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Normal Chest X-ray cannot rule out COVID-19

Chest X-ray is the first imaging method to diagnose COVID-19. However, a normal chest X-ray cannot rule out COVID-19. Review of the severe acute respiratory syndrome (SARS) and middle east respiratory syndrome (MERS), normal chest X-ray does not guarantee that a person has no that disease. To date, several clinical evidence has examined the sensitivity and specificity of chest X-ray in identifying COVID-19. In terms of sensitivity, Chest CT has greater sensitivity than does chest radiography. However, the chest X-ray examination is relatively more affordable in terms of cost and is almost always available in all hospitals.

The gold standard for diagnosis of COVID-19 is Real-Time Reverse-Transcriptase Polymerase Chain Reaction (RT-PCR) then followed by genome sequencing for detection of ribonucleic acid / new RNA coronavirus (SARS-CoV-2). The specimen is sputum by throat swab. 

RT-PCR test for COVID-19 has a high specificity, but the sensitivity is relatively low at around 60-70%. A retrospective study by Fang Y et al. reported that RT-PCR sensitivity for COVID-19 was 71%. Therefore, diagnosing COVID-19 requires several other examinations.

Coronavirus Disease 2019 generally causes viral pneumonia with varying degrees of severity. Therefore, clinical evaluation, including X-ray examination, can be considered, especially when the patient first comes for a check-up, making it easier to assess the disease in the early and advanced stages. In general, thorax roentgen imaging on COVID-19 is pneumonia with findings of ground-glass opacity (GGO), consolidation, both focal and multifocal. Thus, although sometimes patients with COVID-1 have a normal lung picture, the chest X-ray still has a special role in the diagnose and evaluation of COVID-19.

 X-ray Interpretation of COVID-19

The interpretation of COVID-19 thoracic X-ray is thought to have similarities with SARS and MERS since coronavirus causes all of them. In Sars, the lesions are common unilateral with unclear peripheral distribution and airspace areas in the lower lung zone. SARS lesions are generally focal in about half of patients, and the remainder is multifocal, and diffuse is less than 10%.

Normal and Pneumonia Chest X-ray Images
Two Chest X-ray images.
Normal Chest X-ray (left side): two yellow arrows pointing normal Lung (clean with black color)
Pneumonia COVID-19 (right side): two red arrows point the lesions (white color)

Source image: https://radiopaedia.org/

Meanwhile, MERS lesion is multifocal airspace opacities in the lower lung zone. The  MERS progressivity can be characterized by the extension of lesion to the perihilar and upper lobe.

Difficult Chest X-ray interpretation on COVID-19

Difficulties in the Chest X-ray interpretation of COVID-19 are generally due to the many uses of non-standard terminology in several studies that have been carried out, such as airspace disease, pneumonia, infiltrates, patchy opacities, and hazy opacities.

A retrospective study by Wong HYF et al. reported that the most characteristic feature of COVID-19 chest X-ray was 59% consolidation and 41% ground-glass opacity (GGO), with peripheral or posterior distribution, especially in the lower lobes. Also, GGO can be found with thickening of inter / intra-lobular, peripheral, and basal septal. Even in asymptomatic COVID-19 patients, disease progression from focal unilateral to diffuse GGO and consolidation was also found. Pleural effusion was rarely encountered, but if found may be a predictor of poor prognosis.

A retrospective study by Weinstock MB et al. Of 636 chest X-ray films of COVID-19 confirmed patients through RT-PCR examination. It was assessed radiologically with normal, mild, moderate, and severe disease.

This study found 371 (58.3%) chest X-ray showed a normal picture, and 265 (41.7%) chest X-ray showed an abnormal picture. From the abnormal results, each of them consisted of 195 chest X-rays with mild images, 65 chest X-rays with moderate images, and 5 chest X-rays showed severe images. The interstitial changes in the lung were found in 151 chest X-rays (23.7%), and GGO was found in 120 chest X-rays (18.9%). The primary location of the lesion was the lower lobe at 215 chest X-rays (33.8%), bilateral at 133 chest X-rays (20.9%), and multifocal at 154 chest X-rays (24.2%). Effusion and lymphadenopathy were rare.

However, this study had limitations that the method was retrospective, and there was only one chest X-ray in each patient. These made the reliability reduced because each patient who had been examined could show different stages of the disease. It was difficult to know whether a patient with a normal chest X-ray would continue to have a normal chest X-ray when clinical symptoms begin.

