The nurse obtains a sputum specimen from a client with suspected TB for laboratory study. Which of the following laboratory techniques is most commonly used to identify tubercle bacilli in sputum?
A. Acid-fast staining
B. Sensitivity testing
C. Agglutination testing
D. Dark-field illumination
Correct Answer: A. Acid-fast staining
The most commonly used technique to identify tubercle bacilli is acid-fast staining. The bacilli have a waxy surface, which makes them difficult to stain in the lab. However, once they are stained, the stain is resistant to removal, even with acids. Therefore, tubercle bacilli are often called acid-fast bacilli. CDC recommends the historical practice of obtaining three sputum samples with at least one early morning sample. Each specimen increases the sensitivity of testing. The first-morning specimen increases sensitivity by 12%. Sputum smear sensitivity could be increased by centrifugation or sedimentation.
Option B: Two methods of determining drug susceptibility are phenotypic and genotypic testing. Genotypic testing is faster than phenotypic methods. Microscopic observation of drug susceptibility (MODS) assay is a culture-based method to differentiate MTB from NTM and drug susceptibility to rifampin and isoniazid.
Option C: Agglutination tests are based on the presence of agglutinating antibodies in patient sera that can react with specific antigens to form visible clumps. In the agglutination tests, the antibody-antigen reaction can be either a direct or passive agglutination reaction.
Option D: Darkfield illumination is a technique in optical microscopy that eliminates scattered light from the sample image. This yields an image with a dark background around the specimen and is essentially the complete opposite of the brightfield illumination technique.
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