Test : Acid fast bacteria test
Indications : The test is ordered to detect infections caused by acid fast bacteria in a person. Several species of the mycobacterium family cause TB in humans and animals.
An early morning, deep cough sputum specimen is collected. A minimum of 2ml of the sample is required for the test. If transportation takes time refrigerate the sample. Direct the patient to rinse mouth thoroughly before sample collection.
Small amount of the sample is taken and a smear is made on a glass slide. It is then air dried and heat fixed. Flood the slide with carbol fuchsin stain which is a phenolic compound capable of penetrating bacterial cell wall. Cover flooded slide with filter paper and expose to steam for ten minutes. Add more stain as required. Cool the slide and rinse it with water. Once again, flood the slide with acid alcohol for 15 seconds to remove stain from other cells. Tilt the slide over a sink and add the acid alcohol drop by drop until the red color stops running off from the smear. Rinse slide. Stain using methylene blue (counter stain) to add blue color to the non acid –fast cells. Rinse and dry the slide. Observe the slide under oil immersion objective of a microscope.
At least two samples from the same patient must be taken on consecutive days for observation and analysis.
Physiology : The acid-fast stain test for bacteria is a laboratory test to detect the presence of
TB or other pathogenic, acid fast bacteria in a sample of blood, or other body tissues.
The acid fast stain, also known as the Ziehl-Neelsen stain, was first described by two German doctors Ziehl and Neelsen. It is a bacteriological stain used to identify acid fast bacterium, particularly those belonging to the genus
Mycobacteria, as its thick cell makes it resistant to gram stain. Acid fast stain has also been useful in detecting bacteria belonging to the genus Nocardia.
The reagents used in the making of the stain are carbol fuchsin, acid alcohol and methylene blue. After staining, the acid fast bacteria will be bright red.
Normal Range : Negative
Interpretation : Normal
No acid-fast bacteria were found in the stained smear.
Abnormal
Presence of acid fast bacteria indicates the following:
• Tuberculosis / related infections
• Nocardia infection
References :
1. http://health.mo.gov/lab/smearculture.php
2. http://www.nlm.nih.gov/medlineplus/ency/article/003766.htm
3. http://www.slideshare.net/doctorrao/acid-fast-staining-in-tuberculosis
Careers
Mycobacterium Tuberculosis is an aerobic bacteria that is why when a tuberculoma transforms to a cavitatory lesion, the number of bacteria increase to enormus number.If communication between cavity and bronchus is esatablished, then the bacteria coughed out during coughing. Cavitatory pulmonary lesion must be considered sputum +ve until and unless prove otherwise. Always do the microscopy of such patients and even saliva may have bacteria(AFB was detected in saliva of such patient)