Indocyanine Green (ICG) Clearance Test

Abstract

The liver plays a critical role in metabolism, detoxification, synthesis of proteins, and excretion of metabolic waste. Evaluating the functional capacity of the liver is essential for diagnosing liver disease and determining the safety of surgical interventions involving hepatic tissue. Traditional liver function tests such as bilirubin, transaminases, and albumin provide indirect information about hepatic health but do not always accurately reflect the liver’s functional reserve. In contrast, dynamic liver function tests measure the ability of hepatocytes to process specific substances in real time. The Indocyanine Green (ICG) clearance test is one such dynamic assessment. By observing how efficiently the liver removes the dye from circulation, clinicians can estimate hepatic blood flow and hepatocellular function simultaneously. This makes the test particularly useful in evaluating patients with chronic liver disease and those undergoing liver resection or transplantation. This research-based article provides a comprehensive explanation of the physiological basis, testing methodology, interpretation of results, clinical applications, advantages, limitations, and integrative perspectives associated with the Indocyanine Green clearance test.

Introduction

The Indocyanine Green (ICG) Clearance Test is a specialized diagnostic assessment used to evaluate hepatic functional reserve, hepatic blood flow, and overall liver performance. It involves intravenous administration of the dye indocyanine green followed by measurement of its clearance from the bloodstream.Because the dye is exclusively taken up by hepatocytes and excreted into bile without undergoing metabolism, the rate of its elimination directly reflects hepatic function and perfusion. The ICG clearance test is widely used in hepatology, liver surgery, and transplant evaluation to assess liver functional capacity before major surgical procedures. It also helps determine prognosis in chronic liver diseases and cirrhosis.

Historical Background

Indocyanine Green was first developed in the mid-20th century as a medical dye used for cardiovascular and hepatic function testing. Over time, it became widely applied in hepatology due to its unique pharmacokinetic properties.

Its strong binding to plasma proteins and exclusive hepatic clearance make it an ideal marker for assessing liver function without interference from renal or extrahepatic metabolism.

Today, the ICG clearance test is routinely used in several countries for preoperative evaluation of liver surgery and for assessing hepatic functional reserve in patients with cirrhosis or other chronic liver disorders.

Chemical and Pharmacological Properties of Indocyanine Green

Indocyanine Green is a water-soluble tricarbocyanine dye with several unique characteristics that make it suitable for hepatic function testing.

Important Properties

• Rapid binding to plasma proteins after intravenous injection
• Exclusive uptake by hepatocytes
• No metabolic transformation in the body
• Excretion into bile without enterohepatic recirculation
• Short plasma half-life under normal liver function conditions

Because the dye is not metabolized or excreted by the kidneys, its clearance reflects liver performance with high specificity.

Physiological Basis of the ICG Clearance Test

The ICG test evaluates three important physiological aspects of liver function.

1. Hepatic Blood Flow

The dye is delivered to the liver through the portal vein and hepatic artery. Efficient clearance depends partly on adequate hepatic perfusion.

2. Hepatocyte Uptake Capacity

Hepatocytes actively transport ICG from blood into liver cells. Damage to hepatocytes reduces this uptake capacity.

3. Biliary Excretion

After uptake, the dye is excreted into bile canaliculi and eventually eliminated through bile flow. Impairment of bile excretion slows dye clearance.

Thus, the rate of disappearance of ICG from plasma reflects combined effects of hepatic perfusion, hepatocyte function, and biliary excretory capacity.

Procedure of the ICG Clearance Test

Patient Preparation

Patients usually undergo the test after fasting for several hours. This ensures stable baseline circulation and avoids interference from postprandial physiological changes.

Preparation Includes

• Avoid heavy meals before the test.
• Review medications that may influence hepatic blood flow.
• Obtain baseline vital signs.

Dye Administration

A calculated dose of Indocyanine Green is injected intravenously. The standard dose is usually based on body weight.

After injection, the dye rapidly binds to plasma proteins and circulates through the bloodstream until it reaches the liver.

Measurement of Dye Clearance

Blood samples are collected at specific time intervals to measure the concentration of Indocyanine Green remaining in circulation.

Modern techniques may also use non-invasive pulse spectrophotometry sensors placed on the skin to measure dye disappearance continuously.

Commonly Measured Parameters

• ICG Plasma Disappearance Rate (PDR)
• ICG Retention Rate at 15 Minutes (ICG-R15)

Key Parameters of the Test

Plasma Disappearance Rate (PDR)

This value represents the percentage of dye cleared from plasma per minute. Higher PDR values indicate better liver function, whereas lower values suggest impaired hepatic clearance.

Retention Rate at 15 Minutes (ICG-R15)

This parameter measures the percentage of dye remaining in circulation 15 minutes after injection. It is widely used to estimate liver functional reserve.

Interpretation

• Normal Liver Function: Less than about 10% retention
• Moderate Impairment: Approximately 10–20% retention
• Severe Impairment: Greater than about 20–30% retention

Higher retention rates indicate reduced hepatic capacity to remove the dye.

