Chest CT Can Help Identify COVID-19 Cases in Patients with Initial Negative Tests

June 19, 2020

Patients who test negative with RT-PCR show lower levels of pulmonary consolidation on chest CT, indicating earlier disease.

CT scans show lower levels of pulmonary consolidation among patients who initially test negative for COVID-19 with the RT-PCR test, but who, ultimately, test positive for coronavirus.

Based on these findings, new research, published Thursday in the American Journal of Roentgenology, highlighted the role that chest CT scans can play in potentially identifying COVID-19-positive patients during the early stage of the disease when the RT-PCR test is at its least reliable.

“The less pulmonary consolidation found at CT, the greater is the possibility of negative initial RT-PCR results,” said a team of researchers from China, led by Dandan Chen of the Guangzhou First People’s Hospital in China. “Chest CT is important in the screening of patients in whom disease is clinically suspected, especially those who have negative initial RT-PCR results.”

To determine how valuable chest CT can be with this patient group, Chen’s team examined CT scans and RT-PCR results from 21 patients admitted to five hospitals located in four districts of Guangzhou, China. The patients were admitted from Jan. 19, 2020, to Feb. 20, 2020, and they were all eventually confirmed with RT-PCR to have COVID-19 infection.

During the study, patients underwent chest CT and mouth swab tests on the same day and underwent RT-PCR tests within three days. Chen’s team divided them into two groups: those with initial positive results (14 patients) and those who initially tested negative (seven patients). Those patients who had initial negative tests did test positive for the virus via a second RT-PCR test two days later.

According to the team’s analysis of the CT scans, 67 percent of the COVID-19 lesions were located in multiple lobes with 72 percent being identified in both lungs. In addition, the investigators visualized several other CT findings frequently:

  • Ground-glass opacities (95 percent)
  • Consolidation (72 percent)
  • Subpleural distribution (100 percent)

Other CT findings included:

  • Air bronchogram (57 percent)
  • Vascular enlargement (67 percent)
  • Interlobular septal thickening (62 percent)
  • Pleural effusions (19 percent)
IndexNegative RT-PCRPositive RT-PCRp
Left or right lung241.000
Left and right lung510
Single lobe250.743
Multiple lobes59
GGO 7130.469
Consolidation 3120.040
Air bronchogram660.061
Vascular enlargement590.743
Interlobular septal thickening580.525

Among study participants who initially tested negative, the chest CT scan show that most lesions appeared as ground-glass opacities or opacities mixed with a small area of consolidation. This indicates early-stage disease, the team explained. And, as the area of consolidation increases, the likelihood of a false-negative result decreases.

“In these cases,” the team said, “chest CT may be considered a primary tool for detection of current COVID-19 in epidemic areas.”

In comparison with the positive initial RT-PCR results group, CT results of the initially negative RT-PCR were less likely to indicate pulmonary consolidation (p=0.04). This outcome, coupled with the knowledge that RT-PCR is less effective in patients who are in initial disease phases, highlighted the potential importance of chest CT, particularly in patients who initially test negative with RT-PCR, the team contended.

“Although CT is not the final standard for the diagnosis of COVID-19 pneumonia, it nevertheless plays an irreplaceable role,” the team said. “When patients with suspected COVID-19 pneumonia who have an epidemiologic history and typical CT features have negative initial RT-PCR results, repeated RT-PCR tests and patient isolation should be considered.”