Most commonly used laboratory technique used to identify tubercle bacilli in sputum?

KPSC Staff Nurse - 2016

Applied Microbiology & Infection control

Easy

Appeared in: KPSC Staff Nurse - 2016

Explanation

Acid-fast staining is the primary and most rapid method for the initial detection of tubercle bacilli (Mycobacterium tuberculosis) in sputum samples.
The technique, often the Ziehl-Neelsen (ZN) method, relies on the unique property of the mycobacterial cell wall, which is rich in a waxy substance called mycolic acid.
This waxy wall retains the primary stain (carbolfuchsin) even when treated with a decolorizing agent like acid-alcohol, making the bacilli appear red against a blue background, confirming their 'acid-fast' nature.

Option B: Sensitivity testing is not an identification method. It is performed after the bacteria have been identified and cultured to determine which antibiotics will be effective against them.
Option C: This is a serological test that detects antigen-antibody reactions in blood serum. It is not used for identifying bacteria directly from a sputum smear.
Option D: This is a special microscopy technique used to visualize organisms that are too thin to be seen with standard bright-field microscopy. Tubercle bacilli are not in this category.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Laboratory identification of Mycobacterium tuberculosis as background academic context rather than a clinical decision trigger.
A positive Acid-Fast Bacilli (AFB) smear provides a rapid, presumptive diagnosis of TB, allowing for the prompt initiation of treatment and implementation of infection control measures to prevent transmission.
Nurses play a critical role in patient education and ensuring the collection of a high-quality sputum sample (deep cough, early morning), which directly impacts the accuracy of the test.
What if? If a patient is unable to produce sputum, the nurse would anticipate assisting with or preparing the patient for alternative procedures like induced sputum, bronchoscopy with bronchoalveolar lavage (BAL), or gastric aspiration (especially in children).

How to Approach the Question

First, identify the core question: it asks for the 'most common' laboratory technique to 'identify tubercle bacilli' in 'sputum'.
Recall or deduce the key characteristic of tubercle bacilli (Mycobacterium tuberculosis). The defining feature is its unique, waxy cell wall rich in mycolic acid.
Connect this biological feature to a specific laboratory test. The waxy, mycolic acid-rich cell wall is the basis for 'acid-fast' staining, as it resists decolorization by acid.
Evaluate the other options to confirm your choice. Sensitivity testing is for treatment, not identification. Agglutination is for serology. Dark-field microscopy is for different types of bacteria (spirochetes). This process of elimination confirms acid-fast staining is the correct answer for initial identification.

Concept Tested & Keywords

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