Question

Outline the principle of tuberculin testing, chest X rays, and acid fast staining?

Answer 1

Tuberculin testing – it is an intradermal technique of detecting M. tuberculosis infection in an individual. It is also known as Mantoux tuberculin test as it was developed by Charles Mantoux. Tuberculin is derived in the form of purified protein from cultures of M. tuberculosis. A person previously exposed to M. tuberculosis develops hypersensitivity towards tuberculin. A standard dose of five tuberculin units (TU) (0.1ml) is injected intradermally usually on the anterior surface of forearm and observed 48 to 72 h later for a delayed reaction. T-cells sensitized by prior infection are recruited to the skin site where they release lymphokines which cause local vasodilatation, fibrin deposition, edema and recruitment of other inflammatory cells to the area causing a bump. The diameter of indurated area is measured. Medical risk factors determine whether the size of induration (5mm, 10mm or 15 mm) should be considered as a positive indication of tuberculosis.

Chest X ray

Chest X –ray is a non-invasive diagnostic imaging technique of producing images of the chest and spine area which includes heart, lungs, airways, blood vessels and the bones. X rays are electromagnetic rays which are made to passs through the body exposing to a photographic film the internal structures of the body based on their density. Dense matters such as bones and tumors absorb the radiation and appear light while soft tissues and breaks/ fissures in bone let radiation pass through, making them look darker on the photographic plate.

Acid fast staining

It is used for bacteria belonging to the genus mycobacterium. These contain a lipoid capsule (containing mycolic acid) with a high molecular weight and it remains waxy at room temperature thus preventing penetration of gram stains (aqueous). Apart from mycobacterium, structures that can be identified using acid fast staining include – actinomycetes in tissues, cultures of nocardia, oocycts of Cryptosporidium, Isospora and bacterial spores.

Acid fast staining is also known as Ziehl-Neelsen and Kinyoun method.

A dye phenol mixture (basic fuchsin and carbolic acid (phenol) is used to stain the mycobacterial structures since phenol has a high solubility in lipids or waxes. Once staining is complete, decolorization is resisted by weak mineral acid (20% sulfuric acid). Phenol acts as a mordant and the only the background gets decolorized revealing pink structures on the counter-stained (methylene blue or malachite green) background.