Interpreting a covid-19 test resultBMJ 2020; 369 doi: https://doi.org/10.1136/bmj.m1808 (Published 12 May 2020) Cite this as: BMJ 2020;369:m1808
- Jessica Watson, GP and National Institute for Health Research doctoral research fellow1,
- Penny F Whiting, associate professor in clinical epidemiology1,
- John E Brush, professor of internal medicine2
- 1Centre for Academic Primary Care, Bristol Medical School, University of Bristol, Bristol, UK
- 2Sentara Healthcare and Eastern Virginia Medical School, Norfolk, VA, USA
- Correspondence to J Watson
What you need to know
Interpreting the result of a test for covid-19 depends on two things: the accuracy of the test, and the pre-test probability or estimated risk of disease before testing
A positive RT-PCR test for covid-19 test has more weight than a negative test because of the test’s high specificity but moderate sensitivity
A single negative covid-19 test should not be used as a rule-out in patients with strongly suggestive symptoms
Clinicians should share information with patients about the accuracy of covid-19 tests
Across the world there is a clamour for covid-19 testing, with Tedros Adhanom Ghebreyesus, director general of the World Health Organization, encouraging countries to “test, test, test.”1 The availability of the complete genome of covid-19 early in the epidemic facilitated development of tests to detect viral RNA.2 Multiple assays with different gene targets have been developed using reverse transcriptase polymerase chain reaction (RT-PCR).3 These viral RNA tests use samples usually obtained from the respiratory tract by nasopharyngeal swab, to detect current infections. Serology blood tests to detect antibodies indicating past infection are being developed; these will not be considered in depth in this article.
Testing for covid-19 enables infected individuals to be identified and isolated to reduce spread,4 allows contact tracing for exposed individuals,5 and provides knowledge of regional and national rates of infection to inform public health interventions. However, questions remain on how to apply test results to make optimal decisions about individual patients.
This article was produced at speed to address an urgent need to address uncertainties in testing for covid-19. We searched Pubmed using the terms “covid”, “SARS-CoV-2”, “sensitivity”, “specificity”, “diagnosis”, “test”, and “PCR”, and KSR evidence using terms for covid and test. This was supplemented by discussion with colleagues undertaking formal systematic reviews into covid-19 diagnosis.
How accurate are test results?
No test gives a …