An air-fluid level in bowel on X-ray is a common finding that can represent a normal finding all the way to life threatening bowel obstruction. An air-fluid level means that there is both air and fluid in a bowel loop. The air will rise to the top of the bowel loop and the fluid will settle at the bottom. There is a sharp straight interface between the air and fluid.
This finding needs to be interpreted in the context of how the patient is feeling and also if there are any other concerning findings on an X-ray. A few air-fluid levels scattered in the abdomen without other findings may be normal. Multiple air-fluid levels in the abdomen combined with dilated bowel and a sick patients who is vomiting and has abdominal pain usually means bowel obstruction.
A small bowel obstruction or ileus are common conditions which are associated with air-fluid levels in bowel. An ileus of bowel is a non obstructed bowel which is not working or paralyzed. The contents of bowel stagnate and are not pushed along. This can cause a similar appearance to a bowel obstruction. The distinguishing feature is the lack of transition to normal or decompressed bowel.
A small bowel obstruction can be of varying degree and also causes air-fluid levels. Often dilated bowel loops are seen and there is a transition to normal or decompressed small bowel. As opposed to ileus, there is less of a generalized abnormality. At times, X-rays can even be normal when the bowel is not. In bowel obstruction, the patient is often sick with vomiting and pain. The cause of the obstruction will often not be identified on an X-ray.
A CT scan is much better at diagnosing an obstruction and the cause. Sometimes a hernia will cause an obstruction, but most commonly it is seen as a result of adhesions after surgery, or scar formation. They are often not seen on X-ray or CT but are presumed to be present at the site of obstruction where the bowel transitions from dilated and abnormal to normal or decompressed.
Therefore, air-fluid levels can be harmless in a well patient getting an X-ray for an unrelated cause, all the way to a life threatening bowel obstruction. The latter will be associated with other findings and the patient will be sick. A CT scan will be done to further investigate. The CT will be better at determining if there is a blockage of bowel and what the cause may be. A surgeon will often get involved if there a bowel obstruction.