Chest X-Rays Not Effective in Determining When TB Acquired

There is little correlation between the appearance of tuberculosis on chest X-rays and how recently the disease was acquired, according to a study in the June 8 issue of JAMA, a theme issue on tuberculosis.

Co-author Neil W. Schluger, MD, of Columbia University, New York, presented the findings of the study at a JAMA media briefing on tuberculosis at the National Press Club.

Traditionally, active tuberculosis (TB) disease has been classified as either primary or secondary, reflecting the time between initial infection with Mycobacterium tuberculosis (MTB) and the onset of clinical disease, according to background information in the article. That interval can range over many years. Primary and secondary TB are also thought to have different characteristic radiographic (X-ray) and clinical features, though these clinical observations have been based on studies conducted before the availability of molecular fingerprinting techniques for TB. Molecular (DNA) fingerprinting, also known as restriction fragment length polymorphism (RFLP) analysis is a method for comparing strains of MTB from individual patients on a genetic basis. These techniques allow comparison of patients who have recently acquired tuberculosis to those whose tuberculosis was acquired long ago.

The researchers in this study used molecular fingerprinting and conventional epidemiology to test whether recently transmitted cases have radiographic features distinct from distantly acquired infection and secondly, whether the atypical features of the radiograph in HIV-associated TB are due to recent infection or are manifestations of altered immunity in the reactivation of latent infection. The study included 546 patients treated at a New York City medical center between 1990 and 1999. Eligible patients had to have had at least 1 positive respiratory culture for Mycobacterium tuberculosis and available radiographic data.

The researchers found that in clinically well-defined patients with TB that the most significant independent predictor of radiographic appearance is HIV status, the authors write. The altered radiographic appearance of pulmonary tuberculosis in HIV is due to altered immunity rather than recent acquisition of infection and progression to active disease.

Although a clustered fingerprint (a DNA fingerprint from an MTB strain from one patient which has an exact match with an MTB strain recovered from at least one other patient), representing recently acquired disease, was associated with typical radiograph, the association was lost when adjusted for HIV status.

In summary our findings argue that the terms primary and reactivation TB are misleading when used to make inferences linking radiographic findings to epidemiologic characteristics of patients. Radiographic findings have implications regarding host immune status of patients, but whether a patients disease is due to recently transmitted or remotely acquired infection cannot be determined from them, the authors conclude.

Reference: JAMA. 2005;293:2740-2745