The AFB sputum test diagnoses pulmonary tuberculosis

The article was professionally consulted by Dr. Doan Thi Hong Hanh - Head of Laboratory - Laboratory - Vinmec Ha Long International Hospital

The AFB sputum test is characterized by simple, fast results, and low cost. All patients with suspected pulmonary tuberculosis have this test.

According to the World Health Organization, tuberculosis is still one of the major causes of death worldwide, especially in developing countries. The incidence of pulmonary tuberculosis is increasing. According to statistics in 1990, there were 6.6 million cases of tuberculosis in the world, 8.3 million cases in 2000 and 9.24 million cases in 2006. It is estimated that every year around the world there are about 3.5-4 million new patients and about 2 million deaths from pulmonary tuberculosis , the actual number may be much higher than the estimate. Vietnam is ranked 12th out of 22 countries with the highest infection rate in the world, ranked 3rd in Asia. According to the national survey of TB infection rates in 2010, the TB infection rate was 307.2 cases per 100,000 adult population, about 24,000 people die from pulmonary tuberculosis every year.
Tuberculosis is an infectious disease caused by the bacterium Mycobacterium tuberculosis. Mycobacteria are aerobic, alcohol-resistant, acid-fast (abbreviated as AFB: acid fast bacilli) bacteria, with a thick phospholipid coat, difficult to absorb drugs and difficult to treat. Tuberculosis can be found in all parts of the body, in which pulmonary tuberculosis is the most common form, accounting for 85-90% of total cases. Pulmonary TB is also the only form of TB that spreads to people around by contact. Pulmonary tuberculosis is very easily spread from people with tuberculosis to others through tiny droplets of sputum containing bacteria when an infected person coughs, sneezes, or talks. If not detected and treated, on average, one TB patient can infect about 10 other people, which is a major public health risk.

Pulmonary tuberculosis often has clinical manifestations such as cough and expectoration lasting for more than 2 weeks, mild fever in the afternoon, weight loss, night sweats, loss of appetite, general body fatigue and weakness. The most typical symptom is cough, initially dry cough, then coughing up purulent or bloody sputum.
When suspected that a patient has pulmonary tuberculosis, to confirm the diagnosis, the doctor will assign the patient to perform tests such as AFB sputum test, Xpert MTB/RIF test, culture for TB bacteria, etc. .. With the advantage of being fast, simple and easy to perform, the AFB sputum test is the most commonly used routine test. The patient's sputum sample will be stained with the Ziehl-Neelsen technique, which is used to detect alcohol-resistant bacteria. The specimen is then examined to detect bacteria on an optical microscope.

3.1. How to take sputum for testing According to the current guidance of the Ministry of Health, for the convenience of patients who can be diagnosed on the day of medical examination, patients with pulmonary tuberculosis can take two sputum samples at the examination place. then, the time of sampling 1 and 2 must be separated by at least two hours. To ensure accurate sputum test results, patients must perform sputum collection according to the following steps:
Breathe in deeply. Exhale strongly. Take a deep breath for the 3rd time, cough deeply from the lungs. Put the sputum cup (opened) close to your mouth, pour sputum into the bottom of the cup, tighten the lid. Give the sputum cup and test sheet to the medical staff. Rinse your mouth with plain water before taking the sputum. Inhale deeply, exhale forcefully. Repeat 3 times to create a cough Coughing up phlegm deeply from the chest Open the lid of the sputum cup and bring it close to the mouth, spit sputum into the cup Close the lid of the sputum cup, screw the lid tightly In case it is difficult to spit out sputum, support by vibrating, Take an expectorant or nebulize warm saline. Note, if the amount of sputum collected is too small (<2ml) and there is no purulent mucus, the patient must repeat the above steps to have a quality sputum sample.

3.2. Methods of conducting and reading AFB sputum test results Because the cell wall of TB bacteria contains a large amount of mycolic acid, when using the traditional Gram staining method, TB bacteria are difficult to detect. To detect TB bacteria on sputum samples, the test technicians will stain the sputum smears according to the Ziehl-Neelsen method. The dye contains phenol and is heated when dyed, so the fuchsin will penetrate the bacterial wall. Due to its acid-alcohol resistance, after bleaching with 3% alcohol-acid solution, AFB still retains its red Fuchsin color while other cells and bacteria are bleached to lose their red color. The slide was then background stained with 0.3% methylene blue solution to contrast AFB red against a light blue background.

3.3. According to the sputum test results, pulmonary tuberculosis is divided into 2 types: AFB (+) and AFB (-). The diagnosis of AFB pulmonary TB is confirmed (+) when at least one sputum sample is positive for AFB, the test is performed in laboratories accredited by the National Tuberculosis Program.
Patients are diagnosed with pulmonary TB AFB (-) when at least 2 samples of AFB sputum are negative, but when using culture method (or new techniques such as Xpert MTB/RIF) tuberculosis bacteria present in the sputum. Or the patient, although the sputum smear result is negative, but based on the clinical symptoms, the picture of suspected tuberculosis on the chest X-ray, the patient is HIV (+) or does not respond to broad-spectrum antibiotic treatment, the doctors The specialist will consult and determine the patient's AFB (-) TB status.
AFB (+) pulmonary tuberculosis is highly contagious and is a great risk to public health. Although AFB (-) pulmonary tuberculosis is less contagious, it still seriously affects the patient's health. People with pulmonary tuberculosis, both AFB positive and negative, need to be treated according to the protocol of the Ministry of Health, isolated during the treatment period to limit the spread to the community.
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