Bronchitis is usually diagnosed through a physical exam and your symptom history, not a single definitive test. Most cases of acute bronchitis are caused by viruses, so doctors focus on ruling out more serious conditions like pneumonia rather than confirming bronchitis itself. Depending on your symptoms and how long you’ve been sick, your doctor may order additional tests like a chest X-ray or blood work.
The Physical Exam Comes First
Your doctor will start by listening to your lungs with a stethoscope. They’re checking for specific sounds that signal inflammation or mucus in your airways. Wheezing is a high-pitched hissing sound, usually heard when you breathe out, that indicates narrowed airways. Rhonchi are lower-pitched gurgling or snoring sounds caused by mucus in your larger airways. These sounds may shift around your chest when you cough as mucus moves. Crackles (sometimes called rales) sound like rattling or bubbling and suggest fluid or mucus in your smaller airways.
None of these sounds alone confirm bronchitis. Your doctor combines what they hear with your other symptoms, how long you’ve been coughing, whether you have a fever, and your overall health history to piece together a diagnosis. They’ll also check your temperature, look at your throat, and feel your lymph nodes for swelling.
When a Chest X-Ray Is Needed
A chest X-ray isn’t routine for suspected bronchitis. It’s ordered when your doctor suspects pneumonia instead. If you have a high fever, severe shortness of breath, or your lung sounds suggest something deeper than inflamed airways, the X-ray helps distinguish between the two. Pneumonia shows up as a visible area of infection on the image, while acute bronchitis does not. A normal chest X-ray in someone with a persistent cough and mucus production essentially confirms that the problem is bronchitis rather than pneumonia.
Oxygen Levels
Your doctor will likely clip a pulse oximeter onto your finger to measure how much oxygen your blood is carrying. A reading between 95% and 100% is normal for a healthy person. Readings below 92% suggest your lungs aren’t moving enough oxygen into your bloodstream, which could point to a more serious infection or complication. If levels drop to 88% or lower, that warrants immediate medical attention. For people who already have a chronic lung condition, slightly lower baseline readings are sometimes expected.
Blood Tests and Sputum Cultures
Routine blood work isn’t always necessary for a straightforward case of acute bronchitis. But if your symptoms are severe, your doctor may order a complete blood count to look for signs of a bacterial infection or to rule out other conditions. Elevated white blood cell counts can suggest your body is fighting a bacterial rather than viral infection.
A sputum culture, where you cough up mucus into a container for lab analysis, is reserved for cases where a serious bacterial or fungal lung infection is suspected. This test identifies the specific organism causing the infection, which helps your doctor choose the right treatment. You’re more likely to need one if you’re producing large amounts of discolored sputum, have a high fever with chills, or if an imaging test has raised concerns about a deeper lung infection.
Ruling Out Whooping Cough
If your cough is severe, comes in intense fits, or has lasted several weeks, your doctor may test for pertussis (whooping cough), which can look a lot like bronchitis in its early stages. The test involves a nasopharyngeal swab, where a thin, flexible swab is inserted deep into the nose to collect a sample from the back of the nasal passage. The sample is analyzed using PCR testing, which detects bacterial DNA and returns results relatively quickly. This test works best within the first three to four weeks of cough onset. After that, the amount of bacteria in the nasal passage drops significantly, increasing the chance of a false negative.
Breathing Tests for Chronic Bronchitis
If you’ve had a productive cough for at least three months in two consecutive years, your doctor may evaluate you for chronic bronchitis, which falls under the umbrella of chronic obstructive pulmonary disease (COPD). This involves spirometry, a breathing test where you blow as hard and fast as you can into a tube connected to a machine. The test measures how much air you can push out of your lungs in one second compared to the total amount you can exhale. A ratio below 0.7 after using an inhaled medication to open your airways is the threshold for a COPD diagnosis.
Spirometry is painless and takes only a few minutes. You’ll be asked to take a deep breath and then exhale forcefully into the mouthpiece, usually repeating this several times to get consistent readings. Your doctor may also use this test to distinguish chronic bronchitis from asthma, since the two conditions can cause similar symptoms but respond differently to treatment.
Symptoms That Trigger More Testing
Most cases of acute bronchitis resolve on their own within one to three weeks, and the diagnostic process stays simple. But certain warning signs push your doctor toward more extensive testing. These include a fever above 100.4°F, coughing up blood, worsening shortness of breath or wheezing, a bluish tinge to your lips or nail beds, confusion or difficulty concentrating (especially in older adults), and a cough lasting longer than three weeks. Any of these may prompt imaging, blood work, or cultures that wouldn’t otherwise be necessary for a typical case.