The lungs are the most commonly involved organ in TB and, from a public health perspective, the most important. In Canada, in 2017, 69.4% of all notified cases were classified as PTB.1 Despite the increased availability of more-detailed imaging techniques, such as computed tomography (CT), chest radiography remains the mainstay of chest imaging for PTB. This overview is focused on chest imaging in adults.
2.1.1. Chest radiograph
Chest radiographs are important to both the diagnosis and management of PTB. They are accessible and inexpensive in most settings, they can be easily acquired and interpreted at point of care and they are safe: a single chest radiograph exposes the patient to an amount of radiation that is roughly equivalent to what they are exposed to naturally over the course of about 10 days. Serial chest radiographs provide additional information in that they allow one to detect change and therefore help to identify and chart the progress of TB disease.
PTB can manifest on chest radiograph in a variety of ways.2 Key patterns important to the diagnosis of TB disease are summarized in the following sections.
2.1.2. Key diagnostic patterns
The classic radiographic presentation of PTB in immunocompetent adults is upper lung zone disease (ie, involving the apical or posterior segments of the upper lobes or the superior segments of the lower lobes), with or without cavitation but with no discernable adenopathy. This was the pattern observed in 56.2% of culture-positive PTB patients in a recent population-based cohort study in Canada; in 67.0% of those patients with this pattern, the sputum microscopy was positive.8 The presence of cavitation in such patients increases the probability of a higher semi-quantitative smear grade (from 1-2+ to 3-4+),3 which may explain in part why smear-positivity and cavitation are independent risk factors for transmission.4–6 More details are provided in Chapter 2: Transmission and Pathogenesis of Tuberculosis.
Intrathoracic adenopathy, typically involving the left or right hilar and/or right paratracheal lymph nodes, is a common feature of primary PTB in children. This pattern of progressive primary PTB may also present in adolescents or adults.
A calcified pulmonary nodule, usually in the lower part of upper lobes or the upper part of lower lobes and close to the pleura, is consistent with a granuloma and generally indicates remote TB infection.
Unusual or atypical patterns include solitary pleural effusion, lower lobe TB, a diffuse micronodular (miliary) pattern and, occasionally, a normal chest radiograph.
2.1.3. Key factors that can influence the chest radiographic presentation of PTB
Older age and immunosuppressing conditions that are known to increase the risk of TB (eg, human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS), transplant immunosuppression7,8 and renal failure9–11 also increase the likelihood of an atypical radiographic presentation of PTB. Another high-risk medical condition, silicosis, can, itself, alter the appearance of the chest radiograph in such a way as to make it difficult to discern concomitant PTB. Patients with past PTB treated with collapse therapy, such as thoracoplasty, can relapse years later with atypical radiographic abnormalities, most of which are unrelated to the current episode.
2.1.4. Chest radiographs during pregnancy
As a general rule, the risk to the fetus of undiagnosed PTB far outweighs any risk from radiation exposure. Some simple steps, however, can minimize the risk to the fetus of radiation exposure. First, avoid chest radiographs during the first trimester if possible; second, limit the exposure to a single posterior-anterior (PA) view; and third, double-shield the abdomen, both front and back.
2.1.5. Accuracy and limitations of chest radiography
Sensitivity: If any abnormality is considered, the chest radiograph has more than 95% sensitivity;12 if only those key patterns listed above are included, the sensitivity is reduced substantially. A normal radiograph may sometimes occur in someone who is sputum culture-positive and living with HIV, especially those with advanced immunosuppression; close contacts of sputum smear-positive PTB; and patients with extra-pulmonary TB. Such patients may or may not have symptoms referable to the respiratory tract.
Specificity: Specificity is greater when only chest radiographic abnormalities suggestive of PTB are considered.12 If sensitivity is improved, by considering any abnormality as possible TB, then specificity is reduced from 89% to 75%.
Inter-reader variability: One of the greatest problems associated with chest radiograph reading is that interpretation is highly variable. Even with experienced chest radiologists, there is poor agreement between and within readers regarding the presence of cavitation, hilar adenopathy or the likelihood of TB disease.
In summary, chest radiograph is an imperfect tool. The sensitivity in people with symptoms is high, therefore a negative chest radiograph can be a helpful, albeit imperfect, rule-out test. However, it cannot be used as a stand-alone test to rule in PTB.
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