Page 207 - fourth year book

COMMUNICABLE DISEASES (T.B)

B. Acid Fast Bacilli (AFB) smear

 It is the first bacteriologic evidence of the presence of mycobacteria in
a clinical specimen. Mycobacteria are called acid-fast bacilli because:-

They are rod-shaped bacteria that can be seen under the microscope
after a staining procedure where the bacteria retain the color of the stain
after an acid wash (acid-fast).

 Smear microscopy is the quickest and easiest procedure but requires
5000-10000 bacilli per milliliter of a specimen to detect M.bacteria thus
negative smears do not exclude TB disease.

 It is used also to determine degree of infectiousness of the patient as
the acid-fast bacilli can be counted and classified as 4+, 3+, 2+, or 1+.

C. Nucleic acid amplification test (NAA)

 NAA testing used to directly detects the genetic components (DNA
and RNA) of mycobacteria by amplifying/replicating pieces of the
bacterial genetic material.

 It is recommended to be performed on at least one respiratory
specimen from each patient with signs and symptoms of pulmonary

TB
 A single negative NAA test result should not be used as a definitive

result to exclude TB disease.

 Used mostly for confirmation of smear positive cases, provide
reliable, accurate and quick result in 1-2 days compared to 1-2 weeks

in culture and requires few number of bacilli 10/milliliter in the
sample but it is expensive.

D. Specimen culturing

 Culture remains the gold standard for laboratory confirmation of TB
disease and the growing bacteria are also required to perform drug-
susceptibility testing and genotyping.

 Culture examination is also essential for determining whether the TB

is pulmonary or extra-pulmonary disease and it should be done on all
diagnostic specimens, regardless of AFB smear or NAA results.

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