Introduction:
The liver plays a vital role in excretory function, eliminating substances such as bilirubin and various dyes or drugs through the same metabolic pathways. Dye tests were traditionally used to evaluate the liver’s ability to take up, conjugate, and excrete such compounds into bile.
Although these tests have largely been replaced by more accurate enzyme assays and biochemical tests, they are historically significant in assessing hepatic excretory capacity.
Bromsulphthalein (BSP) Test:
Bromsulphthalein (BSP) is a synthetic dye once widely used to measure liver excretory function. After intravenous injection, BSP is taken up by the liver, conjugated, and excreted into bile — following a similar pathway to bilirubin metabolism.
Principle:
The rate at which BSP disappears from the bloodstream reflects the liver’s ability to excrete organic anions. A delay in BSP clearance indicates hepatic dysfunction.
Procedure:
- A measured dose of BSP dye is injected intravenously.
- Blood samples are collected at intervals (typically after 45 minutes).
- The amount of BSP remaining in plasma is measured spectrophotometrically.
Interpretation:
- Normal liver function: Most of the dye is cleared from plasma within 45 minutes.
- Impaired hepatic function: Retention of BSP in the blood indicates reduced liver excretory capacity (as seen in hepatitis, cirrhosis, or biliary obstruction).
Limitations and Risks:
- The BSP test has largely fallen out of use due to potential side effects.
- Injection may cause anaphylactic shock in allergic individuals.
- If the dye accidentally enters tissues instead of the vein, it can cause local tissue necrosis.
Indocyanine Green (ICG) Test
Indocyanine green (ICG) is a safer and less allergenic dye used as an alternative to BSP for testing hepatic excretory function. Like BSP, it is taken up exclusively by the liver and excreted unchanged into bile.
Advantages:
- Lower risk of allergic reactions compared to BSP.
- Does not cause tissue damage if extravasation occurs.
Limitations:
- Not useful in patients with jaundice, since bilirubin and the dye share the same excretory route.
- When bilirubin excretion is impaired, dye excretion will also be abnormally low, making the test results unreliable.
Clinical Relevance
Dye excretion tests like BSP and ICG were once valuable in detecting hepatic excretory defects before jaundice developed. However, because of their potential hazards and limited diagnostic specificity, they have been replaced by safer and more precise tests such as:
- Serum bilirubin estimation
- Liver enzyme assays (ALT, AST, ALP, GGT)
- Imaging techniques for biliary obstruction
Summary Table: Dye Tests of Liver Excretory Function
| Test | Dye Used | Purpose | Advantages | Limitations |
|---|---|---|---|---|
| Bromsulphthalein (BSP) Test | Bromsulphthalein | Assesses liver’s ability to excrete organic anions | Reflects hepatic function similar to bilirubin pathway | Allergic reactions, tissue necrosis, replaced by modern enzyme tests |
| Indocyanine Green (ICG) Test | Indocyanine Green | Measures hepatic excretory capacity with less toxicity | Safer and less allergenic than BSP | Unreliable in jaundiced patients due to bilirubin competition |
Detailed Notes:
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