Abdominal Ultrasound: Preparation, Do You Need to Fast and a Full Bladder

The gallbladder stores bile produced by the liver. After a meal, especially one containing fat, hormones (among them cholecystokinin) are released that make the gallbladder contract and push bile into the duodenum. A contracted gallbladder is small, has a thickened wall and is hard to assess — stones, polyps or wall changes may be invisible or hard to interpret. When fasting, the gallbladder is filled with bile, which gives a black (anechoic) image on ultrasound, against which stones give characteristic echoes and an acoustic shadow.

The second reason is the bowel and stomach: food and swallowed air create gas, which makes it harder to assess the pancreas and deeper structures. That is why long fasting used to be recommended, and sometimes laxatives or anti-gas drugs as well. Whether this advice is supported by studies has been examined only recently.

These two sources are worth reading together, but with caution. The randomised trial involved hospital inpatients at a single centre, and the certainty of evidence in the meta-analysis is low, mainly because of the small number of studies and risk of bias. They do not show that fasting is harmful or that the unit's instructions may be ignored. They show that for a general assessment of abdominal organs the benefit of long fasting is unconfirmed, and that the only organ that clearly responds to a meal is the gallbladder and biliary tract. The authors of the meta-analysis suggest that strict, many-hour recommendations should be reconsidered.

The conclusion for the patient is nuanced. Since in the studies fasting significantly affected the gallbladder and bile duct, and the ultrasound was ordered precisely because of pain in the right upper abdomen, gallstones or changes in the liver and bile ducts, preparing according to the centre's instruction makes sense. If, on the other hand, the scan mainly concerns the kidneys, aorta or pelvic organs, strict fasting has less justification, but the decision is made by the unit, not the patient. In practice recommendations differ between centres — which the review confirms by describing them as highly variable.

In a pelvic ultrasound through the abdominal wall (uterus, ovaries, bladder and, in men, the bladder and prostate), a full bladder acts as an acoustic window. Water conducts ultrasound waves without distortion, so structures lying behind a full bladder can be seen well. In addition, a full bladder pushes loops of bowel to the sides, which would otherwise hide the pelvic organs behind gas. This is how educational materials on pelvic ultrasound technique describe it.

For the patient this means practical guidance: if the unit asks for a full bladder, fill it as instructed, but not so much that the examination is painful. Usually a defined amount of water is drunk some time before the scan and urine is not passed until the examination. If the urge is unbearable, tell the staff — the scan can be done partially or after partial emptying. For an abdominal ultrasound proper (liver, gallbladder, pancreas) a full bladder is usually not required, unless the centre also orders assessment of the pelvis.

Bowel gas is the most common cause of technically unsatisfactory abdominal ultrasound scans, which is why attempts have long been made to reduce it with anti-gas drugs (simethicone, dimethicone) and laxatives. The evidence is surprisingly thin.

This study is very old (1987), assessed a limited number of structures and did not involve modern machines, so it does not close the subject. The lack of confirmed effectiveness of simethicone in the only randomised trial we found does not mean it does harm — it is a drug with a good safety profile. If the centre recommends it in its instructions, it can be used, but significant improvement of the image should not be expected. Simple steps may be more effective: avoiding fizzy drinks, chewing gum and eating quickly before the scan, although hard studies are lacking here too.

Abdominal ultrasound shows well gallstones, cysts and solid masses in the liver and kidneys, dilated bile ducts, hydronephrosis, an enlarged spleen, fluid in the peritoneal cavity or an aortic aneurysm. Some areas are assessed less well: the pancreas is sometimes obscured by gas, and small bowel lesions, tiny ureteric stones or lesions deep in the body may need CT, MRI or endoscopy. The examination also depends on the experience of the operator and on the patient's body build.

The result illustrates an important principle: ultrasound is a good but not a perfect tool — a sensitivity of 71% means that some cases of acute cholecystitis may not be recognised at the first scan. A normal ultrasound therefore does not always rule out disease when symptoms are worrying, and the doctor may recommend repeating the scan or supplementing it with another test. Repeating the scan is also sometimes advised when the report notes 'limited assessment because of gas' or inadequate preparation.

The recommendations for preparing for an abdominal ultrasound rest more on tradition than on strong studies, but that is not a reason to ignore them. A sensible approach is to stick to the centre's instruction, keep a reasonable interval from a meal and say so if a long fast is risky for you. What matters most is the purpose of the scan: when the doctor is looking for gallstones or changes in the bile ducts, preparation matters most. And if the report mentions limited assessment because of gas, that is reason enough to ask the doctor about a repeat or another test.