Am Fam Physician. 2001;63(10):2037
Establishing the diagnosis of tuberculosis (TB) in children can be challenging. Mycobacterium tuberculosis can be detected in samples from fewer than 50 percent of children with TB. The key to diagnosis is linking the child to an adult with confirmed pulmonary TB. The triad confirming the diagnosis is (1) exposure to an infectious case, (2) a positive tuberculin skin test and (3) an abnormal chest radiograph or physical examination. Starke reviews the presentation and diagnosis of TB in children.
Tuberculosis infection occurs when M. tuberculosis is present in a dormant or latent state. The skin test is positive, but the child is asymptomatic. The physical examination is normal, and the chest radiograph may be negative or show a granuloma. TB disease occurs when the organisms rapidly replicate and radiographic or physical signs are present. TB disease in children may develop within weeks or months after the initial infection. Cases of childhood pulmonary TB are usually discovered by health department contact investigation of an adult or adolescent with active infection. TB infection is treated with one drug while TB disease is treated with multiple drugs. The distinction between infection and disease may be blurred because the two conditions exist as a continuum.
A positive Mantoux tuberculin skin test, using 0.1 mL (5TU) of purified protein derivative is induration that usually develops 48 to 72 hours after application. Induration occurring after 72 hours is also significant and should be taken as the result. Up to 10 percent of children with culture-proven TB do not react to tuberculin skin testing at the time of evaluation, but most become reactive after treatment is initiated. A larger number of children with TB meningitis or miliary TB have a skin test that is initially nonreactive. When TB is strongly suspected, any reaction to a tuberculin test should be considered suggestive, although not diagnostic.
Radiographic abnormalities in children with TB disease are caused by a combination of parenchymal infiltrate and mechanical changes caused by airway obstruction. Abnormalities can present in any lobe and may be multilobar. The most common findings are isolated hilar or mediastinal adenopathy, segmental hyperinflation or atelectasis, alveolar consolidation, interstitial densities, pleural effusion or cavitation. Computed tomographic (CT) examination has no role in the evaluation of asymptomatic infected children with normal chest radiographs. In patients with abnormal chest radiographs, a CT scan can clarify endobronchial disease, pericardial invasion, early cavitation or enlarged nodes. TB meningitis is identified by CT scan or magnetic resonance imaging demonstrating communicating hydrocephalus, basilar infiltrate and infarct, especially in the thalamus and middle cerebral artery distribution.
Children rarely produce sputum samples adequate for examination and typically have low bacterial loads. This situation makes mycobacteriologic identification by smear difficult. Acid-fast stains of early morning gastric washing also have a low yield. The best specimen for culture is the early morning gastric washing containing aspirated respiratory secretions. Bronchoscopy can be useful to demonstrate endobronchial lesions or airway compression. Culture confirmation is not necessary if the “triad” mentioned above is present. Sensitivity testing can be performed on specimens obtained from the source case. Nucleic acid amplification (NAA) is a new technique that allows rapid amplification of mycobacterial species. Both DNA and RNA systems are available, but only the DNA systems have been used in published pediatric studies. Specificity is highest on sputum that is positive by acid-fast staining. Results have been disappointing using NAA on gastric aspirates from children. The high false-positive rate suggests that NAA should be used only when TB disease is suspected by abnormalities in the chest radiograph.