A gallbladder ejection fraction is part of a hepatobiliary iminodiacetic acid scan (HIDA), a medical test performed to diagnose acalculous biliary pain (ABP), among other conditions. ABP is characterized by an intermittent crampy pain on the right side of the upper abdomen that is often triggered by eating fatty foods but not caused by gallstones. During the scan, a hormone is injected into the gallbladder, causing it to contract. The rate at which the gallbladder releases bile is the gallbladder ejection fraction. A normal gallbladder ejection fraction is usually greater than 35 percent.

The gallbladder is a sac that stores bile from the liver. Bile is used to help digest fats in the foods people eat. Sometimes painful pebble-like deposits called gallstones form out of cholesterol or bilirubin in the gallbladder. If an ultrasound or other imaging test reveals that a patient experiencing gallstone symptoms does not have gallstones, ABP is often suspected as the cause of pain.

Diagnosing ABP can be difficult because patients experience pain typical of gallstones but do not have them. Delayed gallbladder emptying defined as a gallbladder ejection fraction of less than 35 percent usually indicates ABP because the pain can be related to how the gallbladder muscle contracts. Calculating a patient’s gallbladder ejection fraction is therefore an important part of definitively diagnosing ABP.

Determining a patient’s gallbladder ejection fraction requires a HIDA scan. A HIDA scan is an imaging procedure that creates a series of pictures of an individual’s gallbladder, liver and bile ducts, and small intestine. This is a nuclear medicine scan that uses a radioactive chemical or tracer to highlight organs. Only a small amount of radioactive tracer is used to determine the gallbladder ejection fraction, and it becomes inactive after a few hours.

Once the tracer has been injected, a gamma camera will take continuous pictures of the patient’s abdomen for approximately an hour. Results of the scan are determined by where the tracer is able to go, how freely it flows, and how much of it is absorbed by certain organs. The tracer allows the doctor to measure the size of the patient’s gallbladder.

The patient is also injected with a hormone called CCK if his or her gallbladder ejection fraction will be determined during the HIDA scan. This hormone causes the gallbladder to contract. The gallbladder ejection fraction is the differential between the amount of contraction and the size of the gallbladder before and after the hormone was injected. An ejection fraction of less than 35 percent is associated with ABP.

Although the cause of pain associated with a low gallbladder ejection fraction is not known, many patients with ABP have low-grade inflammation of the lining of the gallbladder. This inflammation is persistent and may be the result of gallstones so small that they cannot be detected on imaging tests. Most people with low gallbladder ejection fractions improve after their gallbladders are removed, but avoiding fatty foods may be sufficient to avoid an operation.

In preparation for a HIDA scan, a patient may be required to fast for at least two hours prior to the procedure. Medications that interfere with the scan results should be avoided. In some cases, a patient may have to take a medication in advance that enhances the scan.

HIDA scans are also used to diagnose other liver and gallbladder problems. Bile duct obstruction, bile leakage, and congenital abnormalities in the bile ducts can all be seen on HIDA scans. Gallstones and gallbladder inflammation are also routinely detected during this test. The scan does not present many risks other than a rash or bruising at the tracer injection site, but pregnant or breastfeeding women should not undergo this procedure. Rarely, a patient may have an allergic reaction to the tracer.