Microscopy (acid fast bacilli)
What is this test?
This test uses a microscope to detect bacteria called acid-fast bacilli in fluid or tissue samples. This test is used to manage mycobacterial infections such as tuberculosis. It may also be used when an atypical mycobacterial infection is suspected. A sample of sputum, bronchial samples, blood, cerebrospinal fluid, urine, stool, or other body fluids or tissues may be collected for this test.
What are related tests?
- Antimicrobial susceptibility test
- Molecular testing, diagnostic
- Mycobacteria culture
- Mycobacterium tuberculosis DNA assay
Why do I need this test?
Laboratory tests may be done for many reasons. Tests are performed for routine health screenings or if a disease or toxicity is suspected. Lab tests may be used to determine if a medical condition is improving or worsening. Lab tests may also be used to measure the success or failure of a medication or treatment plan. Lab tests may be ordered for professional or legal reasons. You may need this test if you have:
- Atypical mycobacterial infection
When and how often should I have this test?
When and how often laboratory tests are done may depend on many factors. The timing of laboratory tests may rely on the results or completion of other tests, procedures, or treatments. Lab tests may be performed immediately in an emergency, or tests may be delayed as a condition is treated or monitored. A test may be suggested or become necessary when certain signs or symptoms appear.
Due to changes in the way your body naturally functions through the course of a day, lab tests may need to be performed at a certain time of day. If you have prepared for a test by changing your food or fluid intake, lab tests may be timed in accordance with those changes. Timing of tests may be based on increased and decreased levels of medications, drugs or other substances in the body.
The age or gender of the person being tested may affect when and how often a lab test is required. Chronic or progressive conditions may need ongoing monitoring through the use of lab tests. Conditions that worsen and improve may also need frequent monitoring. Certain tests may be repeated to obtain a series of results, or tests may need to be repeated to confirm or disprove results. Timing and frequency of lab tests may vary if they are performed for professional or legal reasons.
If sputum or urine samples are collected, you will need to provide early morning samples each day for 3 consecutive days.
How should I get ready for the test?
Before a sputum sample is collected, you may be asked to drink more fluids. Drinking more fluids may help you produce a sputum sample.
A bronchoscopy is a procedure that requires your written consent. Review the consent form with the healthcare worker and ask any questions that you have before signing the consent form. Tell the healthcare worker if you have a history of nosebleeds, throat infections, chest pain, heart conditions, or a recent heart attack. Inform the healthcare worker if you have a medical condition or are using a medication or supplement that causes excessive bleeding. You should also report if you have a history of allergic or other reactions to topical anesthetics.
Before having blood collected, tell the person drawing your blood if you are allergic to latex. Tell the healthcare worker if you have a medical condition or are using a medication or supplement that causes excessive bleeding. Also tell the healthcare worker if you have felt nauseated, lightheaded, or have fainted while having blood drawn in the past.
A lumbar puncture is a procedure that requires your written consent. Review the consent form with the healthcare worker and ask any questions that you have before signing the consent form. You may receive certain medications and need imaging tests done prior to the procedure.
To prepare for giving a urine sample, be sure to drink enough fluids before the test, unless you have been given other instructions. Try not to empty your bladder before the test.
Before giving a stool sample, tell the healthcare worker if you have diarrhea or are using antibiotics, barium, bismuth, oil, iron, magnesium, or medication to stop diarrhea.
Other body fluid or tissue samples:
A different sample other than the samples listed above may be used for this test. Ask your healthcare worker for information about how to prepare for this test. If you have questions or concerns about the preparation for this test, talk to the healthcare worker.
How is the test done?
A sample of sputum, bronchial samples, venous blood, cerebrospinal fluid, urine, stool, or other body fluids or tissues may be collected for this test.
Sputum is mucus that is secreted by the airways and lungs. To collect a sample of sputum, you may be asked to cough forcefully, and spit out sputum into a container. If you are unable to produce a sputum sample, you may need to have the sample induced. To induce a sputum sample, a healthcare worker will prepare a solution in a nebulizer for you to inhale. You will be asked to inhale the solution over a period of time, which may last up to 20 minutes. You will then be asked to cough and spit out sputum into a container.
