Xylose Absorption Test
Timed blood samples drawn from a vein in your arm and a timed urine collection
Fast for 8 hours and avoid foods high in pentose, such as jams, fruits, and pastries, for 24 hours prior to the test. Ask your doctor if there are any changes to your medications that you should make.
D-Xylose is a simple sugar (carbohydrate) that is usually easily absorbed by the intestines. This test determines how well someone absorbs D-xylose, in order to evaluate the person's ability to absorb carbohydrates in general. This is done by measuring the level of D-xylose in the blood and urine after a standard amount is ingested
The body digests foods in several stages. Proteins, fats, and carbohydrates are partly broken down in the stomach by acids and enzymes; they are then broken down further in the small intestines by pancreatic enzymes and bile from the liver. The products of digestion can then be absorbed, primarily in the small intestine. Finally the nutrients are transported throughout the body and used or stored.
If there is not enough bile or pancreatic enzymes available, then carbohydrates and other foods cannot be properly digested. If a condition prevents the intestines from absorbing the nutrients, then they are "lost" by excretion in the stool. In both cases—improper digestion or absorption—the affected person can experience symptoms associated with malabsorption and, in severe cases, symptoms of malnutrition and vitamin deficiency.
The xylose absorption test is used to help determine whether a person is absorbing carbohydrates appropriately. However, this test is not routinely requested and not widely available in the UK. Several major health organisations, including the British Society of Gastroenterology, the World Gastroenterology Organisation, and the American Gastroenterology Association do not include this test in their guidelines for evaluating possible malabsorption.
How is the sample collected for testing?
Prior to starting a xylose absorption test, the patient should have fasted for 8 hrs (a 4 hour fast may be sufficient for a child). The patient should also empty their bladder immediately before beginning the test (this urine is not saved).
A fasting blood sample is obtained by inserting a needle into a vein in the arm. The person is then given a standard dose of D-xylose dissolved in water to drink. Typically the dose is 25 grams of D-xylose, but if the person is unable to tolerate this amount, then a 5 gram dose may be used. For children, the dose is adjusted for weight.
The person is then asked to rest. Another blood sample is collected at 2 hours (1 hour for a child). All urine is collected for 5 hours, starting from the time the dose is given. The fasting blood, timed blood, and 5-hour urine samples are then tested for D-xylose.
Is any test preparation needed to ensure the quality of the sample?
Fast for 8 hours and avoid foods high in pentose, such as jams, fruits, and pastries, for 24 hours prior to the test. Ask your doctor if there are any necessary medication changes.
How is it used?
The xylose absorption test may be requested to help determine whether a person is absorbing carbohydrates normally and to distinguish between malabsorption disorders caused by insufficient pancreatic enzymes or bile and those due to intestinal dysfunction.
This is not a routinely performed test. Its use and availability has declined over time. Major health organisations, including the British Society of Gastroenterology, the World Gastroenterology Organization, and the American Gastroenterology Association do not include this test in their guidelines for evaluating possible malabsorption.
When is it requested?
- Fatty stools that are loose and foul-smelling (steatorrhoea)
- Persistent diarrhoea
- Abdominal pain, cramps, bloating, and gas
- Weight loss
- Failure to thrive (in children)
Testing may be requested when a doctor wants to determine whether the intestines are absorbing carbohydrates normally and/or wants to distinguish between different possible causes of malabsorption.
What does the test result mean?
With the D-xylose absorption testing procedure, high blood and urine levels of D-xylose, after drinking the D-xylose solution, are normal. They indicate good D-xylose absorption by the intestines. This suggests that the tested person's symptoms are likely due to another cause, such as pancreatic or bile insufficiency.
Low levels of blood and urine D-xylose indicate poor absorption. A variety of conditions that affect digestion and absorption may cause decreased D-xylose levels. These may include bacterial overgrowth in the intestines, parasitic infections, a shortened bowel (such as from surgery) and coeliac disease.
Depending upon a person's clinical situation, an abnormal D-xylose test may be followed by additional blood or stool tests to try to isolate the cause and/or by a small-bowel biopsy to look at intestinal cells.
Is there anything else I should know?
Several drugs can affect test results, including aspirin, digitalis, MAO inhibitors, metformin, nalidixic acid, opium alkaloids, atropine, and indomethacin.
Dehydration, exercise, and the rate that the stomach empties may affect test results.
Some people may experience nausea, vomiting, or diarrhoea from the D-xylose dose.
Can this test be performed in my doctor's surgery?
Is it really necessary to save all of my urine over 5 hours?
Yes. In order to accurately interpret absorption and excretion during this time period, you should save all of your urine.
Why is the fasting blood sample collected?
This measurement of D-xylose in a fasting sample is used as a baseline. It should be negative or very low prior to receiving the D-xylose dose.
Is there a reason to repeat a xylose absorption test?
Usually only if the doctor feels that the first set of test results was compromised in some way, such as due to an interfering medication or because the person tested was unable to tolerate and retain the normal D-xylose dose (nausea and vomiting).
Why would malabsorption cause vitamin deficiency?
Because the vitamins cannot be absorbed normally and because fat-soluble vitamins such as A, E, K, and D can be "trapped" in and excreted with fat in fatty stools.