CORADS Score, CT Severity Score

The CORADS Score and CT Severity Score in COVID 19 Infection attempts to determine how likely are a set of CT findings due to a COVID 19 infection and if they are related to a COVID 19 infection,  how severe is it. As the COVID 19 Pandemic sweeps across the world, the whole spectrum of its clinical manifestations is slowly being understood.

It is well known that the COVID 19 virus enters the body primarily via respiratory droplets and aerosols which travel through the oral cavity, down through the larynx, trachea, major airways, bronchi, bronchioles and finally reach the alveoli of the lungs where the virus wreaks its havoc.

One of the important manifestations in the lung is the induction of an exaggerated immune response which can spread to other organs as well (systemic inflammatory response syndrome). This inflammation is also accompanied by thrombosis of blood in the small blood vessels and capillaries in the lung, COVID 19 damages the heart, and other affected organs leading to their dysfunction.

Classical CT findings of COVID 19 infection in the lungs, include, bilateral, multifocal, peripheral (ie subpleural), rounded, lower lobe ground glass opacities, a crazy pavement pattern, a spider web pattern, vascular thickening, or a reverse halo (reverse atoll) sign. CT features that suggest a non COVID 19 etiology include lymphadenopathy, cavitation, a nodular pattern, a mass lesion, pleural effusion, segmental, centrilobar, hilar or apical involvement and pleural thickening. The severity of the manifestations in the lungs has a direct prognostic implication in patient management.

As reports from Radiologists across the globe varied in their respective definitions of “mild disease” and “severe disease”, the need for a standardized, reproducible and uniform system of reporting applicable across the globe was felt. A standardized method of reporting to reduce / eliminate inter-observer variation and ambiguity in reporting was the need of the hour.

Netherlands Radiology CORADS Score CT Severity ScoreIt was in this background that Radiologists in the Netherlands (lead by Dr Mathias Prokop at the Radboud University Medical Center) on behalf of the Dutch Radiological Society (Nederlandse Vereniging voor Radiologie) unveiled their COVID 19 reporting and data system (CORADS for short).

The CORADS score

The CORADS score is a measure of the level of suspicion of a set of CT findings being of a COVID 19 infection etiology. It is not a measure of the severity of the infection. A low score suggests a non COVID 19 etiology and a high CORADS score suggests a COVID 19 etiology. This system of reporting along with a CT severity scoring system (based on either lobar involvement or bronchopulmonary segment involvement) was quickly adopted across the globe and a uniform, reproducible and standardized worldwide reporting system for the CT manifestations of COVID 19 slowly evolved.

The proposers of the CORADS system tested it out initially between eight Radiologists who reported 105 CT scans. It was found that 68% of reports were in complete agreement with each other. It was also noted that 28% reported a 1 category variation in the CORADS score. A difference of more than 2 CORADS categories was noted in only 3.7% of reporters.

A Fleiss Kappa score of the system was then computed. The Fleiss Kappa score is a measure of the reliability of agreement among reporters. It was found to be 0.47 overall. The Fleiss Kappa scores the highest in grade 1 (normal / no pulmonary involvement) and grade 5 (typical findings in COVID 19). This confirmed the high discriminating capacity of the system in ruling out or confirming COVID 19 disease based on CT findings.

The score of the CT findings was graded from 1-5. Scores 1 and 2 were labeled negative (COVID 19 very unlikely). A score of 3 was labeled as indeterminate (COVID 19 etiology possible). A score of 4 or 5 was labeled positive (COVID 19 very likely). A CORADS score of 6 was subsequently added when classical CT findings were accompanied by a positive RT PCR test.

The CORADS system is based on another similar and successful system of reporting for breast imaging namely the BIRADS system. CT scans in patients with a CORADS score of 4 or 5 were noted even when the RT PCR in these patients was negative suggesting that a CT of the chest could diagnose COVID 19 even when the RT PCR was negative but the clinical index of suspicion is high. These initially negative RT PCRs later became positive as the disease progressed.

The CORADS score is only a measure of the index of suspicion of a CT finding to be due to a COVID 19 infection. It is not a measure of the severity of the lung involvement in COVID 19. It is perfectly possible to have a CORAD score of 5 with a CT severity score of <8 (based on the lobar method of calculation) indicating mild disease. It just means that there is a high chance of the “mild” findings on CT to because of a COVID 19 infection.

The obvious worst case scenario is a CORAD score of 5 or 6 associated with a CT severity score of >15/25 (lobar method of calculation) indicating a high chance of the CT findings being a result of a severe COVID 19 infection.

The CORADS score grading is based on the following findings:

CORADS 0 – incomplete or inadequate scan which cannot be reported (usually due to the patient coughing or breathing during the test)

CORADS 1 – normal CT or presence of findings suggestive of a non infectious etiology such as CHF, emphysema, lung tumors, lung fibrosis.

CORADS 2 – findings consistent with infections other than COVID 19, bronchiolitis, tree in bud appearance, cavitation, thickened bronchi. Minimal alveolar involvement.

CORADS 3 – Unclear if COVID 19 is the etiology, bronchopneumonia, lobar pneumonia, septic emboli, and ground glass opacities.

CORADS 4 – highly suspicious of COVID 19, unilateral or centrilobar / non peripheral ground glass opacities, multifocal consolidation.