A bronchial (lower airway) sample is collected during a bronchoscopy. During a bronchoscopy, bronchial cells and secretions may be collected using one or more of several different methods.
A bronchoscopy is done with general anesthesia or conscious sedation. Your vital signs will be monitored and a ventilator may assist your breathing. The bronchoscope, a flexible fiberoptic instrument, is passed through your nose or mouth. If the scope is passed through your mouth, a bite block may be used to protect your teeth. The bronchoscope then passes through your airways and into your lung. A topical anesthetic is often sprayed down the tube to prevent coughing during the procedure.
During the bronchoscopy, your airways are inspected and samples of cells or tissue may be collected using bronchioalveolar lavage, bronchial brushing, and/or bronchial biopsy. A bronchioalveolar lavage is done by placing the tip of the bronchoscope into an area of your lung. Saline solution is injected and drawn out through the scope. This procedure is usually repeated several times to collect samples of cells and secretions from different areas of the lungs. During bronchial brushing, a small brush within the scope is used to collect samples of cells from the lung. When a biopsy is needed, an instrument located within the scope is used to collect samples of tissue from the lung.
When a blood sample from a vein is needed, a vein in your arm is usually selected. A tourniquet (large rubber strap) may be secured above the vein. The skin over the vein will be cleaned, and a needle will be inserted. You will be asked to hold very still while your blood is collected. Blood will be collected into one or more tubes, and the tourniquet will be removed. When enough blood has been collected, the healthcare worker will take the needle out.
Cerebrospinal fluid is the fluid that surrounds the brain and spinal cord tissues. The procedure that collects a sample of cerebrospinal fluid is called a lumbar puncture. For this procedure, you may need to lie on your side facing away from the person performing the procedure. You may be asked to roll up into a ball with your knees brought close to your chest. This position allows your spine to spread apart slightly and helps direct needle placement. This procedure may also be done while you are in an upright sitting position, with your knees drawn up toward your chest.
An area in the lower back will be chosen for the needle insertion site. This is about at the level of the top of your hip bone. Your skin will be cleaned with antiseptic and the area will be draped with sterile cloth. Local anesthetic will be injected with a small needle to numb the area. Anesthetic cream may also be applied. After the anesthetic has taken effect, the lumbar puncture needle is inserted between the vertebrae and into the spinal column. Cerebrospinal fluid is drawn out for testing and the needle is removed. You may need to change positions during the procedure if the healthcare worker is having difficulty collecting a sample of fluid. This procedure may be done more than one time if collecting cerebrospinal fluid is difficult.
To provide a sample of urine, you will be asked to urinate into a container. Fill the container as much as you can, but do not overfill it. Urine samples may also be taken from a catheter.
For a stool sample, you will be asked to have a bowel movement into a special container. Avoid adding urine, water, tissues, or toilet paper to the stool sample.
Other body fluid or tissue samples:
A different sample other than the samples listed above may be used for this test. Methods used to collect other body fluids or tissue samples may vary. Ask the healthcare worker to explain how this sample may be collected. If you have questions or concerns about this test, talk to the healthcare worker.
How will the test feel?
The amount of discomfort you feel will depend on many factors, including your sensitivity to pain. Communicate how you are feeling with the person doing the test. Inform the person doing the test if you feel that you cannot continue with the test.
Generally, collection of a sputum sample is not painful. If the sample is induced, the coughing may be uncomfortable.
During a bronchoscopy, general anesthetic or conscious sedation may be used. General anesthesia is done by an anesthesiologist. You receive medicine that puts you into a deep sleep where you are unable to feel pain. With conscious sedation, you receive medicine that puts you in a dream-like state, where you should not feel pain or remember the procedure. However, you are still awake enough to move and respond to directions. After the procedure, you may have a sore throat or cough for several days
During a blood draw, you may feel mild discomfort at the location where the blood sample is being collected.
