Can CT scans diagnose COVID-19? Experts argue for and against
In addition to RT-PCR testing, some experts argue that CT scans can diagnose COVID-19. Others disagree. Medical News Today spoke to two doctors who present their opposing viewpoints.
As the COVID-19 pandemic continues to claim lives across the globe, early diagnosis of people with SARS-CoV-2 is essential. Once a person has received a diagnosis, limiting their physical contact with others is one way to slow the spread.
What is the best way of establishing a firm diagnosis for COVID-19?
For many, the use of reverse transcriptase polymerase chain reaction (RT-PCR) is the gold standard. This molecular biology technique detects genetic material that is specific for the SARS-CoV-2 virus. Yet, RT-PCR is not 100% accurate, and some experts have raised questions around false-positive and false-negative test results.
Could computed tomography (CT) scans, which combine a series of X-ray images, serve as an alternative or an adjunct to RT-PCR diagnosis?
There have been reports from Wuhan in China of healthcare professionals using CT scans to diagnose COVID-19, yet medical and public health bodies in the United States have not followed suit.
Medical News Today spoke to two doctors on opposing sides of the argument around the use of CT scans during the pandemic.
Arguing for the use of CT is Dr. Joseph Fraiman, M.D., an emergency doctor working in the New Orleans area, LA, and a former Medical Manager for Lousiana’s Disaster Task Force 1 Urban Search and Rescue Team.
On the opposing side is Dr. Mark Hammer, M.D., from the Department of Radiology at Brigham and Women’s Hospital, Harvard Medical School, in Boston, MA. He writes on behalf of a group of doctors who recently published a viewpoint article on this topic in The Lancet.
Dr. Fraiman: The American College of Radiology (ACR) and the Centers for Disease Control and Prevention (CDC) have recommended against routine use of CT scans to diagnose COVID-19.
However, in China, where experts quickly and effectively controlled the disease, professional medical organizations universally agree that CT plays “a vital role in early detection, observation, and disease evaluation.”
The ACR recommend against CT scans based on false-positive rates, scanner contamination, and a lack of change in individual patient management.
However, Chinese experts strongly recommend CT based on rapid identification, improved quarantining, and tailored public health measures.
Why such opposing views?
Early on in the COVID-19 outbreak, it became clear that RT-PCR testing is both slow and unreliable. One Chinese official estimated that swab test sensitivity was as low as 30%, a finding consistent with RT-PCR testing of fluid samples.
More optimistic estimates from the ACR suggest a range of 42–71% for the RT-PCR test. In other words, the swab misses nearly a third of cases — at best.
According to a research article in Radiology, CT is far more sensitive, commonly detecting cases the swab misses. CT’s higher sensitivity (routinely estimated at well over 90%) and real-time results come at the expense of increased false-positives.
But this downside is of far less concern to public health authorities than poor sensitivity. The World Health Organization’s (WHO) strategic objective to stop human transmission of SARS-CoV-2 prioritizes the detection and isolation of potential spreaders above all else.
For the benefit of all
This focus on sensitivity, even at the cost of false-positives, reflects the goals of pandemic medicine. Typically, doctors use diagnostic tests to benefit individuals, but the calculus of testing changes in epidemics.
Experts in nations that endured SARS learned the hard way that rapid, sensitive testing is critical during an infectious outbreak not because it benefits individuals, but because it benefits everyone.
The focus of American experts on individual case management is well-intentioned and morally laudable — usually. But the same logic quickly becomes dangerous during an epidemic. In transitioning to pandemic thinking, nations such as China saw that over-diagnosing COVID-19 was far better than under-diagnosing.
You can’t fight what you can’t see, and CT scans are the best way to see the disease. If false-positives occur, the ‘penalty’ is typically home quarantining additional people who are actively ill with another lung infection.
Moreover, the reason to diagnose COVID-19 is not “patient management,” it is pandemic management. The scans are not for the benefit of the person having the scan; they are for everyone else.
Detecting disease and quarantining is how we keep people without the infection safe. In other words, aggressively finding disease and isolating positives halts transmission, potentially saving not just one life, but many.
