Ultrasound vs CT for abdominal pain

Abdominal pain has dozens of causes, and the scan chosen depends on where and how it hurts. Pain in the right upper abdomen after fatty meals points toward the gallbladder, where ultrasound is the established first test. Pain in the right lower abdomen raises appendicitis, where either test may be used. Colicky flank pain that spreads to the groin suggests a kidney stone, where CT is more sensitive.

A single "best" scan for abdominal pain does not exist. A clinician takes a history, examines you, checks blood and urine, and then chooses imaging to answer the specific question that remains.

Ultrasound is real-time, portable and free of radiation. It shows gallstones, a thickened gallbladder wall, a dilated bile duct, liver changes, kidney swelling from obstruction, ovarian cysts and free fluid. It can show a swollen appendix in slim patients and children, and is often the first test for them.

Its limitations are bowel gas, obesity and operator skill. Gas can hide deep structures, a normal ultrasound does not exclude appendicitis, and the pancreas and bowel can be hard to see.

CT gives a consistent cross-sectional view, unaffected by gas or body shape. It identifies bowel obstruction, perforation, diverticulitis and its complications, abscesses, kidney stones, blood vessel problems and many cancers. Contrast through a vein improves detail. It is the usual test for sick adults when a surgical cause is suspected.

The costs are radiation, the need for contrast in many cases (with kidney and allergy checks), and the fact that it misses many gallstones and has limited value in some gynaecological problems.

The principle in imaging guidance, including the Royal College of Radiologists referral guidance, is to use the test with the highest chance of answering the question for the lowest harm. Radiation sparing is prioritised in children and pregnancy, while in a sick adult with possible perforation or obstruction speed and diagnostic certainty matter more.

Pathways for suspected cancer, such as NICE NG12, also direct certain symptoms like unexplained weight loss with abdominal pain towards urgent imaging or endoscopy rather than a general scan.

• Pain after fatty food on the right upper abdomen, with or without fever: ultrasound, with urgent review if feverish or yellow.

• Sudden severe pain in the right lower abdomen with fever and vomiting: urgent medical assessment; imaging chosen by the surgical team.

• Colicky pain in the flank spreading to the groin, blood in urine: CT or ultrasound, depending on age, pregnancy and local protocols.

• Persistent bloating, weight loss, change in bowel habit: GP assessment and pathway tests, not a self-requested scan.

• Pregnant, or a child: ultrasound first wherever possible.

A normal ultrasound does not mean nothing is wrong; it means the structures it can see look normal. Pain with an unclear cause may need more tests. And CT is not automatically a more thorough test for every abdominal complaint, since it can miss stones and some gynaecological conditions.

Many abdominal pains also settle without any scan, and a scan taken early can show no cause. Clinicians often watch the course of symptoms.