Drug Resistant TB - Advances in TB Diagnostics

Introduction

Tuberculosis (TB) is one of the world's most important infectious causes of morbidity and mortality. Between 8 and 9 million develop TB disease, and approximately 2 million die from TB each year. Despite this enormous global burden, case detection rates are low, posing major hurdles for TB control. Delay in the diagnosis and adequate treatment of TB has contributed to community spread of both drug-susceptible and drug-resistant disease.

Conventional TB diagnosis continues to rely on smear microscopy, culture and chest radiography. These tests have several limitations. They are slow, tedious and difficult to perform in field conditions.

Recent advances in molecular biology and a better understanding of the molecular basis of drug resistance have provided new tools for rapid tuberculosis diagnosis. For diagnosis, new tools include newer versions of nucleic acid amplification tests, immune-based assays and rapid culture systems. For drug resistance, new tools include line-probe assays, bacteriophage-based assays, molecular beacons and microscopic observation drug susceptibility assay.

Although the ideal test for TB is still not in sight, substantial progress has been made in the past decade. With the resurgence in the development of new tools for TB control, it is likely that the next decade will see greater progress and tangible benefits.

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Conventional TB Diagnostic Methods

Accurate and early diagnosis of TB is crucial to effective patient management and to TB control. Moreover, accurate identification of Latent TB Infection (LTBI) is the key to prevention of the disease among persons at risk.

Diagnosis of TB in the laboratory is performed by so-called conventional methods that involve specimen digestion and decontamination, microscopical examination of smears to detect acid-fast bacilli; bacterial isolation is performed by culture on solid or liquid media and, finally, identification and drug susceptibility testing are performed.

AFB Microscopy

Microscopic examination of respiratory specimens for Acid-Fast Bacilli (AFB) plays a key role in the initial diagnosis of tuberculosis, monitoring of treatment, and determination of eligibility for release from isolation. The detection of acid-fast bacilli in smears is the simplest, cheapest and remains the most widely used rapid diagnostic test for tuberculosis. In countries where the resources are limited, sputum smear microscopy is the main route of diagnosing pulmonary TB.

For patients with suspected pulmonary tuberculosis, three sputum specimens, preferably collected early in the morning, should be submitted to the laboratory for AFB smear and mycobacteriology culture.

If tissue is obtained, it is critical that the portion of the specimen intended for culture not to be put in formaldehyde. The use of AFB microscopy on urine or gastric lavage fluid is limited by the presence of mycobacterial commensals, which can cause false-positive results.

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Mycobacterial Culture

Definitive diagnosis depends on the isolation and identification of M. tuberculosis from diagnostic specimen in most cases, a sputum specimen obtained from a patient with a productive cough. Specimens may be inoculated onto egg- or agar-based medium [e.g., Lowenstein-Jensen (U) or Middlebrook 7H10] and incubated at 37 °C under 5% CO2. Because most species of mycobacteria, including M. tuberculosis, grow slowly, 4 to 8 weeks may be required before growth is detected.

Drug Susceptibility Testing

In general, the initial isolate to M. tuberculosis should be tested for susceptibility to isoniazid, rifampicin, and ethambutol. In addition expanded susceptibility testing is mandatory when resistance to one or more of these drugs is found or the patient either fails to respond to initial therapy or has relapse after the completion of treatment. Susceptibility testing may be conducted directly (with the clinical specimen) or indirectly (with mycobacterial cultures) on solid or liquid medium.

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Radiographic Procedures

The initial suspicion of pulmonary tuberculosis is often based on abnormal chest radiographic findings in a patient with respiratory symptoms. Although the 'classic' picture is that of upper lobe disease with infiltrates and cavities, virtually any radiographic pattern may be seen. A radiological lesion does not confirm etiology of  tuberculosis, as there are no pathogno mic radiological signs of  tuberculosis.