Chest x-ray remains crucial for finding drug-resistant TB

Budget cuts have been added to coinfection with human immunodeficiency virus and multidrug-resistant disease as the biggest threats to the prevention of tuberculosis in the U.S.

San Francisco and neighboring Bay Area communities saw TB incidence decline until 2006, thanks to a stringent control program funded by the Centers for Disease Control and Prevention and the state of California. But TB is poised to once again become a major healthcare issue in Northern California. The CDC plans to trim its budget by 25% in the next five years, and the state has announced that it will cut $1 million from its surveillance program.

The region has seen a 20% uptick in active TB cases between 2006 and 2007, according to Dr. Masae Kawamura, director of the Tuberculosis Control Section of the San Francisco Department of Public Health. The most recent cases of the highly infectious disease have been documented in immigrant workers, many of whom live together in close quarters.

Chest x-rays are integral to TB surveillance. All suspected TB patients in the Bay Area undergo chest x-ray, generally to confirm disease. But even when patients have positive results on skin tests, x-ray results are often misread as a cold or pneumonia, according to Dr. Lisa Goozé, TB Controller for the San Mateo County Health Services Agency, just south of San Francisco.

Given the subjectivity involved in reading these x-ray exams, it is not surprising that the disease can be missed, according to a paper in the International Journal of Tuberculosis and Lung Disease (2005;9:1088-1096). Experts in the field concur that chest x-ray is not the best way to unmask active TB; quantiferon TB (QFT), purified protein derivative (PPD) test, sputum smear microscopy, and nucleic acid amplification (NAA) assay are on the diagnostic frontline (Acta Reumatologica Portuguesa 2008;33:77-85). Still, radiography remains an important part of the TB diagnostic armamentarium.

In a March 6, 2008, paper in the online journal BMC Infectious Diseases (www.biomedicentral.com/1471-2334/8/32), public health officials from the U.S., Tanzania, and Finland identified signposts on chest x-ray that indicate the presence of TB, especially in the absence of other symptoms.

"An abnormal x-ray is defined as the presence of a focal infiltrate, cavity formation, hilar adenopathy, or a miliary pattern . . . [C]hest x-ray and sputum microbiology will identify a significant number of cases of presumptive TB even among those who deny fever or cough," wrote lead author Dr. C. Fordham von Reyn from Dartmouth-Hitchcock Medical Center in Lebanon, NH.

In another study, Saskia den Boon, Ph.D., and colleagues found that chest x-ray had a high sensitivity (97%) but low specificity (69%) for bacteriologically positive TB. Den Boon's group concluded that chest x-ray could be used as a screening tool in TB prevalence surveys, particularly in regions of the world where sputum testing was not widely available (Int J Tuberc Lung Dis 2006;10:876-882).

In an interview with Diagnostic Imaging, den Boon emphasized x-ray's shortcomings in detecting TB, a disease she gained experience with as project manager for the Mulago Inpatient Noninvasive Diagnosis of Pneumonia Study (MIND) in Kampala, Uganda. Patients with latent TB may still manifest abnormalities on chest x-ray but may not have active disease, she said. The presence of HIV has changed the typical picture of TB, making diagnosis a little tougher.

Den Boon's group has developed a TB chest x-ray reading and recording system designed to reduce interobserver variability. The system categorizes abnormalities on chest x-ray into parenchymal, nodular, pleural, and central structure abnormalities; any abnormalities; and any abnormalities associated with TB.

The system makes provisions for recording TB-related complications such as mycetomas and granulomas, as well as lobular volume loss, collapse, and bronchiectasis. It also eliminates descriptions of parenchymal abnormalities by shape or any quantitation of pleural disease in an effort to make readings more objective.

The aim is to produce a reading method that is less complicated than those advocated by the International Union Against Cancer and the National Institute for Occupational Safety and Health, den Boon's group wrote. This reading and recording system can be used by anyone with clinical experience in chest radiology (Int J Tuberc Lung Dis 2005;9:1088-1096).

-By Shalmali Pal