Chest X-rays are often done for respiratory symptoms like cough. Chest X-rays done for cough do not always show abnormalities to explain the cough. This is because many causes of cough do not show up on an X-ray.
What conditions that cause cough do not show up on chest X-ray?
Causes that may not show up on a chest X-ray can include : asthma, heart burn or reflux, medication use, post nasal drip, emphysema, bronchitis, damage or inflammation of airways. There are other conditions that do not show up on X-ray as well.
What does show up on a chest X-ray?
Infections may show up on a chest X-ray to explain symptoms. This can be a pneumonia which is associated with cough and fever. More chronic infections can show up as well. Many infections, particularly viral ones will not show on chest X-ray. Infection on chest X-ray will often be white areas or spots amidst the black of the normal lung. This can be isolated to one spot or throughout both lungs.
Cancer of the lung can cause cough. An early or small lung cancer may not be visible on a chest X-ray. A larger cancer may appear as a nodule or mass in the lung. Advanced cancer which has spread may show up as multiple nodules, fluid, and enlarged lymph nodes.
Other less common causes can include heart failure. This will show up on X-ray as an enlarged heart, fluid and edema in the lungs. Inflammatory conditions of the lung like sarcoidosis can show up as white areas and enlarged lymph nodes. Scarring of the lungs can be seen when it is advanced. We can sometimes see findings of emphysema on a chest X-ray.
A normal chest X-ray does not mean all is normal
I think a chest X-ray is a good start to evaluating a cough. It can reveal infections, cancer and other conditions of the lung which can cause cough. When a chest X-ray is normal, it certainly does not mean that the work up of your cough should stop.
Chest X-rays do not show all infections, cancers and lung diseases. A chest CT is a much better test for that. Additionally, there are many conditions outlined above that have no findings on chest X-ray or CT. In these cases, your doctor will have to make a diagnosis based on additional testing and his clinical findings.
A chest X-ray is a quick and relatively inexpensive test to screen the lungs for abnormalities. A negative chest X-ray however should not be the end of the workup. There are many infections, cancers, and other lung diseases which will not be seen on chest X-ray. It is therefore important to workup cough with more then just a chest X-ray.