Davis's Lab & Diagnostic Tests
d-Xylose Tolerance Test
General
Synonym/Acronym:
N/A
Rationale
To assist in the differential diagnosis of small intestine malabsorption syndromes such as celiac, tropical sprue, and Crohn disease.
Patient Preparation
Instruct the patient to fast for at least 12 hr before the test. In addition, the patient should refrain from eating foods containing high amounts of pentose sugars, such as fruits, jams, jellies, and pastries, for 24 hr before the test (d-xylose is a pentose sugar). Some medications (e.g., acetylsalicylic acid, atropine, indomethacin, metformin, nalidixic acid, and neomycin) interfere with the test and should be withheld, by medical direction, for 24 hr before testing. Activity should be restricted for the duration of the test. Protocols may vary among facilities. The test should be started between 0600 and 0800, if possible. As appropriate, provide the required urine collection container and specimen collection instructions.
Normal Findings
Method: Spectrophotometry.
| Dose Given | Conventional Units | SI Units (Conventional Units × 0.0666) |
|---|---|---|
| Plasma | Plasma | |
| Infant dose 0.5 g/kg (max. 25 g) | Greater than 15 mg/dL after 2 hr | Greater than 1 mmol/L |
| Pediatric dose 0.5 g/kg (max. 25 g) | Greater than 20 mg/dL after 2 hr | Greater than 1.3 mmol/L |
| Adult dose | ||
| 25 g | Greater than 25 mg/dL after 2 hr | Greater than 1.7 mmol/L |
| 5 g (given if patient is known or expected to have severe symptoms) | Greater than 20 mg/dL after 2 hr | Greater than 1.3 mmol/L |
| Urine | |
| Children | Greater than 16%–40% of dose in 5-hr urine sample |
| Adults | Greater than 16% or greater than 4 g of dose in 5-hr urine sample |
| Older adults (65 yr and older) | Greater than 14% or greater than 3.5 g of dose in 5-hr urine sample |
Critical Findings and Potential Interventions
N/A
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