Are serological tests effective in diagnosing tuberculosis?
Q: I am a 28 years old male. I would like to know about the sensitivity and specificity of serology for diagnosis of tuberculosis?
A:Tuberculosis is usually diagnosed by physicians using a combination of information derived from the patients history, epidemiological context, clinical signs, radiological features (X-ray or CT scan), and laboratory markers (microscopic examination of clinical specimens such as sputum, bacteriological culture of such specimens, blood tests, etc). There are conflicting reports in the medical literature about the role of serological tests in the diagnosis of pulmonary tuberculosis (affecting the lungs) and extra-pulmonary tuberculosis affecting (organs of the body apart from lungs such as lymph nodes, pleura, brain, bone and joints, kidney, liver, intestine, eyes, reproductive organs, etc). Serological tests generally look for antibodies against various antigens of the tubercle bacilli whose scientific name is Mycobacterium tuberculosis. Two relatively recent systematic reviews have given very useful insight on the role of serological tests in the diagnosis of tuberculosis. Systematic reviews are scientific studies of the other studies which look at all the evidence available, analyze the validity of individual studies, and come to a conclusion based on overall objective, rational analysis taking into account the findings from all studies based on their individual merit.
The systematic review on the role of serological tests in pulmonary TB (based on findings from 68 studies) stated that
- overall, commercial serological tests for pulmonary tuberculosis vary widely in performance;
- sensitivity of these tests is higher in smear-positive than smear-negative cases (this means that the chance of detecting TB by these tests is higher if the patients sputum samples are positive for TB when examined microscopically);
- in studies of smear-positive patients (which means microscopy positive for TB), one of the commonly used serological test (which detected IgG by enzyme-linked immunosorbent assay) showed limited sensitivity (range 63% to 85%) and inconsistent specificity (range 73% to 100%);
- specificity of these serological tests is higher in healthy volunteers than in patients in whom tuberculosis disease is initially suspected and subsequently ruled out; and
- there were insufficient data to determine the accuracy of most commercial tests in smear microscopy-negative patients (which means those patients where microscopy is negative for TB), as well as their performance in children or persons with HIV infection. The authors concluded that none of the commercial tests evaluated performed well enough to replace sputum smear microscopy. Thus, these tests have little or no role in the diagnosis of pulmonary tuberculosis (Steingart et al. PLoS Medicine 2007).
In a different systematic review by the same group of investigators on the role of serological tests in the diagnosis of extra-pulmonary tuberculosis (based on findings from 21 studies) the authors stated that:
- all commercial tests provided highly variable estimates of sensitivity (range 0 to 100%) and specificity (range 59 to 100%) for all extrapulmonary sites combined;
- for all tests combined, sensitivity estimates for both lymph node tuberculosis (range 23% to 100%) and pleural tuberculosis (range 26% to 59%) were poor and inconsistent; and
- there were no data to determine the accuracy of the tests in children or in patients with HIV infection, the two groups for which the test would be most useful. The investigators concluded that at present, commercial antibody detection tests for extrapulmonary tuberculosis have no role in clinical care or case detection (Steingart et al. Thorax 2007).