Meanwhile, a systematic review by a meta-analysis by Rodriguez-Morales et al. involving 780 patients reported that the findings on the chest X-ray of patients confirmed positive for COVID-19 were pneumonia, which generally occurred bilaterally (72.9%), with GGO 68.5 images %.

Comparison of Chest X-ray and Thoracic CT Scan on COVID-19

The findings on the chest X-ray and chest CT scan are generally the same, but the chest CT scan is generally clearer to confirm findings that were previously present on the chest X-ray. Dominant findings from CT scans occur bilaterally and peripherally with GGO, consolidation, or both.

Thoracic X-ray Sensitivity in COVID-19 Patients

A descriptive study by Shi H et al. found that the initial thoracic X-ray examination in hospitalized COVID-19 patients had a sensitivity of 69% for each abnormality encountered. Meanwhile, the initial chest X-ray examination when the patient first comes to the ER has a lower sensitivity. This is presumably because the severity of the disease in newly arrived patients is generally lighter than in hospitalized patients.

A study by Wong et al. compared the sensitivity between chest X-ray and RT-PCR in the diagnosis of COVID-19. Six of the patients (9%) had abnormal chest X-rays before they were confirmed positive by RT-PCR. The sensitivity of the initial PCR RT was higher at 91%, while the chest X-ray was 69%.

Thoracic CT-Scan Sensitivity in COVID-19 Patients

A study by Hosseiny et al. involving 24 COVID-19 patients reported an initial thoracic CT scan (0 to 4 days after symptom onset) that 17% of them showed no pulmonary opacity, 42% showed focal or consolidated GGO, and 42 % shows multifocal pulmonary opacity.

Meanwhile, serial thoracic CT scan on days 5-13 shows the progression of pulmonary opacity. The findings of the thoracic CT scan at an advanced stage (over 14 days) show varying degrees of improvement, but no resolution was found even up to 26 days.

A retrospective study by Fang Y et al. compared the sensitivity between chest CT scan and RT-PCR in the diagnosis of COVID-19. The study reported that 50 out of 51 (98%, 95% CI 90-100%) patients showed a picture of viral pneumonia on the initial examination compared with RT-PCR, which was positive in 36 of 51 patients (71%, 95% CI 56-83% ). This study also supports the examination of the thoracic CT scan as a COVID-19 screening tool for patients who meet the COVID-19 criteria but show a negative RT-PCR.

Patients with clinical symptoms of COVID-19 or patients who have been in contact with positive patients with COVID-19 should continue to undergo RT-PCR examination to confirm new coronavirus RNA (SARS-CoV-2) and CT scan for the detection of more sensitive pneumonia.

Advantages and disadvantages of X-ray and CT scan

In terms of cost, a chest X-ray is cheaper than a CT scan. Moreover, considering that all equipment used by COVID-19 patients must be carefully sterilized, this makes the thorax CT scan relatively impractical. This disadvantage makes the chest X-ray examination widely carried out at diagnosis and follow-up for COVID-19.

Candidates for patients who should undergo a chest X-ray examination

In 2020, The Fleischner Society, an international organization based in the United States, has released the thoracic radiology consensus guidelines on COVID-19. According to the consensus of candidates for patients who should undergo a chest X-ray examination include:

Patients with mild clinical symptoms of COVID-19: Radiological examination will be recommended if the risk factors for the disease become progressive, carried out after a positive RT-PCR.

Patients with moderate-severe COVID-19 clinical symptoms: radiological examination will be recommended if the patient experiences a deterioration in pulmonary status, the examination is carried out after a positive RT-PCR. If tests for COVID-19 are not available, the radiological examination can be considered if there is an alternative diagnosis such as lobar pneumonia.

Chest X-ray examination may be an essential component in the management of patients with COVID-19. Surely further investigation will be needed to broaden understanding of the findings of the chest X-ray in each course of the disease. Learning from the SARS and MERS pandemics,  on individuals who are considered cured of COVID-19 based on the RT-PCR's results needs chest X-ray follow-up to look for chronic lung involvement evidence (such as interlobular thickening, water trapping or fibrosis).

Medical personnel needs to remain vigilant to prevent human-to-human transmission. In addition, prevention of nosocomial infections can play an important role in reducing the spread of disease. Therefore, the Radiology Team must be aware of all precautions and strategies to minimize the risk of infection between staff and patients during the examination and afterward.


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