Tuberculosis Diagnosis

Recently, in vitro T-cell-based methods, the Tuberculin Skin Test alternative, have been made available to diagnose TB infection; they include interferon γ assays (QuantiFERON-TB, Quantiferon- TB Gold, T-SPOT-TB), which use Purified Protein Derivate (PPD), Early Secretory Antigen Target 6 (ESAT6), and Culture Filtrate Protein 10 (CFP10). The latter two antigens are coded by genes of region of difference 1 (RD1) locus of M. tuberculosis and are more specific for MT (exception is Mycobacterium marinum, Mycobacterium kansassi and Mycobacterium sulzagi (1, 7, 8).

Kinyoun modification, auramine O, and rhodamine stains are used for direct examination of M. tuberculosis (1, 2).

However, the gold standard for diagnosis is a positive culture of MT from clinical specimens collected from the suspected cases (1, 9). Various culture media used to detect and grow MT include (10):

1) BACTEC radiometric system

2) Mycobacteria growth indicator tube (MGIT)

3) BACTEC MGIT 960 system

4) MB-Redox based on the reduction of a tetrazolium salt indicator, ESP culture system 2 based on the detection of pressure changes and MB/BacT system that relies on colorimetric detection of carbon dioxide

5) Phage-based assay based on the ability of MT to support production of mycobacteriphage (like FASTPlaque TB test) (1, 2, 8).

It is prudent to perform drug susceptibility testing (DST), especially in areas with a high rate of MDR TB. Various methods are devised for DST including the proportion, the resistance ratio, the absolute concentration methods and BACTEC radiometric system divided into genotypic and phenotypic methods. The former include molecular nucleic acid amplification (amplified MT direct test, AMTD and Amplicor MT), real-time Polymerase Chain Reaction (PCR), and strand-displacement amplification methods (1, 2, 9, 10).

The latter include various metabolic tests such as color indicators and oxygen consumption, visualization of micro-colonies, nitrate reduction assay, and use of phages. Examples are Mycobacteria growth indicator tube (MGIT), E-test, MB/bacT system, and ESP culture system 2 (1, 2, 11).

Recent advances of molecular epidemiology use techniques which include DNA finger printing, like the restriction fragment length polymorphism, RFLP, spacer oligotyping and spoligotyping, genomic deletion analysis, and Mycobacterial interspersed repetitive units (MIRU) (1, 2).

The main purpose of these tests is increasing the ability to diagnose latent infection (1, 2).

Detection of adenosine deaminase (ADA) is an easy, rapid, low cost test but can be used as an extra tool for Tuberculosis pleural effusions and meningitis according to its low accuracy (1, 2). The major problems encountered in rapid diagnosis of TB and differentiation of infection from the disease includes unavailability of advanced tests and inaccuracy of available tests. In immune compromised patients, especially those with HIV infection, results of diagnostic tests are equivocal, thus making a definite diagnosis becomes even more difficult.

1) In every setting, high index of suspicion is required for rapid diagnosis of TB, and physicians should take the epidemiological, clinical and paraclinical data into account to gauge the probability of active disease to order feasible, accurate and cost effective diagnostic tests.

2) In high endemic areas, TB should be suspected early and steps should be taken for prompt diagnosis and treatment.

3) In every setting, PPD, X-ray, smear, and culture are requested in suspected cases and if available more sensitive tests are required according to the site of infection. In resource-limited countries the priority is to ensure a network of quality-controlled microscopy for AFB in clinical samples, most often sputum.

4) It is recommended to request HIV testing in every child with Tuberculosis.

5) It is better to request drug susceptibility test on all obtained isolates, unless the result of this test in isolates from the index case is available. This policy can help to prevent the emergence of MDR and its transmission.

6) The decision to treat a child should be carefully considered, and once such a decision is made the child should be treated with a full course of therapy.

7) A majority of TB diagnostic studies are focused on test accuracy and not on outcomes such as accuracy of diagnostic algorithms. Revised clinical algorithms for TB diagnosis may demonstrate the impact of these new tools.