Bronchitis is diagnosed primarily through a physical exam and your symptom history, not through a single definitive test. Most cases of acute bronchitis are caused by viruses, so the diagnostic process focuses less on confirming bronchitis itself and more on ruling out something worse, particularly pneumonia. Chronic bronchitis follows a different path, requiring breathing tests to measure how well air moves through your lungs.
Acute vs. Chronic: Two Different Diagnostic Paths
The distinction between acute and chronic bronchitis shapes everything about how a diagnosis unfolds. Acute bronchitis is a temporary inflammation of the airways, usually following a cold or respiratory infection. A cough that lasts up to three weeks, sometimes with mucus production, is the hallmark. Your doctor can typically diagnose it in a single office visit based on your symptoms and a physical exam.
Chronic bronchitis is a more serious, long-term condition. It’s defined by a productive cough (one that brings up mucus) lasting at least three months in two consecutive years. Because chronic bronchitis falls under the umbrella of chronic obstructive pulmonary disease (COPD), diagnosing it requires lung function testing to confirm that your airways are persistently narrowed.
What Happens During the Physical Exam
The most important tool in diagnosing bronchitis is a stethoscope. When your doctor listens to your lungs, they’re checking for specific abnormal sounds that indicate inflamed or mucus-filled airways.
Rhonchi are low-pitched, continuous sounds sometimes described as snoring or gurgling. They’re caused by mucus rattling in the larger airways and often shift location when you cough, as the mucus moves around. Wheezing, a higher-pitched hissing sound, is more common when you breathe out and signals that your airways have narrowed. Both sounds are typical of bronchitis.
What your doctor does not hear matters just as much. Crackles (a popping or crackling sound), decreased breath sounds in one area, or sounds that suggest fluid has consolidated in part of your lung all point toward pneumonia rather than bronchitis. This difference in lung sounds is one of the primary ways the two conditions are separated at the bedside.
When a Chest X-Ray Is Needed
Most people with bronchitis do not need a chest X-ray. The purpose of imaging isn’t to confirm bronchitis. It’s to rule out pneumonia when certain warning signs are present. Guidelines from the American Academy of Family Physicians identify specific red flags that warrant an X-ray:
- Rapid heart rate above 100 beats per minute
- Rapid breathing above 24 breaths per minute
- Fever above 100°F (37.8°C)
- Shortness of breath, bloody sputum, or rust-colored mucus
- Abnormal lung exam findings suggesting consolidation
Adults over 75 get special consideration. Older adults with pneumonia are less likely to develop a fever or rapid heart rate, so their symptoms can look deceptively mild. Doctors often have a lower threshold for ordering imaging in this group.
If a chest X-ray comes back clear, that supports a bronchitis diagnosis. If it shows a patch of white in one area of the lung, suggesting fluid or infection has settled into the air sacs, the diagnosis shifts to pneumonia.
How Bronchitis Differs From Pneumonia
This is the central question your doctor is trying to answer, because the two conditions look similar early on but require very different treatment. Bronchitis inflames the tubes that carry air into your lungs. Pneumonia goes deeper, inflaming the tiny air sacs where oxygen actually enters your bloodstream. That’s why pneumonia causes more systemic illness: your body isn’t getting enough oxygen.
Pneumonia tends to produce high fever (sometimes reaching 105°F), chills, rapid breathing, and a rapid heart rate. Bronchitis can cause a low-grade fever but rarely pushes temperatures that high. Shortness of breath at rest is uncommon with bronchitis and more concerning for pneumonia. Your oxygen levels, checked with a small clip on your finger called a pulse oximeter, help clarify the picture. Normal readings suggest your lungs are exchanging oxygen effectively, which points away from pneumonia.
Blood Tests and When They’re Used
Routine blood work isn’t part of a standard bronchitis diagnosis, but it can help in ambiguous cases. Two markers are particularly useful for sorting out whether a bacterial infection is involved.
Procalcitonin is a blood marker that rises when bacteria are actively causing infection. Levels below 0.10 ng/mL suggest a bacterial infection is not present, which is the expected finding in viral bronchitis. Levels between 0.10 and 0.25 make bacterial infection unlikely, while readings above 0.50 suggest bacteria are probably involved. This test is most valuable because it helps doctors decide whether antibiotics would actually help. Since most bronchitis is viral, a low procalcitonin level reinforces the decision to skip antibiotics.
C-reactive protein (CRP) is a general inflammation marker. In respiratory infections, levels below 5 mg/L are considered low and consistent with a mild or viral process. Levels above 20 mg/L suggest more significant inflammation and may prompt further investigation for pneumonia or a bacterial cause. CRP alone can’t diagnose bronchitis, but it adds context when combined with your symptoms and exam findings.
Diagnosing Chronic Bronchitis
If your cough has persisted for months and keeps coming back, your doctor will likely order spirometry, a breathing test that measures how much air you can blow out and how quickly. You’ll breathe into a mouthpiece connected to a machine, first taking a deep breath in and then blowing out as hard and fast as you can.
The key measurement is the ratio between two values: how much air you can force out in one second versus the total amount you can exhale. According to the Global Initiative for Chronic Obstructive Lung Disease (GOLD), a ratio below 0.7 after using an inhaled medication to open the airways confirms airflow obstruction. This is the spirometric threshold for diagnosing COPD, the disease category that includes chronic bronchitis.
The “after bronchodilator” part is important. Everyone’s airways narrow slightly during a forced exhale. The medication opens the airways first, so the test measures your best possible airflow. If the ratio is still below 0.7 after that, the obstruction is considered fixed rather than reversible, which distinguishes COPD from asthma.
Your doctor may also order a chest X-ray or CT scan to look for structural changes in the lungs, and blood work to check your oxygen and carbon dioxide levels if chronic bronchitis has been present for a long time.
What Bronchitis Diagnosis Doesn’t Include
There’s no throat swab, sputum culture, or rapid test that confirms “bronchitis” the way a strep test confirms strep throat. The color of your mucus, despite common belief, doesn’t reliably distinguish viral from bacterial infection. Green or yellow mucus can appear with any respiratory infection as your immune system responds.
Antibiotics are prescribed far too often for acute bronchitis. Because the diagnosis rests on clinical judgment rather than a clear-cut test result, understanding what your doctor is looking for (and what they’re ruling out) can help you make sense of the visit and the treatment plan that follows.