AFB culture

To help identify a mycobacterial infection; to diagnose tuberculosis (TB); to monitor the effectiveness of treatment

If you have symptoms, such as a chronic cough, weight loss, fever, chills, and weakness, that may be due to TB or another mycobacterial infection; if your doctor suspects that you have active TB; if your doctor wants to monitor the effectiveness of TB treatment

Usually, three sputum samples are collected early in the morning on different days. If the patient is unable to produce sputum samples, a bronchoscope may be used to collect fluid during a procedure called a bronchoscopy. In children, gastric washings/aspirates may be collected. Depending on symptoms, urine, cerebral spinal fluid (CSF), other body fluids, or biopsied tissue samples are sometimes tested.

Acid-fast bacillus (AFB) are rod shaped bacteria that can be seen and counted under the microscope on a specially stained sample on a glass slide, called an AFB smear. The most common acid-fast bacilli are members of the genus Mycobacterium.

Mycobacterium tuberculosis is the most prevalent mycobacteria, and the most infectious. Most samples that are submitted for AFB smears and cultures are collected because the doctor suspects that the patient has tuberculosis. Only a few of the more than 60 species of mycobacteria that have been identified cause infections in humans. They include:

• M. africanum - causes a disease similar to TB in other parts of the world
• Mycobacteria avium-intracellulare complex (MAC) - can cause a lung infection in immunosuppressed patients, such as the elderly and those with AIDS; this infection is not contagious, but it can be difficult to treat as it tends to be highly resistant to antibiotics
• Other mycobacterial species grow in water, such as fish tanks, and can cause skin infections, while others can infect wounds and artificial body parts.
• A few mycobacteria, such as M. bovis - can sometimes be transferred from animal to human.

Several AFB smears, from different samples, are frequently evaluated. If acid-fast bacilli are present on any of the smears, a mycobacterial infection is likely. Since TB is the most common cause, a presumptive diagnosis of TB can be made but other follow-up testing must be done to positively identify the acid-fast bacilli as either M. tuberculosis, or as another mycobacteria.

AFB cultures are the next step; they are used to grow acid-fast bacilli in the laboratory. Body fluid or tissue samples are decontaminated, digested, and concentrated before being introduced into a nutrient environment and incubated. Since mycobacteria grow slowly, positive identification of the species that is/are present may take days to several weeks, while negative results (no mycobacterial growth) can take up to 6 to 8 weeks to confirm.

Since TB (and M. avium) most frequently infects the lungs (pulmonary), sputum is the most commonly tested sample. Sputum is phlegm, thick mucous that is coughed up from the lungs. Usually, three to five early morning samples are collected (on consecutive days) in individual sterile cups.

If you are unable to produce sputum samples, your doctor may collect respiratory samples using a procedure called a bronchoscopy. Bronchoscopy allows your doctor to look at, and collect samples from, your bronchi and bronchioles. These branching tubes connect the trachea in your throat to your lungs, providing a pathway for air to enter your lungs. Once a local anaesthetic has been sprayed onto the trachea, your doctor can insert a tube into your bronchi and smaller bronchioles and aspirate fluid samples for testing. Sometimes, they will introduce a small amount of saline through the tubing and into the bronchi and then aspirate it to collect a bronchial washing.

In children, gastric washings/aspirates may be done. This involves introducing saline into the stomach through a tube, followed by fluid aspiration.

If your doctor suspects extrapulmonary TB (outside of the lungs, fairly common in AIDS patients), he may test the body fluids and tissues most likely affected. For instance, you may collect one or more urine samples if he suspects TB has infected your kidneys. Or a needle may used to collect fluid from your joints or from other body cavities, such as the pericardium (lining around the heart) or abdomen. Occasionally, your doctor may need to use a needle to collect a sample of cerebral spinal fluid (CSF) or do a minor surgical procedure to obtain a tissue biopsy. A blood culture may be done if your doctor suspects that the infection has spread through the blood.