Abdominal X-rays are sometimes a starting point for symptoms of abdominal pain both in the outpatient and emergency settings. Your doctor may decide to start with an abdominal X-ray since it is fast and inexpensive and can provide a diagnosis of the cause of your abdominal pain in some cases.
The cause of abdominal pain is broad and abdominal X-rays can only sometimes provide a cause. I think abdominal X-rays are best for detecting kidney stones, bowel obstruction, and perforated bowel. There are numerous conditions which will not be seen on an abdominal X-ray such as appendicitis, pancreatitis, cancer and many other serious conditions. These will often be diagnosed with a CT scan or other tests.
Therefore, abdominal X-rays are just a starting point. If your doctor is concerned about a bowel blockage or twist called a volvulus, then an abdominal X-ray is a start, but can also miss these diagnosis. A CT scan will often be need to not only diagnose but confirm suspicion on an X-ray.
A patient who comes in with flank pain and blood in the Urine, may have a kidney stone which they are passing. Sometimes an X-ray is ordered. A stone will look like a small white spot along the expected course of the ureters by the spine. The ureters will allow urine to pass from the kidneys to the bladder.
A kidney stone can be found anywhere along the urinary tract from the kidneys to the bladder. Unfortunately, not every kidney stone is identified on an X-ray. Additionally, so,times other benign calcifications can look like kidney stones. If there is continued suspicion or confusion then a ct scan will be needed.
A perforated bowel can often be diagnosed on abdominal X-rays. Air normally is only located within the bowel. If there is air outside the bowel in the abdomen, then this often means that there is perforated bowel, or air has escaped from the bowel due to a tear of the wall. This will be seen as air under the diaphragms. The X-ray has to be done upright for best diagnosis. This is often a surgical emergency.
Foreign bodies can be detected on an abdominal X-ray. Often these are ingested. Some foreign bodies which do not show up on X-ray will be missed. The X-ray will show how far the foreign body has passed and if it has caused a blockage or perforation.
Abdominal X-rays may also occasionally show other important findings. For example, an aneurysm of the aorta may sometimes be identified which may need treatment. Various calcifications such as gallstones can be identified which may be responsible for your symptoms. A lot of vascular calcifications indicates plaque build up and atherosclerosis which can lead to bowel ischemia or not enough blood getting to bowel.
An abdominal X-ray is therefore just a starting point, and does not provide a diagnosis in many cases. A ct scan is much better for diagnosing the cause of abdominal pain. Your doctor will also take a detailed history and physical exam and obtain lab testing in some cases. In some cases, referral to a surgeon or gastroenterology may be appropriate as well.