The Sputum for AFB (Tuberculosis) test is a laboratory diagnostic test that looks for acid-fast bacilli (AFB)—a group of bacteria that includes Mycobacterium tuberculosis, the germ that causes tuberculosis (TB). The sample used is sputum, which is thick mucus coughed up from the lungs (not saliva from the mouth). Because TB most commonly affects the lungs, examining sputum is a key way to evaluate an active TB lung infection.

Most often, the first step is an AFB smear, where a lab stains the sputum and checks it under a microscope for AFB. If AFB are seen, it suggests a higher bacterial load and can support a diagnosis of pulmonary TB. However, an AFB smear does not prove TB on its own, because some non-tuberculous mycobacteria can also appear acid-fast.

Depending on the laboratory and your clinician’s plan, sputum may also be sent for mycobacterial culture (to grow and identify the organism) and/or NAAT/PCR testing (molecular testing that can detect TB DNA more quickly and may identify drug resistance in some settings). These additional tests help confirm whether TB is present and guide treatment.

In plain language, what results mean:

AFB smear positive means AFB were seen in your sputum. This raises concern for an infectious mycobacterial lung disease, including TB. Your care team usually correlates this with symptoms, imaging (like a chest X-ray), and confirmatory tests (culture/NAAT). People with smear-positive pulmonary TB are more likely to be contagious.

AFB smear negative means AFB were not seen on the microscope exam. This does not completely rule out TB—especially if you have strong symptoms or risk factors—because some people have low numbers of bacteria in sputum. Further testing (culture/NAAT) may still be needed.

Culture positive for TB confirms tuberculosis and can help determine the best medications (including checking for drug-resistant TB). Culture negative

A sputum test for AFB is usually ordered when a clinician is evaluating respiratory symptoms that could be caused by tuberculosis or another mycobacterial infection. It is especially important because TB is an infectious disease that can spread through the air, and early diagnosis supports timely treatment and helps protect close contacts.

Your doctor may recommend this sputum test if you have symptoms such as:

• Chronic cough (often lasting 2–3 weeks or longer)
• Coughing up mucus (sputum) or coughing up blood
• Chest pain or shortness of breath
• Fever, night sweats, chills
• Unexplained weight loss or fatigue
• Ongoing or recurrent lung infection symptoms that do not improve as expected

It is also commonly done when there are risk factors for TB, including:

• Close contact with someone who has active TB
• Living or working in higher-risk settings (e.g., shelters, correctional facilities, long-term care)
• Recent travel to or residence in areas where TB is more common
• Weakened immune system (for example, HIV, certain cancers, transplant medications, or long-term steroid use)
• Abnormal chest imaging suggestive of TB

Because TB bacteria may not appear in every sample, clinicians often request multiple sputum samples (commonly collected on different mornings). Following collection instructions—such as producing a deep cough specimen from the lungs—improves accuracy.

• Active pulmonary tuberculosis (TB)
• Extrapulmonary tuberculosis (TB outside the lungs, when respiratory samples are part of the evaluation)
• Non-tuberculous mycobacterial (NTM) lung disease (e.g., MAC—Mycobacterium avium complex)
• Drug-resistant tuberculosis (evaluated with culture and/or molecular testing)
• Chronic or recurrent lung infections requiring evaluation for atypical causes

• Diagnosing the cause of chronic cough and persistent respiratory symptoms
• Early detection of active TB to start treatment promptly and reduce complications
• Protecting family and community health by identifying potentially contagious pulmonary TB
• Guiding targeted therapy by confirming TB vs. other mycobacterial infections (and assessing possible drug resistance)
• Supporting pulmonary health monitoring when chest imaging or symptoms suggest a serious lung infection