Interpretation of Test Results

Normal Results

Efficient uptake and excretion of ICG indicates healthy hepatocytes, adequate hepatic blood flow, and intact biliary excretion pathways.

Mildly Abnormal Results

Moderate delay in dye clearance may indicate early hepatic dysfunction or mild impairment of hepatic blood flow.

Markedly Abnormal Results

High retention levels indicate severe hepatocellular dysfunction, advanced fibrosis, cirrhosis, or impaired hepatic perfusion. Such findings may influence decisions regarding surgery or transplantation.

Advantages of the ICG Clearance Test

Several features make the Indocyanine Green (ICG) Clearance Test valuable in hepatology and liver surgery.

• Direct assessment of hepatic functional reserve
• Rapid results
• Non-radioactive and relatively safe dye
• Highly specific to liver function
• Useful for surgical risk assessment
• Provides quantitative measurements

Because the test reflects dynamic liver function, it offers information that conventional static biochemical liver function tests cannot provide.

Limitations

Despite its advantages, the ICG Clearance Test has certain limitations that should be considered during clinical interpretation.

• Requires intravenous dye administration
• Results may be influenced by changes in hepatic blood flow
• Less commonly available in some healthcare settings
• Interpretation may vary depending on the patient’s clinical condition
• Does not identify the underlying cause of liver disease

Therefore, the test should always be interpreted alongside clinical examination, laboratory findings, imaging studies, and other diagnostic investigations.

Safety and Adverse Effects

Indocyanine Green has an excellent safety profile and is generally well tolerated. However, rare adverse reactions may occur in susceptible individuals.

Possible Adverse Reactions

• Mild allergic reactions
• Nausea or flushing
• Hypersensitivity in individuals with iodine allergy

These reactions are uncommon, and the test is generally considered safe when performed under appropriate medical supervision.

Comparison With Other Liver Function Tests

Traditional liver function tests measure biochemical markers such as liver enzymes, bilirubin, albumin, and coagulation factors. While these tests provide valuable clinical information, they do not always accurately reflect the liver’s functional reserve.

In contrast, dynamic tests such as the ICG Clearance Test directly assess liver performance by evaluating:

• Hepatic blood flow
• Hepatocyte uptake capacity
• Biliary excretion efficiency

For this reason, the ICG Clearance Test plays an important role in preoperative liver assessment and surgical planning.

Integrative Hepatic Support: Ayurvedic Perspective

Traditional Ayurvedic hepatoprotective herbs have long been used to support liver metabolism, antioxidant defense, and cellular resilience. Modern scientific research has identified several botanicals that may help maintain healthy liver function by protecting hepatocytes from oxidative stress and supporting normal metabolic processes.

1. Bhumi Amalaki (Phyllanthus niruri)

Bhumi Amalaki contains lignans such as phyllanthin and hypophyllanthin, which demonstrate hepatoprotective activity. Research suggests these compounds help maintain hepatocyte membrane stability while enhancing antioxidant defense mechanisms.

2. Kalmegha (Andrographis paniculata)

Kalmegha contains the bioactive compound andrographolide, known for its anti-inflammatory and hepatoprotective properties. Studies indicate that it supports detoxification pathways and helps protect liver cells against oxidative injury.

3. Kutki (Picrorhiza kurroa)

Kutki is rich in picrosides, compounds that help maintain healthy hepatic enzyme activity and protect liver tissue against toxin-induced damage. Experimental studies also suggest a role in supporting liver regeneration and metabolic regulation.

4. Bhringaraja (Eclipta alba)

Bhringaraja contains wedelolactone and various flavonoids that promote antioxidant activity and support hepatocyte regeneration. Traditionally, it has been valued for maintaining liver vitality and overall hepatic wellness.

5. Punarnava (Boerhavia diffusa)

Punarnava possesses antioxidant and anti-inflammatory properties that may support healthy microcirculation and metabolic balance within liver tissue, contributing to overall hepatic health. These herbs are traditionally used to promote liver wellness and should be considered supportive measures rather than substitutes for professional medical diagnosis or treatment.

Conclusion

The Indocyanine Green (ICG) Clearance Test is a valuable dynamic assessment of liver function that evaluates the liver’s ability to remove a specialized dye from the bloodstream. Since the dye is exclusively taken up by hepatocytes and excreted into bile without undergoing metabolism, its clearance provides direct insight into hepatic blood flow, hepatocellular function, and biliary excretory capacity. The test is widely used in hepatology, particularly for evaluating patients before liver resection or transplantation, where accurate assessment of hepatic functional reserve is essential. Although it does not replace conventional laboratory investigations or imaging studies, it provides unique information that static liver function tests cannot offer. When interpreted alongside clinical findings and complementary diagnostic tests, the ICG Clearance Test serves as an important tool for comprehensive liver evaluation, prognosis, and surgical decision-making.