Before a lumbar puncture, a local anesthetic is given to the procedure site to numb the area. You may feel mild discomfort or stinging when the numbing medicine is injected. You may feel a pressure, a popping sensation, and discomfort when the procedure needle is inserted. Tell the person doing the test if you feel pain or numbness down your leg during the procedure. You may have back discomfort for several days after the procedure.
This test usually causes no discomfort.
This test usually causes no discomfort.
Other body fluid or tissue samples:
A different sample other than the samples listed above may be used for this test. This test may feel different depending on many factors, including the sample needed and how it is collected. Ask the healthcare worker what to expect during this test.
What should I do after the test?
After a sputum sample is collected, call the healthcare worker if you experience a new onset of pain in your throat, trouble swallowing, or if you are coughing up blood.
Following a bronchoscopy and collection of a sample of cells or tissue, you will need to rest until healthcare workers say that you are able to leave the facility. You can usually eat and drink as you normally do soon after the procedure. You may have a sore, dry throat for a short time, and you may develop a slight fever the evening after the procedure.
Contact your healthcare worker if you cough up significant amounts of bright red or dark-colored blood, or have a high fever, which remains for several days. Contact your healthcare worker immediately if you have sudden or a new onset of chest pain, shortness of breath, wheezing, or other difficulty breathing.
After a blood sample is collected from your vein, a bandage, cotton ball, or gauze may be placed on the area where the needle was inserted. You may be asked to apply pressure to the area. Avoid strenuous exercise immediately after your blood draw. Contact your healthcare worker if you feel pain or see redness, swelling, or discharge from the puncture site.
After the lumbar puncture is complete, a bandage will be placed over the site and pressure held until the bleeding or drainage has stopped. You will need to lie flat for at least 1 to 2 hours after the lumbar puncture. Healthcare workers will monitor for drainage from the puncture site for a period of time after the test. You may be able carefully turn from your back to your side. You will be offered fluids to drink.
After collecting a urine sample, close the container if it has a lid. Place the container where the healthcare worker asked you to put it. Clean your hands with soap and water.
After giving a stool sample in a healthcare facility, close the container if it has a lid, and place the container where the healthcare worker instructed. Clean your hands with soap and water. If you have been asked to collect the stool sample while at home, follow the directions provided.
Other body fluid or tissue samples:
A different sample other than the samples listed above may be used for this test. Instructions for what to do after a collection of other body fluid or tissue samples may vary. Ask the healthcare worker to instruct you on what to expect after this test is completed. If you have questions or concerns about what to expect after the test is completed, talk to the healthcare worker.
What are the risks?
Sputum: If you are unable to produce a sputum sample on your own, it may need to be induced. Risks with collection of an induced sputum cell sample include minor damage to the throat and trachea. The person doing this test may need to perform it more than once. Talk to your healthcare worker if you have any concerns about the risks of giving a sputum sample.
Bronchial samples: A bronchial (lower airway) sample is collected by a procedure called a bronchoscopy. This procedure may require sedation (putting you to sleep), which has its own risks. If the bronchoscope is passed through your nose, there is a risk of damage to the inside of your nose. If the bronchoscope is passed through your mouth, there is a risk of damage to your mouth and throat. A bronchoscopy can irritate the airways and lungs. Commonly, a mild sore throat or cough is experienced after the procedure. You may have a fever that comes and goes for a few days. You may also feel chest or back discomfort for a few days.
Less common risks of a bronchoscopy include lung bleeding, hemoptysis (coughing up blood), lung infection, and bronchospasm (sudden, short-term narrowing of the airways). Additionally, there is a risk of damage to the airways and lungs, including the possibility of a pneumothorax (collapsed lung) or decreased lung function. You may have difficulty breathing after this procedure. If you have a medical condition, or are using a medication or supplement that causes excessive bleeding, you are at a higher risk of bleeding from this procedure. The person doing this procedure may need to perform it more than once. Talk to your healthcare worker if you have any concerns about the risks of this procedure.
