The Indocyanine Green (ICG) test is a diagnostic procedure that uses a special fluorescent dye to visualize and assess various physiological functions. This test allows medical professionals to observe blood flow, organ function, and tissue perfusion in real-time. By tracking the dye’s movement, clinicians gain insights into the health and performance of specific organs and systems, making it a valuable tool for diagnosis and guiding treatment across multiple medical specialties.
Understanding Indocyanine Green
Indocyanine Green (ICG) is a water-soluble cyanine dye with unique fluorescent properties. When injected into the bloodstream, ICG rapidly binds to plasma proteins, primarily albumin. This keeps it confined within the vascular system, preventing leakage into surrounding tissues and allowing for clear visualization of blood flow.
ICG absorbs and emits light in the near-infrared (NIR) spectrum, specifically absorbing around 800 nanometers and fluorescing at approximately 830 nanometers. This is useful because NIR light can penetrate deeper into tissues than visible light and minimizes interference from the body’s natural autofluorescence. The dye is exclusively cleared from the body by the liver into bile, without undergoing any metabolism. This makes ICG an effective indicator for evaluating liver function. ICG has a short half-life in the body, typically ranging from 150 to 180 seconds, ensuring its rapid removal.
Clinical Applications of the ICG Test
The ICG test finds broad application across several medical disciplines due to its unique properties. In assessing liver function, it is a standard test for evaluating hepatic health and blood flow. The rate at which the liver clears ICG from the bloodstream directly reflects its functional capacity, making it useful for pre-operative assessment before major liver surgeries like resections or transplants, and for monitoring patients in intensive care.
The test is also widely used in ophthalmology, particularly for visualizing the choroidal vasculature of the eye. Indocyanine green angiography (ICGA) helps diagnose and monitor conditions such as age-related macular degeneration, polypoidal choroidal vasculopathy, and central serous chorioretinopathy by providing detailed images of blood vessels beneath the retina that other dyes may not reveal as clearly. The dye’s near-infrared fluorescence allows it to penetrate ocular pigments and fluids, offering enhanced visualization.
ICG also plays a role in guiding surgeries, especially in oncology and reconstructive procedures. In cancer surgery, it assists in sentinel lymph node mapping, helping surgeons identify the first lymph nodes to which cancer cells are likely to spread. For reconstructive surgery, ICG helps evaluate tissue perfusion and viability of flaps, ensuring adequate blood supply to transplanted tissues. ICG can also be utilized to measure cardiac output, though other methods are now more common.
The ICG Test Procedure
Patients undergoing an ICG test may be asked to fast or avoid specific medications that could interfere with the dye’s clearance. Upon arrival, a healthcare professional will explain the procedure. Patients should inform staff of any allergies.
A small tube, known as a cannula, is inserted into a vein to inject the ICG dye. For ophthalmic applications, eye drops may be administered to dilate the pupils. A measured dose of the ICG dye is then injected intravenously.
Following the injection, specialized equipment, such as a near-infrared camera or a device that measures dye concentration in blood samples, monitors the dye’s circulation and clearance. Depending on the application, this might involve taking blood samples at precise intervals to assess liver function, or capturing real-time images of blood flow using fluorescence imaging. The entire procedure typically lasts about 30 minutes.
Interpreting ICG Test Outcomes and Safety
The interpretation of ICG test outcomes depends on the specific clinical application. For liver function assessment, clinicians evaluate parameters such as the ICG plasma disappearance rate (PDR) or the retention rate at 15 minutes (ICG-R15). A faster disappearance rate or a lower retention percentage indicates better liver function, while a higher ICG-R15 might suggest impaired liver function. In surgical contexts, the intensity and pattern of ICG fluorescence help surgeons identify anatomical structures, assess blood flow to tissues, or locate tumors.
The ICG test is generally considered safe, with a low incidence of adverse reactions. Most patients experience no significant side effects, though some might notice temporary blurring of vision or light sensitivity if eye drops were used, or mild bruising at the injection site. The dye is rapidly processed and eliminated by the liver, minimizing long-term side effects.
However, as ICG contains a small amount of sodium iodide, it is contraindicated in individuals with a known allergy to iodine or shellfish. Rarely, severe allergic reactions, including anaphylaxis, can occur. Patients with severe liver impairment may also experience slower dye clearance, which can affect image quality. Caution is also advised when interpreting results in patients with hyperbilirubinemia, as high bilirubin levels can interfere with ICG kinetics.