Current best evidence for clinical care (more info)
INTRODUCTION: Early differentiation between emergency department (ED) patients with and without corona virus disease (COVID-19) is very important. Chest CT scan may be helpful in early diagnosing of COVID-19. We investigated the diagnostic accuracy of CT using RT-PCR for SARS-CoV-2 as reference standard and investigated reasons for discordant results between the two tests.
METHODS: In this prospective single centre study in the Netherlands, all adult symptomatic ED patients had both a CT scan and a RT-PCR upon arrival at the ED. CT results were compared with PCR test(s). Diagnostic accuracy was calculated. Discordant results were investigated using discharge diagnoses.
RESULTS: Between March 13th and March 24th 2020, 193 symptomatic ED patients were included. In total, 43.0% of patients had a positive PCR and 56.5% a positive CT, resulting in a sensitivity of 89.2%, specificity 68.2%, likelihood ratio (LR)+ 2.81 and LR- 0.16. Sensitivity was higher in patients with high risk pneumonia (CURB-65 score =3; n = 17, 100%) and with sepsis (SOFA score =2; n = 137, 95.5%). Of the 35 patients (31.8%) with a suspicious CT and a negative RT-PCR, 9 had another respiratory viral pathogen, and in 7 patients, COVID-19 was considered likely. One of nine patients with a non-suspicious CT and a positive PCR had developed symptoms within 48 hours before scanning.
DISCUSSION: The accuracy of chest CT in symptomatic ED patients is high, but used as a single diagnostic test, CT can not safely diagnose or exclude COVID-19. However, CT can be used as a quick tool to categorize patients into "probably positive" and "probably negative" cohorts.
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There is nothing new here. Both CT and PCR are mediocre tests, with sensitivity problems. Since most studies consider all positive PCR tests true positive, it's difficult to know what the specificity is.
This practical study sheds light on the relative utility of two tests for COVID: the PCR and the chest CT. Each test has its failings but the good news is that for the more seriously ill patients, the CT had excellent sensitivity even if it lost some specificity. It would be great news if, going forward, we had better tests to rule in or rule out this virus. But recognized reasons for discordant results will help both therapeutic decisions as well as appropriate infection prevention protocols. Studies like this will eventually help direct diagnostics more efficiently. This will be especially useful in the winter respiratory virus season since we are learning that a number of possible pathogens other than coronavirus can produce radiographic abnormalities.