare provider via phone regarding clinical management

          • Clinicians are strongly encouraged to test for other causes of respiratory illness (e.g. influenza) [2]

      • New York

        • Testing for COVID-19 shall be authorized by a healthcare provider when:

          • An individual has come within proximate contact (same classroom, office, or gatherings) of another person known to be positive; or 

          • An individual has traveled to a country that the CDC has issued a Level 2 or Level 3 Travel Health Notice, and shows symptoms of illness; or 

          • An individual is quarantined (mandatory or precautionary) and has shown symptoms of COVID-19 illness; or 

          • An individual is symptomatic and has not tested positive for any other infection; or 

          • Other cases where the facts and circumstances warrant as determined by the treating clinician in consultation with state and local department of health officials.

        • Sources & useful links:

Where to Test for COVID-19?


(1).      Laboratory testing for 2019 novel coronavirus (2019-nCoV) in suspected human cases https://www.who.int/publications-detail/laboratory-testing-for-2019-novel-coronavirus-in-suspected-human-cases-20200117 (accessed Mar 19, 2020).

(2)       CDC. Coronavirus Disease 2019 (COVID-19) https://www.cdc.gov/coronavirus/2019-ncov/hcp/clinical-criteria.html (accessed Mar 18, 2020).

(3)       Wang, M.; Wu, Q.; Xu, W.; Qiao, B.; Wang, J.; Zheng, H.; Jiang, S.; Mei, J.; Wu, Z.; Deng, Y.; Zhou, F.; Wu, W.; Zhang, Y.; Lv, Z.; Huang, J.; Guo, X.; Feng, L.; Xia, Z.; Li, D.; Xu, Z.; Liu, T.; Zhang, P.; Tong, Y.; Li, Y. Clinical Diagnosis of 8274 Samples with 2019-Novel Coronavirus in Wuhan. medRxiv 2020, 2020.02.12.20022327. https://doi.org/10.1101/2020.02.12.20022327.

(4)       Ai, T.; Yang, Z.; Hou, H.; Zhan, C.; Chen, C.; Lv, W.; Tao, Q.; Sun, Z.; Xia, L. Correlation of Chest CT and RT-PCR Testing in Coronavirus Disease 2019 (COVID-19) in China: A Report of 1014 Cases. Radiology 2020, 200642. https://doi.org/10.1148/radiol.2020200642.

(5)       Kanne, J. P.; Little, B. P.; Chung, J. H.; Elicker, B. M.; Ketai, L. H. Essentials for Radiologists on COVID-19: An Update—Radiology Scientific Expert Panel. Radiology 2020, 200527. https://doi.org/10.1148/radiol.2020200527.

(6)       COVID-19. EMCrit Project.

(7)       Fang, Y.; Zhang, H.; Xie, J.; Lin, M.; Ying, L.; Pang, P.; Ji, W. Sensitivity of Chest CT for COVID-19: Comparison to                     RT-PCR. Radiology 2020, 200432. https://doi.org/10.1148/radiol.2020200432.

(8)       Guan, W.; Ni, Z.; Hu, Y.; Liang, W.; Ou, C.; He, J.; Liu, L.; Shan, H.; Lei, C.; Hui, D. S. C.; Du, B.; Li, L.; Zeng, G.; Yuen, K.-Y.; Chen, R.; Tang, C.; Wang, T.; Chen, P.; Xiang, J.; Li, S.; Wang, J.; Liang, Z.; Peng, Y.; Wei, L.; Liu, Y.; Hu, Y.; Peng, P.; Wang, J.; Liu, J.; Chen, Z.; Li, G.; Zheng, Z.; Qiu, S.; Luo, J.; Ye, C.; Zhu, S.; Zhong, N. Clinical Characteristics of Coronavirus Disease 2019 in China. N. Engl. J. Med. 2020, 0 (0), null. https://doi.org/10.1056/NEJMoa2002032.

(9)       Shi, H.; Han, X.; Jiang, N.; Cao, Y.; Alwalid, O.; Gu, J.; Fan, Y.; Zheng, C. Radiological Findings from 81 Patients with COVID-19 Pneumonia in Wuhan, China: A Descriptive Study. Lancet Infect. Dis. 2020. https://doi.org/10.1016/S1473-3099(20)30086-4.

(10)     Huang, Y.; Wang, S.; Liu, Y.; Zhang, Y.; Zheng, C.; Zheng, Y.; Zhang, C.; Min, W.; Zhou, H.; Yu, M.; Hu, M. A Preliminary Study on the Ultrasonic Manifestations of Peripulmonary Lesions of Non-Critical Novel Coronavirus Pneumonia (COVID-19); SSRN Scholarly Paper ID 3544750; Social Science Research Network: Rochester, NY, 2020. https://doi.org/10.2139/ssrn.3544750.

(11)     AHS. COVID-19 Frequently Asked Questions for Primary Care. https://www.albertahealthservices.ca/assets/info/ppih/if-ppih-covid-19-primary-care-faq.pdf (accessed April 2, 2020).

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