Mycobacterium tuberculosis Lab Diagnosis Stain Vs Culture Vs PCR

A most sensitive method for lab diagnosis of tuberculosis for detection of etiological agent,  Mycobacterium tuberculosis-

Mycobacterium tuberculosis can be detected by following methods and are-

  • Acid Fast staining
  • Cultivation of  organisms from suspected sites specimens ( sputum, urine, bone marrow, etc.)
  • PCR test (GeneXPert test)

For Positive Sample

  • Sputum smear minimum contains 10000 bacilli per ml.
  • It means to see acid-fast bacilli in Ziehl-Neelsen stain the Mycobacterium load must be at least 10000 bacilli per ml.
  • Sputum culture→ 10-100 bacilli per ml
  • Similarly to be culture-positive of Mycobacterium; there must be at least 10 bacilli per milliliter.
  • Polymerase Chain Reaction(PCR)→1-10 bacilli per ml
  • In PCR i.e. gene Xpert to be Mycobacterium tuberculosis (MTB) detected if there is the presence of at least one bacillus.

Note: From above, it verifies that PCR is the most sensitive method for laboratory diagnosis of Mycobacterium tuberculosis than those two  ( AFB stain and M. tuberculosis).

Acid-Fast Stain/ Ziehl-Neelsen Stain

It is the most common stain uses to find out the etiological agent of tuberculosis ( Mycobacterium tuberculosis). The test reports can be provided on the same day of the test request.

M. tuberculosis Culture

Culturing this organism is much more complex and expensive to carry out than sputum smear microscopy for acid-fast bacilli. This is because it requires specific equipment and laboratory facilities (Bio-safety Laboratory 3 due to being bacteria in risk group 3). Culture can also take weeks because of the slow growth of M. tuberculosis and on averages 4 weeks to get a conclusive test result. It can then be a further 4-6 weeks to produce drug resistance results.

PCR Test for M. tuberculosis ( GeneXpert test)

The Genexpert test is a molecular test for tuberculosis. This test diagnoses tuberculosis by detecting the presence of Mycobacterium tuberculosis, as well as testing for resistance to the antimicrobial drug Rifampicin.

Further Readings

  1. Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
  2. Clinical Microbiology Procedure Handbook, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
  3. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
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