Indican Test

Your gut bacteria leave a chemical fingerprint behind. When they break down tryptophan, an amino acid that arrives from your diet, they produce a compound your liver then converts and your kidneys excrete in urine. That compound is called indican, and the amount you put out reflects what your bacteria are doing inside you.

Most blood tests for digestive health look at inflammation or red flags for disease. Indican looks at something more upstream: whether the wrong kinds of bacteria are taking over the wrong parts of your intestines, and whether they are working overtime on the protein you eat. That is a different question, and it can be answered while standard panels still look fine.

What Indican Actually Measures

Indican is the common name for 3-indoxyl sulfate (3-IS), a small molecule made through a specific pathway. Tryptophan in your gut gets converted to indole by colon bacteria that carry a particular enzyme. Your liver then attaches a sulfate group, and your kidneys clear it into urine. The whole chain is one of three major routes your body uses to handle tryptophan, alongside the serotonin pathway and the kynurenine pathway.

Because the very first step depends on bacteria, urinary indican is treated as a quantitative readout of activity in this bacterial branch of tryptophan metabolism. It is not a diagnosis of any one condition. It is a window into one slice of what your microbiome is doing.

This is an exploratory marker rather than a guideline-driven clinical test. Standardized clinical cutpoints do not yet exist, and a single reading should not drive a medical decision on its own. That makes serial testing, alongside other gut and metabolic markers, the most useful way to interpret it.

What High Levels Can Signal

In people with mixed-type irritable bowel syndrome (IBS-M, the form where constipation and diarrhea alternate), urinary indican runs higher than in healthy controls. In a study of 36 IBS-M patients compared with 36 healthy adults, indican rose roughly 17% higher during diarrheal phases than during constipation phases (93.7 vs 80.2 mg per gram of creatinine), and the shift tracked with changes in breath testing that suggested altered gut bacteria.

In postmenopausal women with chronic functional constipation, urinary indican is also elevated compared with healthy women. The severity of abdominal symptoms moves in the same direction as the indican number and with breath-test evidence of bacterial overgrowth. The pattern fits a shift in tryptophan handling away from the serotonin route and toward the bacterial indole route.

People with Parkinson's disease show higher urinary indican than controls, regardless of whether they also have constipation. The signal is interpreted as gut bacterial imbalance (dysbiosis) that is increasingly recognized as part of Parkinson's biology, not just a side effect of slow gut transit.

What Low Levels Can Signal

Indican is not a straightforward higher-is-worse marker. Very low urinary indican has been linked to outcomes you would not expect from a simple toxin reading. In people who have just received a stem cell transplant, low urinary indican in the first weeks reflects a disrupted microbiome and is associated with a higher chance of developing graft-versus-host disease of the gut. In intensive care patients, low urinary indican at 72 hours after admission has been associated with fewer days alive and out of the ICU and with higher one-year mortality.

This is not a contradiction once you reframe what indican is. It is not a toxin level. It is a readout of bacterial activity in the colon. Activity that is too high points toward overgrowth or putrefaction of protein. Activity that is too low points toward a bacterial community that has been wiped out or reshaped by illness, antibiotics, or near-starvation. Both extremes carry meaning, and the middle range is the goal.

What Indican Does Not Tell You

Urinary indican does not reliably separate Crohn's disease from ulcerative colitis, and it does not predict how severe inflammatory bowel disease is. If you have known IBD, this test should not be used to grade your flare. It also does not diagnose specific gut infections, and it cannot tell you which species are over- or under-represented in your colon. For those questions, stool microbiome testing and breath testing for bacterial overgrowth are better tools.

Reference Ranges

There are no universally agreed clinical cutpoints for urinary indican. The values below come from research cohorts, are normalized to creatinine, and should be read as research-reported orientation rather than diagnostic thresholds. Your lab will likely report different numbers depending on assay and normalization.

Source: Chojnacki et al. 2024, IBS-M cohort. Compare your results within the same lab over time for the most meaningful trend. Different methods (gas chromatography, liquid chromatography mass spectrometry, ELISA) can produce different absolute numbers for the same sample.

Tracking Your Trend

A single indican reading is a snapshot of a system that fluctuates with what you ate yesterday, where you are in your IBS cycle, and what your bacteria did overnight. The most useful pattern is the trend across several measurements, especially before and after a deliberate change such as a new diet, a course of probiotics, or treatment for bacterial overgrowth.

A practical cadence is a baseline test, a follow-up at 8 to 12 weeks if you are making changes, and then at least once a year to watch for drift. If you have an active gut condition or you are working through a structured intervention, retesting every 3 to 6 months is reasonable while you are still optimizing.

When Results Can Be Misleading

• Recent high-protein meal: dietary tryptophan loads have been shown to raise urinary indican output, so a single result after a protein-heavy day or a tryptophan supplement may overstate your usual baseline.
• Recent antibiotics: a course of antibiotics can wipe out the bacteria that make indole, producing a transiently low result that reflects bacterial collapse rather than improved gut health.
• Kidney function: indican is cleared by the kidneys, so reduced kidney function can shift how much shows up in urine versus stays in blood. If you have known kidney disease, this test is harder to interpret on its own.
• Collection timing and stabilization: indican is typically measured in a morning fasting sample with an acid stabilizer added to the container. Off-protocol collection can degrade the sample and produce inaccurate numbers.

What to Do With an Abnormal Result

If your indican is elevated, the next step is not to label yourself with a diagnosis. It is to triangulate. Consider a breath test for small intestinal bacterial overgrowth (which gives a direct functional answer about hydrogen and methane production in the small bowel), a stool calprotectin to rule out active gut inflammation, and a stool microbiome panel if you want to see which communities are over- or underrepresented. A gastroenterologist or a clinician comfortable with functional gut workups is the right person to involve, especially if you have symptoms or a family history of inflammatory bowel disease.

If your indican is unusually low and you have recently been on antibiotics, in the hospital, or through a major gut event, the priority is rebuilding the bacterial community, not chasing the number down further. Retesting after a structured period of fiber, fermented foods, and time is more informative than treating the low value itself.