AFB (Acid Fast Bacilli) Culture Test

AFB culture test is performed when symptoms of Mycobacterium infection are present. It is generally recommended in the following situations: 

• Pulmonary tuberculosis infection with symptoms such as cough, fever, chills, malaise and weight loss
• Extra-pulmonary tuberculosis in the spinal cord, kidneys or bones, with common symptoms of infections such as back pain, paralysis, anaemia, headache, coma and joint or abdominal pain
• High-risk groups, such as persons with HIV/AIDS and exposure to tuberculosis cases
• People who have stayed in hospitals and jails
• To monitor the progress of treatment in people undergoing treatment for tuberculosis

Since fasting samples are needed, preparation should be done to take the test early in the morning before breakfast. Mouth should be rinsed thoroughly before coughing up phlegm.

Samples for the AFB test are collected as mentioned below:

• Sputum: Expectorated early morning, deep cough sputum should be collected with minimal saliva content; 5 to 10 mL of sample is collected on three consecutive days for correct diagnosis. It is advised to rinse the mouth with water before collecting sputum. This will ensure that no food particles, mouthwash or oral drugs contaminate the specimen, which tends to inhibit the growth of Mycobacteria.
• Induced sputum: If there is difficulty in coughing sputum, an aerosol inhalation of sterile 5-10% sodium chloride in water is given. This will trigger a cough that will generate sputum.
• Bronchoscopy: A bronchoscope is inserted in the nose or mouth to gain access to lungs for obtaining a sputum or tissue sample. This procedure is done in case of poor sputum production. (Read more: Preparing for bronchoscopy)
• Urine, cerebrospinal fluid and tissue samples are taken if tuberculosis is suspected outside the lung.

Sputum collection is done for a minimum of two days consecutively. Additional time will be needed if a bronchoscope is needed to collect sputum. There are no side effects when a phlegm or urine sample is taken.

Bleeding, infection, pain and local discolouration may occur if a blood sample is taken. AFB culture is monitored for a few weeks, and the sample may be collected again to monitor treatment.

AFB culture can be positive or negative, which indicates the presence or absence of an infection. Results are reported when growth occurs or when no growth is seen at six weeks.

Normal results: A negative result indicates no AFB in the sample taken. This means that there is no infection.

Abnormal results: A positive result indicates the presence of AFB in the sample. The commonest AFB  is Mycobacterium tuberculosis. An additional antibiotic susceptibility test will guide the physician towards the appropriate drugs for effective treatment. If there is a positive culture even after several weeks of continuing treatment, it is an indication of the inefficiency of the treatment. 

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. This information is purely from an educational perspective and is in no way a substitute for medical advice from a qualified doctor.