Chest CT scan best for coronavirus diagnosis

By Dr. Liji Thomas, MDFeb 27 2020

With the coronavirus (COVID-19) epidemic in China spreading to other countries around the world, with numerous fatalities, scientists are looking for new methods for more efficient and accurate diagnosis. A new study published in the journal Radiology in February 2020 reports that performing a CT (computed tomography) scan of the chest increases the diagnostic rate and outperforms the reverse-transcription polymerase chain reaction (RT-PCR) tests currently utilized. The researchers recommend using chest CT to screen symptomatic patients for COVID-19 infections rather than the less sensitive RT-PCR.

CT scan of chest and abdominal cavity. Image Credit: Shidlovski / Shutterstock

The problem

It is estimated that the COVID-19 epidemic has now affected almost 78,500 cases in China (confirmed cases). About 9,100 of these were severe, and over 2,744 patients have died of COVID-19 pneumonia. In other countries, the number of confirmed cases is approximately 4,258, with 70 deaths.

Without specific drugs or vaccines targeting the virus, early detection and isolation of cases are the only available route of containment. Detection, according to the latest Chinese guidelines, is confirmed by RT-PCR or by sequencing genes from blood or sputum secretions. The difficulties in collecting samples under ideal conditions and transporting them to qualified laboratories for examination, the time taken to obtain a result, as well as the limitations of the test kit itself, have restricted the sensitivity of the RT-PCR test done on throat swab samples taken at the first visit to about 30% to 60%. This is far from an acceptable rate, considering that many cases may be allowed to leave undiagnosed and untreated. Not only are these patients at risk, but they may infect the population around them as well.

In this context, the researchers in the current study decided to look into the diagnostic utility of chest CT, which is already in use to diagnose pneumonia. In the vast majority of patients with COVID-19 pneumonia, the chest CT shows typical findings, namely, ground glass four opacities, multiple patches of consolidation, with or without interstitial changes at the lung periphery. Moreover, such features were also found in patients with symptoms but with a negative initial RT-PCR.

The current study was carried out in order to test the diagnostic performance of chest CT as a complementary measure in clinically symptomatic patients with an initial negative RT-PCR result.

The study

The study was conducted in Wuhan, China, from January 6, 2020, on over 1,000 patients suspected to have COVID-19 infection. Serial RT-PCR tests and CT scans were examined.

The results of both tests were extracted from the electronic database of a single hospital in China. Multiple RT-PCR testing on the day of presentation and up to 3 days afterward was treated as diagnostic testing. Repeated testing after that date was used to check for negative to positive or positive to negative, conversion of the result.

Similarly, CT scans taken within 7 days of the diagnostic RT-PCR assay were included in the study. This included the first of multiple scans taken in this time period. Scans following the initial scan were used to assess the change in disease activity over time.

The results

About 60% tested positive with the RT-PCR, but 88% with chest CT scan. Of this 60%, it is possible to pick up 97% of cases using chest CT alone. Moreover, when the RT-PCR test is negative, the positive chest CT scan made it possible to pick up approximately 300 more patients who were very likely to have the infection, and 33% of these were thought to have COVID-19 infection.

Among the RT-PCR negative patients, 75% showed chest CT findings, which suggested COVID-19 infection. In all, there were about 900 patients with positive chest CT findings, with an average age of 60 years. Just over 300 of them had positive chest CT findings but negative RT-PCR results. Of these patients, 83% showed characteristic lung signs.

About half of these were highly likely cases, and another third probable cases – which means that, in the words of the author, “About 81% of the patients with negative RT-PCR results but positive chest CT scans were re-classified as highly likely or probable cases with COVID-19.”

Early detection

When the patients had multiple RT-PCR testing because of an initial negative test, the investigators showed that it took about five days before the RT-PCR results converted from negative to positive. 15 patients seroconverted, and in 2 out of 3 patients, the chest CT changes preceded the RT-PCR conversion, with 93% showing typical imaging signs either before or simultaneous with a positive RT-PCR result.

In 57 cases, the first chest CT scan showed signs of COVID-19 infection before or in parallel to the initial positive RT-PCR results, and almost 100% had a positive chest CT scan either before or within six days of the first positive RT-PCR result. In 42%, the first signs of improvement were on the chest CT, with less than 4% showing the reverse trend.

Concerning monitoring, the positive RT-PCR cases became negative in about seven days, whereas follow-up CT scans showed resolution of chest signs before the RT-PCR results did.

Implications

The chest CT scans showed a higher sensitivity for the diagnosis of COVID-19 infection than initial RT-PCR results. It picked up almost all the cases detected by RT-PCR, as well as 75% of cases that were initially missed with the serologic test. The detection rate of 88% could help in containing the disease more effectively.

Chest CT thus has a positive predictive value and a negative predictive value of 65% and 83%, respectively. While there is a relatively high number of false positives, in an emergency situation such as the present, the priority is rightly on identifying the most significant number of cases as quickly as possible rather than on absolute accuracy of diagnosis. Moreover, some false-positive cases in this study may well have been true positives, given the low sensitivity of RT-PCR assay.

The researchers recommend the use of multiple parameters in such cases, including exposure history, clinical symptoms, CT changes, and changes evolving over time, saying, “Early diagnosis of COVID-19 is crucial for disease treatment and control. Compared to RT-PCR, chest CT imaging may be a more reliable, practical, and rapid method to diagnose and assess COVID-19, especially in the epidemic area.”