As of March 16th, so in just the first three months of the outbreak, there have been 167,511
cases of COVID-19 and 6,606 deaths, working out to a mortality rate of 3.9%. On top of
that, every case of COVID-19, leads to roughly 2.3 new cases, meaning that the outbreak is
still spreading. Many countries have been affected, and there are numerous cases of
community spread, meaning that individuals are getting sick without being around any
sick contacts or having traveled to outbreak areas. In that context, healthcare workers
need diagnostic tools to investigate cases of potential COVID-19 that are both sensitive
and specific.
The gold standard for diagnosis of COVID-19 is RT-PCR, or reverse transcription polymerase
chain reaction. In the early days of the epidemic in China RT-PCR was only 30 to 70% sensitive
whereas Chest CT was reportedly much more sensitive in that context. However, more recent
data from the US labs at the University of Washington suggests that second generation
COVID RT-PCR tests are faring much better, with 95%+ sensitivity. Despite its usefulness
in the early days of the epidemic in China, Chest CT findings are no longer part of the
diagnostic criteria for COVID.
There are still clearly issues with access to RT-PCR and related wait times in the US
and elsewhere. Nevertheless, over the past several weeks, several major US radiology
societies have come out with statements making it clear that CT should be used sparingly
in COVID and only when it will impact management.
However, because there will be certain cases when imaging is indicated, as well as patients
imaged for other reasons whose scans reveal findings potentially suggestive of COVID,
it is essential that healthcare providers be familiar with the imaging features of the
infection.
Typically, when a patient has symptoms of COVID-19, like fever, cough, or shortness
of breath, they may get a chest X-ray. The most common abnormal finding is “ground
glass” opacities, meaning that some portions of the lungs look like a “hazy” shade
of gray instead of being black with fine white lung markings for blood vessels. It looks
a bit like frosted glass, like a window in the wintertime. It is important to note, however,
that chest X-rays are not very sensitive for COVID-19 and can yield false negatives.
Now compared to chest X-rays, chest CT gives a much more detailed view. The most common
CT finding in COVID-19 are those ground-glass opacities scattered throughout the lungs.
They represent tiny air-sacs or alveoli getting filled with fluid, and turning a shade of
grey on a CT scan.
In severe or more advanced infections, more and more fluid will accumulate in the lobes
of the lungs, so the ground glass appearance will progress to a solid white “consolidation.”
Finally, there’s a finding called the “crazy paving” pattern due to swelling of the interstitial
space along the walls of the lung lobules. This makes the walls look thicker, like white
lines against the hazy, ground glass background. The appearance is similar to irregular shaped
stones used to pave a street, hence the name “crazy paving.”
These three CT findings - ground glass opacities, consolidations, and crazy paving patterns,
can be seen in isolation or in combination with one another. Ground glass is usually
the first sign and is followed later by one or both of the others.
These findings usually occur in multiple lobes throughout both lungs, and more commonly affect
the outermost or periphery of the lungs. Having said that, in mild or recovering cases of
COVID-19, the findings can be isolated to just one lobe.
It’s no surprise then, that disease severity is proportionate to the lung findings. The
sickest individuals typically have the most severe findings on chest CT. And as patients
improve, there’s gradual resolution of the ground glass and consolidations.
Now, it’s worth mentioning that there are some key CT findings that are usually not
seen in n COVID-19, or at least seen less often. You don’t usually see pleural effusions,
which are fluid collections in the pleural space right outside of the lungs. These are
more common in conditions like congestive heart failure and bacterial pneumonia. You
also don’t see large lymph nodes in the mediastinum or near the hilum or central part
of the lung, which are often seen in other types of pneumonia. Finally, you don’t typically
see lung cavities, which usually develop in bacterial and fungal pneumonia due to necrosis
of the lung tissue later turning into an air-filled cavity.
Now, even though a chest CT is very sensitive for COVID-19, the key findings, ground glass
opacities, consolidations, and crazy paving, can also be seen with other causes of viral
pneumonia like influenza and adenovirus. They can also be seen in a variety of non-infectious
conditions. That means that chest CT is sensitive, but not very specific for COVID-19, and that
individuals with these findings should get further clinical evaluation and laboratory
tests done to exclude other causes.
So as a quick recap. Chest CT is sensitive for severe cases of COVID but not specific
for COVID and should be performed only when the referring physician feels it will impact
management.d. The key findings on chest CT include ground glass opacities, consolidations,
and crazy paving. Individuals presenting to hospitals with these chest CT findings may
need isolation, and should get thorough confirmatory testing and appropriate treatment.