Indigo carmine is a synthetic dye known for its deep blue color. This organic compound is a water-soluble sodium salt. It has been utilized across various fields, serving as a biological stain, a food colorant, and a medical diagnostic agent.
Medical Diagnostic and Surgical Uses
Indigo carmine plays a significant role in medical diagnostics and surgical procedures, primarily as a visual enhancer. In gastrointestinal endoscopy, particularly chromoendoscopy, it is applied topically to highlight subtle changes in the mucosal surface. The dye seeps into tissue grooves and pits, accentuating contours and highlighting abnormal areas like neoplastic lesions or those affected by inflammatory bowel disease. This technique improves the detection rate of adenomas and aids in differentiating between neoplastic and non-neoplastic colorectal lesions by emphasizing specific “pit patterns” on the mucosal surface. It also helps in assessing early gastric cancer and can be applied circumferentially throughout the colon during screening, typically sprayed directly through the endoscope’s air/water channel or via a spray catheter.
In urological procedures, indigo carmine is administered intravenously to assess kidney function and ensure the integrity of the ureters. Once injected, the dye is rapidly filtered by the kidneys and excreted, coloring the urine blue as it passes through the urinary tract. This allows surgeons to visually identify ureteral orifices during cystoscopy and to detect potential injuries or leaks in the ureters during complex abdominal and pelvic surgeries. The dye’s appearance at the ureteral orifices, typically within 5 to 10 minutes following intravenous injection in patients with normal kidney function, confirms urinary flow and tract patency. This immediate visualization is particularly useful in procedures where accidental damage to the ureters is a concern.
How Indigo Carmine Works
Indigo carmine’s function in the body depends on its method of administration. When applied topically, as in chromoendoscopy, it acts as a non-absorbable contrast stain. The dye does not react chemically with or get absorbed by the mucosal tissue; instead, it physically pools in surface irregularities like grooves and crevices. This pooling effect enhances the topographical definition of the tissue, making subtle architectural changes more apparent.
When administered intravenously, indigo carmine is absorbed systemically and rapidly processed. Its mechanism of action relies on its swift and almost exclusive excretion by the kidneys. The dye maintains its blue color throughout its passage, allowing for direct visualization of the urinary system as it is filtered from the blood and concentrated in the urine.
The compound is a water-soluble sodium salt of indigotindisulfonate. This solubility enables its rapid filtration by the glomeruli in the kidneys. The dye essentially acts as a vital stain, highlighting anatomical structures through its physical presence rather than chemical reaction with cells. Its high water solubility and lack of significant metabolism ensure that it passes through the renal system efficiently, appearing in the urine quickly. This property makes it an effective indicator for assessing renal function and tracing the path of urine flow.
Safety Profile and Administration
Indigo carmine is typically administered via intravenous injection, with a common adult dosage of 5 mL for diagnostic purposes. It can also be applied topically as a spray during endoscopic procedures. While intramuscular injection is possible, larger quantities are needed, and the dye’s appearance in urine is delayed compared to intravenous administration.
Side effects from indigo carmine are rare, but some reactions have been noted. Patients may experience a mild, transient increase in blood pressure or a reflex slowing of the heart rate, particularly when under general or spinal anesthesia. Other reported effects include occasional bronchoconstriction, severe bradycardia, or hypotension.
Allergic reactions, though uncommon, can manifest as hives, difficulty breathing, or swelling of the face, lips, tongue, or throat. Caution is advised for patients with a history of allergic reactions to the dye, and it is contraindicated in those who have previously experienced one. Temporary discoloration of the urine to blue or green is an expected, harmless effect following intravenous administration.
It is generally not recommended during pregnancy, and caution is advised for nursing women due to limited data on excretion into breast milk. Patients with severe renal impairment should avoid its use as elimination can be significantly delayed. Indigo carmine received approval for medical use in the United States in July 2022, sold under the brand name Bludigo, and is also approved as a food additive (E132 in Europe, FD&C Blue No. 2 in the US).