Gold standard for diagnosing coronavirus covid-19 infection
The gold standard test for diagnosing coronavirus infection is RT-PCR (Reverse transcription – Polymerase Chain Reaction) test.
However there is a problem with it. Current data suggests that RT-PCR is sensitive for around 30-70% of cases only in acute infection. Which means we can have a 70% false negative with it.
In simple words, if we test 10 confirmed cases of coronavirus infection, 7 of those confirmed covid-19 cases can get negative results from RT-PCR test. Which makes it practically unreliable.
This can be due to several reasons, including that in acute phase of covid-19, there is not much virus in the blood to be detected by this lab technique. It can also be due to poor laboratory techniques in non-standardised hospital and labs setups around the globe.
Chest CT is more reliable than RT-PCR in diagnosing Covid-19 currently
Around 95% of cases can be screened more accurately using chest ct findings for coronavirus infection, which makes it more reliable than current gold standard technique of RT-PCR.
Chest CT can thus be used especially in setups where laboratory techniques are not efficient or kits are not readily available and result in long wait times for RT-PCR test results.
Chest X-Ray findings vs Chest CT in Covid-19
When someone acquires flu like symptoms or symptoms related to coronavirus infection, they get chest x-ray in most setups as an initial checkup.
Chest X-ray findings in Covid-19
On chest xray, a covid-19 patient would likely show Ground Glass Opacities. Which means that some portion of their lungs will have a hazy grey opacity instead of normal appearance.
Chest CT scan findings in Covid-19
Chest CT will also show ground glass opacities.
Are chest xray sensitive to covid-19?
No. Chest x-ray are not very sensitive to coronavirus infection and thus are not a reliable diagnostic or screening tool for coronavirus patients.
Thus patients with high suspicion of covid-19 should get a chest ct-scan, even if their initial chest xray was negative for any ground glass opacities or signs of coronavirus infection.
What is the cause of ground glass opacities in covid-19?
Ground glass opacities represent the fluid accumulation in alveoli, the air sacs in lungs.
As the infection progresses and becomes more severe, more lobes of the lungs will get damaged and show such opacities on chest x-ray and ct scan. However it can also progress to solid white opacities or consolidations.
Crazy Paving Pattern
However in much later stages and as a result of continuous damage, the interstitial space gets swelled along the walls of lobules of the lungs. This results in much thicker walls, which appear as thick white lines in front of the white background of ground glass appearance.
Note that these findings in chest ct scan of covid 19 patient can be isolated or in combination with each other. While ground glass opacities are mostly the first sign, and is then followed by any one or both of the other two findings shown in image above.
These findings usually occur in both lungs and in multiple lobes. Most likely the peripheral or outermost part of the lungs get damaged more often than the other parts.
Can CT scan finding be seen in one lobe only?
Yes. In mild cases of recovering cases, the finding may be isolated and appear in only one lobe of the lungs too.
Chest CT scan findings proportion to covid-19 severity
The disease severity is proportional to the lungs findings in chest ct scan of a coronavirus infection patient.
Thus it can also be used as a prognostic test in covid 19 patients, however x-rays exposure should be kept to minimum.
Chest CT findings Absent in Covid-19
We commonly do not see these findings in coronavirus patients’ ct scan:
- Pleural effusion – fluid collected in pleural space outside the lungs, more common in bacterial pneumonia or CCF (congestive cardiac failure)
- Enlarged mediastinal or hilar lymph nodes (in center part of the chest cavity) – commonly seen in other types of penumonia
- Cavities in lungs – usually seen in bacterial or fungal pneumonia, as a result of lung tissue necrosis resulting in air filled cavities
Covid-19 CT scan findings can be seen in other diseases
It’s also important to note that the ct-scan findings of covid-19 can also be seen in other types of viral pneumonias. For example influenze and adenovirus.
Also note that these findings can be seen in non-infectious diseases of the lungs too.
Chest CT is sensitive or specific for Covid-19?
Chest CT is sensitive to Covid-19. However it is not specific to coronavirus infection as the findings of covid-19 can also be seen in some other disease conditions.
Thus anyone who gets chest ct-scan done for coronavirus infection, should also get detailed history and other tests done for a confirmation to help identify the cause of the disease.
Role of CT scan in different severity & prevalence of covid-19
Patients with mild symptoms or asymptomatic ones, might have a 50% sensitivity from chest CT and get a negative chest ct, however they might get a positive RT-PCR.
Also chest CT is not a good screening tool for areas with very low prevalence of coronavirus, as findings might be due to other causes and infections in that area too.
However in areas where covid-19 is highly prevalent, if a patient gets acutely ill and is symptomatic, then chest CT scan is a more reliable tool for screening. Patients showing ct scan findings in areas of outbreak should be considered a presumptive covid-19 case, even if their initial RT-PCR is negative.
Conclusion
RT-PCR test is the gold standard test for coronavirus infection screening. However the test is currently evolving and adapting, and during this phase it’s not accurate or reliable in many cases. In those cases, CT scan can be more sensitive, especially in regions where prevalence of covid-19 is high.