As for scanner contamination, China has no monopoly on cleaning techniques or technology. Their cleaning protocol allowed for the safe scanning of 200 patients each day on a single scanner with multiple clinics documenting zero transmissions to CT suite staff.
This was one of many integral and necessary adaptations that yielded inarguable success. Their curve swiftly flattened, which saved countless lives.
Rapid results lead to false negatives
Now, with the emergence of a rapid RT-PCR test, there is cause for new fear. In the excitement generated by finally achieving swab results as quickly as CT scan results, providers and authorities could forget how dangerously inaccurate the PCR test is.
Rapid results mean rapid false-negatives and rapid false reassurance. This also means the rapid release of people with COVID-19, allowing them to mingle with people without the infection who may be potentially vulnerable.
Using both tests together would increase sensitivity and improve quarantine efficacy, an impact neither could achieve alone.
The ACR and other organizations have raised concerns that CT scans “may result in false-negative cases, and the risk of missing COVID-19 has broad implications.”
This is a perplexing statement. While the rushed literature we must use to inform COVID-19 testing is methodologically flawed, there is little dispute about whether CT is more sensitive—it is, and the ACR acknowledges this.
A dual approach
Missing from arguments against CT is any comparison to the alternative, RT-PCR, which is both less supported by existing literature, and significantly less sensitive in virtually all comparisons. Moreover, the approach proven successful in China, suggested here, and described in studies, uses both CT and RT-PCR.
Thus any mention of false-negatives should first acknowledge that a CT-based dual approach is mathematically guaranteed to reduce the risk of missing COVID-19 when compared to the default alternative of RT-PCR alone.
Recommending against CT diagnosis for COVID-19, particularly during a shortage of swab tests in the U.S., was an unfortunate error that contributed to a period of unchecked, invisible spread of the virus. The U.S. now has the ignominious distinction of being the world leader in both cases and fatalities, with numbers still growing. In epidemics, a failure to control is virtually always a failure to detect.
There remains, however, a chance for redemption as we move into a phase of carefully repopulating public spaces while attempting to flatten the resultant mini-curves. Success will depend entirely on meticulous detection and quarantining of those infected.
The CDC and ACR should strongly recommend widespread, routine CT-plus-RT-PCR testing for COVID-19.
Why CT scans are not a good way to diagnose or triage people with suspected COVID-19
Dr. Hammer: COVID-19, the disease caused by the SARS-CoV-2, has tested our healthcare system more than any disease in modern times.
Rapid diagnosis of COVID-19 is desirable, as it allows optimal use of protective equipment and isolation rooms in the hospital and accurate quarantine of outpatients.
If and when proven therapies become available, a faster diagnosis would have the additional benefit of quicker treatment.
Unfortunately, the current testing standard, nasal swab RT-PCR, is limited by a lack of supplies (e.g., swabs and test reagents), slow turnaround times (up to several days depending on the lab), and possible false-negative tests. Consequently, some researchers in China have promoted imaging, particularly CT scans, as a first-line means of triaging and diagnosing patients with COVID-19.
COVID-19 has myriad clinical manifestations. The best known and most severe is pneumonia. Experts know that CT scans of the lungs are very sensitive in the diagnosis of pneumonia, but are they useful in the context of COVID-19?
Several research studies from China have argued that CT is useful. Articles from Dr. Yicheng Fang and colleagues at Affiliated Taizhou Hospital in Zhejiang Province, and Dr. Tao Ai and colleagues at Tongji Medical College in Wuhan, published in the journal Radiology suggested that the sensitivity of CT scans for COVID-19 is 97–98%.
In other words, these articles purport to show that only 2–3% of patients with COVID-19 have normal CT scan results.
Unfortunately, these findings are misleading for several reasons.
First, the selection criteria for patients in these studies are unclear, but there appears to be a bias toward people with more severe illness, those in the hospital, or both. It is likely that the figures do not include people with milder (or no) symptoms who would probably have normal CT scans.
Second, the authors do not offer criteria for determining when a CT scan is deemed “positive.” Instead, they appear to include any CT abnormality whatsoever.