CORADS 5 – Typical of COVID 19, multifocal, peripheral ground glass opacities, crazy pavement patterns.

CORADS 6 – CORADS 5 with a positive RT PCR.

It is important to take into consideration the number of days since the onset of symptoms during reporting as CT findings evolve / change with the passage of time. Findings present during the early phase of the disease may not be present during the later stages of the disease and vice versa. The approximate time lines for CT findings during the initial and later phases of the disease are as follows:

0-4 days – Normal CT or Ground glass appearanceCORADS Score, CT Severity Score

5-8 days – Ground glass appearance and crazy pavement appearances

9-13 days – Consolidation

14 days – Varying degrees of fibrosis or resolution of lesions.

The CT severity score

The CT severity score is a measure of the severity of lung involvement in COVID 19 infections. The severity score can be calculated based on the lobar method (score between 0 – 25), or based on the bronchopulmonary segment method (score between 0 – 40)

CORADS Score CT Severity scoreLobar method of CT Severity Score Calculation

In the lobar method of calculation, each lobe of the lung (3 lobes in the right lung and 2 lobes in the left lung) are inspected and the extent of involvement done by a visual assessment and a score is given.

<5%  involvement = Score 1

5%  –  25%  involvement = Score 2

26%  –  49%  involvement = Score 3

50%  –  75%  involvement = Score 4

>75%  involvement = Score 5

Total score 0-25 (score of 1-5 for each lobe of both lungs)

Total score < 8 indicates mild disease. A score between 8 – 15 indicates moderate disease and a score of >15 indicates severe disease. A few examples of scoring are set out below

Example1 (Severe Disease)

Right Upper lobe            26 – 49% involvement       3

Right Middle lobe            <5% involvement             1

Right Lower Lobe           >75% involvement            5

Left Upper Lobe            26 – 49% involvement        3

Left Lower Lobe            >75% involvement              5

Total 17/25 (Severe Disease)


Example 2 (Moderate Disease)

Right Upper lobe            5 – 25% involvement         2

Right Middle lobe            <5% involvement             1

Right Lower Lobe           26 – 49%% involvement   3

Left Upper Lobe            <5%%                                    1

Left Lower Lobe            26 – 49% involvement       3

Total 10/25 (Moderate Disease)


Example 3 (Mild Disease)

Right Upper lobe            <5% involvement             1

Right Middle lobe            <5% involvement            1

Right Lower Lobe           5 – 25% involvement        2

Left Upper Lobe            <5%  involvement              1

Left Lower Lobe            5 – 25% involvement          2

Total 7/25 (Mild Disease)


Segmental method of CT Severity Score Calculation

A CT severity score can also be calculated based on the number of bronchopulmonary segments that are affected in each lobe of bothCORADS Score CT Severity Score lungs.

The right lung has 10 bronchopulmonary segments.

Upper lobe (3) – apical, anterior and posterior

Middle lobe (2) – medial and lateral

Lower Lobe (5) – superior, antero basal, posterobasal, medial basal and lateral basal segments

The Left Lung has 8 BP segments. It is subdivided into 10 segments for ease of calculation

Left upper lobe (5) – apico posterior (subdivided into apical and posterior) and anterior, superior and inferior lingular segments

Left lower lobe (5) – superior, anteromedialbasal (subdivided into anterobasal and medial basal), posterobasal, lateral basal.

Out of the 20 bronchopulmonary segments (10 + 10) in both lungs, the degree of parenchymal involvement is visually assessed and a score between 0 and 2 is given.

No parenchymal opacification = score 0

<50% parenchymal opacification = score 1

>50% parenchymal opacification = score 2

Each segment is scored as above and the total score is added. A total score of <20 is classified as “mild disease”. A total score of >20 is classified as “severe disease”. It is perfectly possible to have a CORADS score of 5 with a CT severity score of <20 indicating mild disease. It just means that there is a high chance of the “mild” findings on CT to because of a COVID 19 infection.

The obvious worst case scenario is a CORADS score of 5 or 6 associated with a CT severity score of 40/40 (based on the broncho pulmonary segment calculation above) indicating a high chance of the CT findings being a result of a severe COVID 19 infection.

Clinical significance of the CORADS and CT  Severity Scores

Major heart / lung surgery in patients with a preoperative CORADS score of 4 or 5 with a high CT severity score (based on either the lobar or broncho pulmonary segmental method of calculation) is associated with greater morbidity and higher mortality compared to patients with lower scores. The detrimental effects are more in patients who are smokers or those who have pre-existing lung disease such as chronic bronchitis or emphysema. They are also likely to need a longer duration of ventilation post-operatively. Weaning these patients off the ventilator may also be difficult.

CORADS Score CT Severity scoreIt is well known that there is an approximately 25% false negative testing rate for the RT PCR test. When the clinical index of suspicion for COVID 19 is high but the RT PCR IS negative, CT imaging is a useful tool to determine the likelihood of the lung infection is due to COVID 19 (CORADS Score) and what its severity (CT Severity Score) is. All these have a direct bearing on the decision regarding the need and timing of surgery in these patients particularly when the surgery involves the heart or the lungs. Even relatively less major procedures such as VATS may be fraught with complications














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