Citrulline Test

Your small intestine is one of the hardest-working tissues in your body. It completely rebuilds its inner lining every few days, and that lining is where nearly every nutrient you absorb crosses from food into blood. When it starts failing, either from disease, surgery, or chronic inflammation, standard blood tests often look unremarkable for years.

This amino acid is the closest thing to a blood-based report card for that absorptive lining. It is made almost exclusively by the cells of your small intestine, and the amount circulating in your blood tracks how much of that tissue is still healthy and functional.

What Your Level Actually Reveals

The cells lining the small intestine, called enterocytes, produce citrulline (the full name of this amino acid) from glutamine and release it into the blood. The liver largely ignores it. The kidneys then convert it to arginine, the building block your body uses to make nitric oxide, a signaling molecule that relaxes blood vessels.

Because essentially all circulating citrulline comes from the small intestine, a low blood level is a strong signal that your enterocyte mass, the total healthy surface area inside your gut, has shrunk. A high or normal level suggests your absorptive tissue is intact.

Short Bowel Syndrome and Intestinal Failure

In adults who have lost a large portion of their small intestine through surgery or disease, blood citrulline reflects how much functional bowel remains. Healthy adult controls averaged around 40 micromoles per liter, while people with short bowel syndrome averaged about 20, roughly half.

A threshold of 20 micromoles per liter classified permanent intestinal failure with high sensitivity and specificity, meaning this test correctly identified most people with failure and correctly cleared most of those without it. In a larger cohort, a cutoff of 31 distinguished short bowel syndrome from healthy status, while 14 flagged the most severe cases requiring intravenous nutrition.

What this means for you: if you have had any substantial small-bowel surgery or suspect chronic absorption problems, a citrulline level well below 20 warrants a full intestinal failure workup, including imaging and a consultation with a gastroenterologist who specializes in nutrition.

Celiac Disease and Villous Atrophy

The small intestine absorbs nutrients through tiny finger-like projections called villi. In celiac disease and some other conditions, these villi flatten, a process called villous atrophy. Citrulline falls as the villi disappear, and it rises again as the lining heals on a gluten-free diet.

Research has proposed rough tiers to match the severity of damage: below 10 micromoles per liter suggests diffuse total villous atrophy, 10 to 20 suggests proximal total atrophy, 20 to 30 suggests partial atrophy, and 30 or above largely rules out significant villous disease. In studies, citrulline outperformed routine labs like albumin and hemoglobin for detecting the actual damage to the gut lining.

If you are managing celiac disease on a gluten-free diet, citrulline offers something that celiac antibody tests cannot: a functional measure of whether your gut has actually healed, rather than just whether your immune system has calmed down.

Critical Illness and Gut Barrier Failure

In sepsis, septic shock, and after major surgery, low citrulline signals that the gut is failing as a barrier and absorber. In a study of 68 ICU patients with sepsis, lower citrulline was associated with septic shock, gastrointestinal failure, and higher 28-day mortality. In people resuscitated after cardiac arrest, low citrulline at admission independently predicted worse neurologic outcome.

This is not a test most people order while feeling well. But if you or a family member is in critical care, a citrulline trend can help clinicians judge whether the gut is recovering or continuing to fail.

Cancer Therapy and Mucosal Injury

Chemotherapy and radiation commonly strip the gut lining, and citrulline tracks that damage. In advanced non-small cell lung cancer treated with the immune checkpoint drug nivolumab, a baseline citrulline of 20 micromoles per liter or higher predicted better treatment response, longer progression-free survival, and longer overall survival. Low citrulline was an independent adverse prognostic factor.

If you are preparing for or recovering from gut-damaging cancer therapy, tracking citrulline gives a window into whether your intestinal lining is holding up or needs support.

Cardiovascular Disease: Not a Predictor

A study of 1,036 hospitalized cardiovascular patients followed for five years found that decreasing citrulline levels did not predict long-term mortality overall. Slightly lower levels appeared in some unadjusted analyses of acute coronary deaths, but the signal disappeared after accounting for other risk factors.

This is not a heart disease risk marker. If that is your main concern, apolipoprotein B, lipoprotein(a), and high-sensitivity C-reactive protein (a blood test for inflammation) offer far more predictive power.

Reference Ranges

Research-based ranges come from adult clinical studies measuring fasting plasma citrulline by amino-acid analyzer or mass spectrometry. They are illustrative orientation, not universal clinical targets. Your lab may report different numbers depending on its assay, and children have different expected values than adults.

Compare your results within the same lab over time. Absolute cutoffs can shift with the assay method, and kidney function and recent food or supplementation can skew a single reading.

When Results Can Be Misleading

• Reduced kidney function: the kidneys clear citrulline to arginine, so poor kidney function can raise blood citrulline and blunt its value as a pure gut marker.
• Recent oral L-citrulline or arginine supplements: these directly raise the blood level without changing your actual enterocyte mass, so a supplement-inflated reading can look reassuringly normal when your gut is actually damaged. Stop supplements for at least 48 hours before testing.
• Non-fasting sample: clinical interpretation is based on fasting, post-absorptive values. Recent food, especially protein-rich meals, can shift the number.
• Recent intense exercise: a study of healthy young men found that high-intensity exercise transiently altered citrulline release from the gut. Avoid a hard workout in the 24 hours before your draw.

Tracking Your Trend

A single citrulline reading is useful, but the trend is where the real information lives. In premature infants with intestinal injury, citrulline fell over the first 48 hours and then either recovered or stayed low depending on the clinical course. In children weaning from long-term intravenous nutrition, a rising citrulline over time was a reliable sign that the intestine was adapting and building new absorptive tissue.

If you are managing celiac disease, inflammatory bowel disease, or recovery from bowel surgery, get a baseline, retest in 3 to 6 months after any significant change (starting a gluten-free diet, adjusting therapy, shifting your diet), and then at least annually. Serial tracking catches a declining trajectory before it crosses a disease threshold.

What to Do With an Abnormal Result

A single low citrulline should prompt a second confirmatory draw, fasting, off supplements, and with kidney function checked. If the low reading repeats, the next step depends on your situation:

• With GI symptoms or suspected celiac: order tissue transglutaminase IgA (a celiac antibody), total IgA, and consider a small-bowel biopsy with a gastroenterologist.
• With prior bowel surgery: work with a gastroenterologist who specializes in intestinal failure or nutrition to quantify absorptive function and parenteral nutrition needs.
• With cancer therapy: share the trend with your oncologist; persistent low citrulline may guide decisions about gut-supportive care.
• With signs of absorption trouble but no obvious cause: pair citrulline with fecal calprotectin (a gut inflammation marker), vitamin B12, folate, and iron studies to map where the system is failing.