In real-world practice, claiming a person has COVID-19 based on the presence of a minor abnormality on a CT both ignores the common subclinical lung inflammation that radiologists frequently encounter and the other diseases that people may have instead of COVID-19.
Third, the results of these articles are at odds with other publications, including a paper by Dr. Shohei Inui and colleagues from the Japan Self-Defense Forces Central Hospital in Tokyo published in the journal Radiology: Cardiothoracic Imaging.
In this study, the researchers studied 104 people with COVID-19 from the infamous Diamond Princess cruise ship. They found that nearly half of asymptomatic people and one-fifth of symptomatic patients had normal CT scans.
When taken collectively, these early results suggest that CT scans produce an unacceptably high false-negative rate and thus will fail to pick up a significant fraction (up to half) of people with COVID-19.
COVID-19 and other causes of pneumonia
Could CT scans of the lungs distinguish COVID-19 from other causes of pneumonia?
A study published in the journal Radiology by Dr. Harrison Bai and colleagues from Xiangya Hospital in Hunan, China, purports to show that radiologists can distinguish COVID-19 from other viral pneumonias with high accuracy. We believe that the results of this article are also misleading.
First, the researchers gloss over the fact that one of the radiologists in the study was only around 50% accurate at diagnosing COVID-19.
Second, and more importantly, this study neglects all of the other diseases that occur in patients except for types of viral pneumonia.
In daily practice, numerous conditions can cause the abnormalities seen on CT scans in patients with COVID-19, including autoimmune lung diseases, pulmonary embolism, mucus aspiration, and bleeding within the lung. It would be tragic if a patient’s real diagnosis was missed or delayed because radiologists were focused on diagnosing COVID-19 at the expense of other diseases.
We believe that the accuracy of CT scans in making a diagnosis of COVID-19 as opposed to other lung diseases may be much lower than these early publications suggest. The false-positive rate of CT will be unacceptably high in most settings where many other diseases are present.
To summarize, several studies purport to show the high accuracy of CT scans in making a diagnosis of COVID-19, even superior to RT-PCR. However, as we showed, these studies may be misleading.
An ineffective screening tool
Even if the goal of CT scans is “pandemic management” rather than “patient management,” the high rate of normal scans in COVID-19 patients (up to half) makes it an ineffective screening tool. If we rely on negative CT scans to allow people to mingle in society, we may cause unchecked spread of the infection.
While early reports from China suggested that RT-PCR had a sensitivity of close to 70%, and much less than CT scans in early studies, a more recent meta-analysis put the sensitivity of RT-PCR much higher at around 89%.
Regardless, the advice of nearly every public health agency around the world, including the CDC, is for people with respiratory symptoms to self-isolate. This does not rely on testing of any kind, whether RT-PCR or CT scans.
We agree with the CDC and major radiology societies, such as the ACR, that RT-PCR is the only way to diagnose COVID-19 definitively.
The Chinese National Health Commission has recently removed chest CT as a criterion for diagnosing COVID-19, and the University of Washington has nearly eliminated its use of CT in people with suspected COVID-19.
Not without risk
Importantly, CT scans themselves are not without risk, particularly to other healthcare providers, such as radiology technologists, who may find themselves exposed to people who have COVID-19.
Moving these individuals around the hospital to the CT scanner risks exposing other patients and parts of the hospital to contamination with COVID-19.
All of this is not to say that CT scans cannot play any role in the diagnosis and management of patients with COVID-19. For example, a person may come to the hospital with abdominal complaints and a CT scan of their abdomen detects abnormalities on their lungs. This may lead to the diagnosis of COVID-19 in someone who came to the hospital for an unrelated reason.
Patients with COVID-19 may also become extremely sick and develop complications that may require a CT scan for diagnosis. In these situations, the benefit of the CT scan outweighs any potential risk of exposure to the radiology technologists. That same calculus does not hold in patients with milder disease.
We strongly agree with the American College of Radiology that CT scans should not be a routine part of the evaluation of patients with COVID-19 but may be used in complicated cases.