How Are Gallstones Diagnosed? Ultrasound, Blood Tests, MRCP and HIDA Scans

Gallstones are diagnosed mainly with an ultrasound scan, which finds stones in the gallbladder in over 90% of cases, supported by blood tests and, when a stone may have slipped into the bile duct, an MRI scan of the ducts called an MRCP.¹ For most people the whole thing is settled by a single ultrasound, and the extra tests come in only when something suggests the problem has moved beyond the gallbladder.

When my own gallbladder was finally scanned, after a fortnight of arguing with myself that it was only indigestion, the stones showed up almost at once, and the sonographer said it so matter-of-factly that I felt faintly silly for having waited. What nobody had told me was how orderly the testing is: one scan does most of the work, and each further test answers a specific question rather than piling on for its own sake. This is the plain map of how gallstones are found, which test does what, and why you might be sent for one and not another.

When gallstones are looked for

A scan for gallstones is usually prompted either by a recognisable attack of pain high under the right ribs, or, very often, by chance during a scan being done for something else entirely. Gallstones are common: roughly 10 to 15% of adults in Western populations have them, more often women, and about 80% of those cause no symptoms at all². That silent majority is why so many are discovered by accident, on a scan of the kidneys or the liver, in someone who never suspected them.

The other route in is the attack itself, the gripping pain that sends people to a doctor asking what is wrong. What the stones are and why they form is set out in gallstones explained; this page picks up at the point where a doctor decides to go looking, and the first thing they reach for is almost always an ultrasound.

Ultrasound: the first-line test

An abdominal ultrasound is the first-line investigation for gallstones and detects stones in the gallbladder in over 90% of cases, which is why it is the first scan arranged once a gallstone problem is suspected.¹ A probe is moved over the upper abdomen, sound waves build a live picture of the gallbladder, and stones show up as bright spots that cast a shadow behind them. It takes minutes, uses no radiation, and needs nothing more than a few hours of not eating beforehand so the gallbladder is full.

NICE recommends ultrasound as the first imaging test for suspected gallstone disease, precisely because it is so good at the main job of seeing stones in the gallbladder³. The scan can also show a thickened, swollen gallbladder wall, which is a clue that the organ is inflamed rather than simply holding quiet stones. For the great majority of people, this single test is the whole diagnosis, and the conversation moves straight on to whether the stones need treating.

Blood tests: what your liver readings show

Blood tests cannot see the stones, and your liver readings can be entirely normal even with a gallbladder full of stones, so their real value is flagging a stone that has moved into the bile duct. A stone sitting quietly in the gallbladder blocks nothing, so the liver blood tests (LFTs) often read normal; it is when a stone slips into the main duct and backs bile up that a raised bilirubin and raised liver readings appear¹.

Up to about 10 to 15% of people with gallbladder stones also have one that has passed into the bile duct, and it is this group the blood tests help to catch³. That is a more serious situation, because a blocked duct can cause jaundice, an infected duct, or inflammation of the pancreas, and it is covered in full in gallstones in the bile duct. So a normal set of bloods is reassuring about the duct, but it says nothing about whether the stones in the gallbladder itself are the kind that need removing.

MRCP and endoscopic ultrasound: looking inside the duct

When a stone is suspected in the main bile duct, the duct is examined with an MRI scan called an MRCP, which pictures the ducts accurately without any tube being passed into the body, since a standard ultrasound often cannot see that far down. MRCP correctly identifies bile-duct stones in over 90% of cases, which is why it is used to look before anything is done to clear the duct³.

An alternative, where the anatomy is awkward or a very small stone is suspected, is an endoscopic ultrasound, in which the ultrasound probe is taken close to the duct from inside the gut. Both are looking tests rather than treatments: they answer the question of whether a duct stone is there, and if one is found it is usually cleared afterwards by a separate endoscopic procedure. The point of the distinction is that seeing a duct stone and clearing it are two different steps, done with different tools.

HIDA scan: how well the gallbladder empties

A HIDA scan measures how well the gallbladder fills and empties rather than simply looking for stones, and it is used mainly when scans and bloods are clear but the symptoms still point at the gallbladder, or when the organ may be acutely inflamed. A small amount of harmless radioactive tracer is followed through the liver and gallbladder over an hour or two, and a normal gallbladder squeezes out a good proportion of its contents; an ejection fraction below roughly 35 to 40% can point to a gallbladder that is not working properly⁴.

It is far from a routine test, and most people diagnosed with gallstones never have one, because their ultrasound has already answered the question. Where it earns its place is the harder middle ground: someone with convincing gallbladder-type symptoms whose ultrasound looks normal, or a suspected inflamed gallbladder where the diagnosis is not clear cut. Those inflamed cases, where the gallbladder has turned hot and tender, are their own subject with their own urgency.

What the diagnosis leads to

Finding gallstones is only the first half of the story; the diagnosis leads straight into a separate question, which is whether those particular stones need treating at all, and that turns on symptoms rather than the scan. Silent stones causing nothing are generally left alone, while symptomatic stones carry a steady risk of roughly 1 to 4% per year of a complication such as a blocked duct or inflammation, which is why they are usually removed².

So the scan that finds your stones does not, by itself, decide anything. It hands over a clear picture, and the real decision, laid out in whether you actually need your gallbladder out, is made by weighing that picture against how much trouble the stones are actually causing you⁵. What the tests cannot do is tell you which kind of stones are yours, or how urgent they are. That is a judgement for a surgeon who can examine you, read your own scans, and follow you up, not for a scan report read alone at a kitchen table.