Journal Article

Interpreting a covid-19 test result

Whiting, Penny F, Watson, Jessica, Brush, John E

2020

Overview

Correspondence to J Watson [e-mail] 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.” A systematic review of the accuracy of covid-19 tests reported false negative rates of between 2% and 29% (equating to sensitivity of 71-98%), based on negative RT-PCR tests which were positive on repeat testing.6 The use of repeat RT-PCR testing as gold standard is likely to underestimate the true rate of false negatives, as not all patients in the included studies received repeat testing and those with clinically diagnosed covid-19 were not considered as actually having covid-19.6 Accuracy of viral RNA swabs in clinical practice varies depending on the site and quality of sampling. In one study, sensitivity of RT-PCR in 205 patients varied, at 93% for broncho-alveolar lavage, 72% for sputum, 63% for nasal swabs, and only 32% for throat swabs.7 Accuracy is also likely to vary depending on stage of disease8 and degree of viral multiplication or clearance.9 Higher sensitivities are reported depending on which gene targets are used, and whether multiple gene tests are used in combination.310 Reported accuracies are much higher for in vitro studies, which measure performance of primers using coronavirus cell culture in carefully controlled conditions.2 The lack of a clear-cut “gold-standard” is a challenge for evaluating covid-19 tests; pragmatically, clinical adjudication may be the best available “gold standard,” based on repeat swabs, history, and contact with patients known to have covid-19, chest radiographs, and computed tomography scans. Inevitably this introduces some incorporation bias, where the test being evaluated forms part of the reference standard, and this would tend to inflate the measured sensitivity of these tests.11 Disease prevalence can also affect estimates of accuracy: tests developed and evaluated in populations with high prevalence (eg, secondary care) may have lower sensitivity when applied in a lower prevalence setting (eg, primary care).11 One community based study of 4653 close contacts of patients with covid-19 tested RT-PCR throat swabs every 48 hours during a 14 day quarantine period.

• Publisher: British Medical Journal Publishing Group,BMJ Publishing Group LTD
• Subject: Accuracy / Alveoli / Betacoronavirus / Cell culture / Clinical Laboratory Techniques / Clinical medicine / Computed tomography / Controlled conditions / Coronavirus Infections - diagnosis / Coronaviruses / COVID-19 / COVID-19 diagnostic tests / COVID-19 Testing / Estimates / Genomes / Humans / Infections / Pandemics / Pneumonia, Viral - diagnosis / Polymerase chain reaction / Practice / Probability / Radiography / Respiratory tract / Ribonucleic acid / Risk assessment / RNA / RNA-directed DNA polymerase / SARS-CoV-2 / Severe acute respiratory syndrome coronavirus 2 / Sputum