long

chain mycolic acids, unique to the mycobacterial cell wall

are responsible for their:

• Acid fastness

• Failure to react with Gram stains

• Resistance to the actionof antibodies andcomplement.

The four species in the Mycobacterium tuberculosis

complex are M. tuberculosis, M. microtic, M. africanum

and M. bovis. Laboratories can use biochemical tests for

differentiation between isolated strains.

Diagnosis of Mycobacterium Tuberculosis

Infection

The diagnosis of tuberculosis is often made on the basis

of clinical symptoms, chest X-ray and sputum AFB, since

available tests based on immunological principles for

Mycobacterium tuberculosis diagnosis have yet to overcome the problem of poor sensitivity and specificity

associated with them. For the time being, speedy and

appropriate laboratory diagnosis of tuberculosis infection

through AFB staining, culture and sensitivity have more

and more important role to play in sensitive detection

and appropriate treatment of patients with tuberculosis.

However, sample collection, preparation, processing

techniques and detection methods employed have a

profound effect on the sensitivity and specificity of the

results for the detection of Mycobacterium tuberculosis

infection by AFB and culture methods.

Specimen Selection

A critical factor in the ability of laboratories to isolate

Mycobacterium tuberculosis is obtaining appropriate

specimen for AFB smear and culture. Approximately 85%

of the TB cases are pulmonary. However, many patients

cannot produce sputum spontaneously and alternative

respiratory tract specimens such as induced sputum,

gastric lavage or fiberoptic bronchoscopy may be needed.

As the proportion of patients with extrapulmonary form

of tuberculosis is increasing, adequate specimen from

extrapulmonary sites need to be provided.

Sample Concentration and Decontamination

Specimens obtained from sterile sites such as CSF,

peritoneal or pleural fluids do not require decontamination.

However, most specimens for AFB smear and culture are

from the respiratory tract and do contain mixed microbial

flora. Successful recovery of mycobacteria depends

upon properly collected specimen and suppression of

contaminating bacteria.