Blood: During a blood draw, a hematoma (blood-filled bump under the skin) or slight bleeding from the puncture site may occur. After a blood draw, a bruise or infection may occur at the puncture site. The person doing this test may need to perform it more than once. Talk to your healthcare worker if you have any concerns about the risks of this test.
Cerebrospinal fluid: Cerebrospinal fluid is collected by a procedure called a lumbar puncture. The most common risk of a lumbar puncture is spinal fluid leakage from the puncture site. This procedure may cause a mild to severe headache, which may last for several days. Although rare, other risks include infection, nerve injury, bleeding in the spinal canal, and damage to the discs in between your spine. If you have a medical condition, or are using a medication or supplement that causes excessive bleeding, you are at a higher risk of bleeding from the puncture site. If you have a condition where you have increased pressure within your skull, such as a traumatic head injury or a large brain tumor, this procedure carries a risk of brain herniation (where the skull contents are pressed down on to the spinal cord causing brain damage). The person doing this test may need to perform it more than once. Talk to your healthcare worker if you have any concerns about the risks of this procedure.
Urine: A urine test is generally considered safe. Talk to your healthcare worker if you have questions or concerns about this test.
Stool: Giving a stool sample is generally considered safe. Talk to your healthcare worker if you have questions or concerns about this test.
Other body fluid or tissue samples: A different sample other than the samples listed above may be used for this test. Ask your healthcare worker to explain the risks of this test to you. If you have questions or concerns about this test, talk to the healthcare worker.
What are normal results for this test?
Laboratory test results may vary depending on your age, gender, health history, the method used for the test, and many other factors. If your results are different from the results suggested below, this may not mean that you have a disease. Contact your healthcare worker if you have any questions. The following is considered to be a normal result for this test:
- Negative 
What follow up should I do after this test?
Ask your healthcare worker how you will be informed of the test results. You may be asked to call for results, schedule an appointment to discuss results, or notified of results by mail. Follow up care varies depending on many factors related to your test. Sometimes there is no follow up after you have been notified of test results. At other times follow up may be suggested or necessary. Some examples of follow up care include changes to medication or treatment plans, referral to a specialist, more or less frequent monitoring, and additional tests or procedures. Talk with your healthcare worker about any concerns or questions you have regarding follow up care or instructions.
You may experience headaches for hours to days, or even weeks after a lumbar puncture. The headaches are usually worse when sitting or standing upright and are relieved by lying down. Headaches are often associated with nausea, a stiff neck, tinnitus (ringing in the ears), photophobia (abnormal sensitivity to or intolerance of light), vertigo (dizziness), and blurred vision. Contact your healthcare worker if your symptoms worsen.
Other body fluid or tissue samples:
A different sample other than the samples listed above may be collected for this test. Ask the healthcare worker for follow up care instructions after this test.
Where can I get more information?
- National Institute of Allergy and Infectious Diseases - www3.niaid.nih.gov
- Centers for Disease Control and Prevention (CDC) - http://www.cdc.gov/
- CDC National Prevention Information Network - http://www.cdcnpin.org
 Anon: American Thoracic Society/Centers for Disease Control and Prevention/Infectious Diseases Society of America: Treatment of tuberculosis. Am J Respir Crit Care Med 2003; 167:603-662.
 Watterson SA & Drobniewski FA: Modern laboratory diagnosis of mycobacterial infections. J Clin Pathol 2000; 53(10):727-32.
 Laszlo A: Tuberculosis bacteriology laboratory services and incremental protocols for developing countries. Clin Lab Med 1996; 16(3):697-716.
 Daniel TM: The rapid diagnosis of tuberculosis: a selective review. J Lab Clin Med 1990; 116(3):277-82.
 Henry JB: Clinical Diagnosis and Management by Laboratory Methods, 20th ed. Saunders, 2001.
Last Updated: 5/4/2018