The indocyanine green (ICG) test is a diagnostic tool that uses a special fluorescent dye to assess organ function and visualize blood flow within the body. When injected into the bloodstream, this compound acts as a tracer, allowing medical professionals to monitor its movement and clearance. The test provides information on how well organs like the liver are functioning, and offers real-time visualization of vascular structures during complex procedures. This non-radioactive technique has become a standard method in various medical fields for obtaining precise, dynamic physiological data.
How Indocyanine Green Dye Works
Indocyanine green is a water-soluble dye that possesses unique optical and biological properties, making it an excellent agent for medical diagnostics. Upon intravenous injection, the ICG dye rapidly binds to plasma proteins, primarily albumin. This binding confines the dye to the intravascular space, preventing it from leaking out into the surrounding tissues.
The dye is then selectively taken up by the liver’s parenchymal cells. The liver secretes the ICG entirely into the bile without undergoing significant metabolism or elimination by the kidneys. The rate at which the liver clears the dye from the bloodstream provides a direct, quantitative measure of its functional capacity and blood flow.
ICG’s defining characteristic is its fluorescence in the near-infrared (NIR) spectrum. When exposed to NIR light, the dye absorbs the energy and re-emits it. This emitted light is captured by specialized cameras, creating a real-time image of the dye’s distribution. This NIR property allows for deeper tissue penetration than visible light, providing clearer visualization.
Clinical Uses of the ICG Test
The ICG test is utilized across multiple medical disciplines, primarily for functional assessment and real-time visualization. In gastroenterology and hepatology, the test is a standard method for quantitatively assessing liver function and reserve. The rate of ICG clearance from the blood, often measured as the retention rate after 15 minutes (ICG-R15), serves as a precise index of overall hepatic health. This data is valuable for planning major liver resections or transplants, allowing surgeons to determine the maximum amount of liver tissue that can be safely removed.
The second application involves using the dye’s fluorescence for real-time visualization during surgery and diagnostic imaging. In ophthalmology, ICG angiography visualizes the choroidal vasculature beneath the retina. This capability is helpful for diagnosing conditions like age-related macular degeneration, where deeper vessel abnormalities are present.
In complex surgical procedures, the dye is used as a fluorescent tracer to map blood flow, a technique called fluorescence angiography. Surgeons use this to assess tissue perfusion in structures like the bowel or in organ grafts during transplantation, ensuring adequate blood supply. ICG also assists in identifying sentinel lymph nodes in cancer surgery and visualizing the anatomy of the bile ducts during minimally invasive procedures.
What to Expect During the Procedure
The ICG test procedure is relatively straightforward, though the specifics depend on whether the test is for functional assessment or imaging. For most applications, a small intravenous line is placed, usually in the arm or hand. Patients may be asked to fast or temporarily stop certain medications beforehand.
The indocyanine green dye is injected quickly into the IV line, often followed by a saline flush. If the test is for liver function, the procedure involves timed blood sampling at specific intervals. These samples are analyzed to calculate the ICG clearance rate, which reflects the liver’s efficiency.
If the test is for visualization, specialized near-infrared cameras are used for real-time monitoring. These cameras capture the fluorescent signal as the dye circulates through the targeted tissues. The entire procedure typically takes between 20 and 30 minutes, though late-phase imaging may extend to an hour.
Safety Profile and Potential Risks
Indocyanine green generally has a favorable safety profile, with adverse effects occurring in a very small percentage of patients. The dye’s rapid clearance by the liver and lack of metabolism contribute to its minimal toxicity. Safety considerations are primarily related to the dye’s composition.
The most significant contraindication is a known hypersensitivity to iodine or iodine-containing substances, including certain shellfish. ICG contains sodium iodide, and in patients with a severe iodine allergy, there is a risk of anaphylaxis, though this is rare. A detailed allergy history is always taken before the test is administered.
Other potential side effects are typically mild and transient, such as nausea, localized pain or bruising at the injection site, or a mild skin rash. Medical staff closely monitor patients during the procedure to quickly manage any reactions. Patients may notice their stool having a greenish tint following the procedure, which is a harmless result of the dye’s excretion.