ICG Retention Rate (R15) Test
🟢 ICG Retention Rate (R15) Test: A Complete Guide
One of the most reliable quantitative assessments of liver function
The Indocyanine Green (ICG) Retention Rate at 15 minutes (R15) is one of the most widely used quantitative tests to evaluate hepatic blood flow, hepatocyte function, and biliary excretion capacity.
It is especially crucial in preoperative assessment before liver resection and in evaluating the prognosis of patients with cirrhosis or advanced liver disease.
This guide explains the purpose, test principles, reference ranges, clinical interpretation, and pitfalls—written for clinicians, trainees, and medical professionals.
1. 🧬 What Is Indocyanine Green (ICG)?
Indocyanine Green (ICG) is a non-toxic, water-soluble dye that binds rapidly to plasma proteins (mainly albumin).
Its characteristics:
- Taken up exclusively by hepatocytes
- Excreted unchanged into the bile
- No enterohepatic circulation
- Not metabolized in the liver
- Clearance depends on liver blood flow + hepatocyte function
💡 Therefore, ICG retention directly reflects the liver’s capacity to process and clear substances.
2. 🎯 Purpose of the ICG Retention Rate (R15) Test
✔ 1) Quantitative Evaluation of Liver Function
Used to assess the functional reserve in:
- Chronic hepatitis
- Cirrhosis
- Nonalcoholic steatohepatitis (NASH)
- Alcoholic liver disease
- Cholestatic disease
✔ 2) Preoperative Risk Assessment for Liver Resection
One of the most important tools for predicting post-hepatectomy liver failure (PHLF).
- R15 < 10% → Major hepatic resection safe
- 10–20% → Limited resection recommended
- > 20% → High risk of postoperative liver failure
Widely used in HCC (hepatocellular carcinoma) surgical planning.
✔ 3) Prognostic and Functional Monitoring
Useful in:
- Cirrhosis prognosis
- Acute-on-chronic liver failure (ACLF)
- Monitoring changes during hospitalization
- Pre-TIPS evaluation
✔ 4) Outpatient or Inpatient Follow-up
Simple, widely available, and reproducible—suitable for regular assessment of functional decline or improvement.
3. 🔬 Test Method: Colorimetry-Based Measurement
✔ Step-by-step procedure
- ICG is injected intravenously at 0.5 mg/kg.
- Blood sampling at 15 minutes.
- Plasma ICG concentration is measured.
- R15 is calculated as:
(Remaining ICG concentration at 15 min / initial concentration) × 100%
✔ Principle: Absorbance at 805 nm
- ICG has a characteristic absorption peak in the near-infrared spectrum (~805 nm).
- The device measures absorbance → absorbance correlates with concentration → yields R15%.
✔ Alternative: Pulse Dye Densitometry (Noninvasive)
Some centers use Pulse Dye Densitometry (PDD):
- Noninvasive
- Continuous monitoring
- Useful in the OR or ICU
4. 📊 Reference Range
| ICG R15 (%) | Interpretation |
|---|---|
| < 10% | Normal liver function |
| 10–20% | Mild to moderate impairment |
| > 20% | Significant dysfunction, high surgical risk |
Reference values vary slightly by institution and device.
5. 🩺 Clinical Interpretation (Increase / Decrease)
🔺 When ICG R15 is Increased → Impaired clearance
Seen in:
1) Cirrhosis
- Reduced hepatocyte mass
- Portal hypertension
- Collateral circulation formation
2) Chronic hepatitis / NASH / Fibrosis
Inflammation and fibrosis reduce uptake and excretion.
3) Acute hepatitis
High AST/ALT often accompany impaired ICG clearance.
4) Hepatocellular carcinoma (HCC)
Due to surrounding liver dysfunction + altered perfusion.
5) Drug-induced liver injury (DILI)
6) Reduced hepatic blood flow
- Heart failure
- Shock
- Budd–Chiari syndrome
7) Cholestasis / Biliary obstruction
- Impaired biliary excretion
🔻 When ICG R15 Decreases
Equivalent to:
- Normal hepatic perfusion
- Improved hepatocyte function
- Recovery from acute injury
- Clinical improvement after treatment
6. 📚 Diseases Related to Abnormal ICG Values
| Condition | Mechanism | ICG R15 |
|---|---|---|
| Cirrhosis | ↓ hepatocyte mass + ↓ perfusion | ↑ |
| Chronic hepatitis | Inflammation → ↓ function | ↑ |
| NASH | Steatosis → fibrosis | ↑ |
| Alcoholic liver disease | Toxic injury | ↑ |
| Biliary obstruction | Impaired excretion | ↑ |
| HCC | Reduced functional reserve | ↑ |
| Heart failure | ↓ hepatic blood flow | ↑ |
| Shock | Perfusion failure | ↑ |
7. 🩻 Interpretation Pitfalls & Precautions
⚠ 1) Hyperlipidemia
Lipemic plasma may interfere with photometric readings.
⚠ 2) Hyperbilirubinemia
Bilirubin absorbs light in similar wavelength ranges → false elevation of R15 possible.
⚠ 3) Hemodynamic instability
Low blood pressure or heart failure can elevate R15 despite preserved hepatocyte function.
⚠ 4) Strict timing is essential
Sampling errors (not exactly at 15 minutes) directly affect results.
⚠ 5) Drug effects
Any medication that alters bile flow or liver blood flow may affect clearance.
8. 📚 References
- Paumgartner G. The use of indocyanine green in liver function tests. Gastroenterology.
- De Gasperi A et al. ICG clearance as a predictor of postoperative liver failure. Ann Surg.
- Sakka SG. Assessing liver function: ICG plasma disappearance rate. Crit Care.
- AASLD Practice Guidelines for Liver Disease.
- Japanese Society of Hepato-Biliary-Pancreatic Surgery: ICG R15 criteria.
- Benya R et al. Pharmacokinetics of indocyanine green. J Clin Pharmacol.