Can CT be used to diagnose COVID-19?
The diagnosis of COVID-19 often needs a combination of four things:
- Epidemiological history of travel to an endemic area or having direct contact with a confirmed case of COVID-19.
- Clinical picture of lower respiratory tract illness, although other symptoms may be present, or the patient may be totally asymptomatic.
- Lab test results showing leukopenia, lymphocytopenia, and elevated C-reactive protein and erythrocyte sedimentation rate.
- Presence of viral ribonucleic acid (RNA) on the real-time reverse transcription polymerase chain reaction (RT-PCR) nucleic acid test.
Figure 1. Diagnosis of COVID-19 often requires a combination of travel history to endemic areas or direct contact with confirmed cases, clinical signs of lower respiratory tract illness, laboratory testing, and the real-time reverse transcription polymerase chain reaction (RT-PCR) nucleic acid test.
The sheer volume of tests being processed during this pandemic can often lead to delays in obtaining RT-PCR results of up to several days in many areas. So, when treating a patient with suspected COVID-19, other methods of diagnosis often need to be used.
Another option for the diagnosis of COVID-19—chest CT
As you might have guessed, chest imaging, including a chest x-ray and / or CT scan, play a cornerstone role in the diagnosis and management of patients with COVID-19. In fact, these patients often show signs of pneumonia.
While both chest x-ray and CT scan may be used for chest imaging in these patients, CT is much more sensitive and consistent in terms of the description of imaging findings and monitoring the changes over the disease course and in relation to treatment.
In fact, CT is also superior to RT-PCR. It has three important advantages over RT-PCR:
- CT is more sensitive for diagnosing COVID-19.
- CT can diagnose COVID-19 earlier than RT-PCR.
- CT can detect improvement earlier than RT-PCR.
Figure 2. Chest CT is superior to real-time reverse transcription polymerase chain reaction (RT-PCR) nucleic acid test for detecting the presence and progression of pneumonia in COVID-19 patients.
CT is more sensitive for diagnosing COVID-19
While RT-PCR is considered the gold standard for the diagnosis of COVID-19, successive studies have reported variable sensitivity of RT-PCR ranging from 30–70%. This means it will catch all the positive cases only 30–70% of the time.
On the other hand, CT was highly sensitive to detect the presence and progression of pneumonia in COVID-19 patients—the sensitivity of CT was often higher than 90%, although it may be as low as 50% in the first 48 hours after symptom onset, as described by some studies.
So, we can conclude that chest CT is much more sensitive than nucleic acid testing for COVID-19, but RT-PCR is better able to rule out the disease and can be used for confirmation of a COVID-19 diagnosis.
CT can diagnose COVID-19 earlier than RT-PCR
Imaging changes often precede the nucleic acid testing positivity, meaning that we can see pneumonia on CT even before the RT-PCR becoming positive.
In endemic areas, this has led to the isolation of patients with pneumonia changes until further testing could definitively determine the causative pathogen. This strategy is especially essential for infection control and it has been carried out in Wuhan, resulting in a good outcome in terms of controlling the transmission of the virus.
CT can detect improvement earlier than RT-PCR
In addition, the changes in chest CT associated with COVID-19 also often show signs of resolution before RT-PCR turns negative, which is another benefit of CT that helps monitor the changes and guide the management of these patients.
What is the correlation between CT changes and the clinical features of COVID-19 patients?
Finally, let’s have a look at the correlation between CT changes and clinical features of COVID-19 patients.
One study found that in patients with COVID-19, the pulmonary inflammation index, which is based on the distribution and size of lung lesions, was significantly correlated with the clinical and laboratory markers such as the number of days from illness onset and body temperature, as well as lymphocyte count, monocyte count, C-reactive protein, and procalcitonin. So, this points to the strong predictive value of chest CT for the clinical situation of these patients.1
But we need to be aware that this correlation is mostly observed in the early and moderate stages of the disease—the first and second weeks of disease. In late stages, the clinical improvement happens earlier than the complete clearing of the lesions on CT, which may take more than a month, with some cases even having some residual fibrotic changes on CT.
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