The "Xylose test" to assess gastrointestinal absorption in the elderly: a pharmacokinetic evaluation of the literature.

A pharmacokinetic evaluation has been made of the literature that has examined xylose absorption as a function of age. Urinary xylose recovery during a 5-hour collection declines with age after oral and intravenous dosing. Calculated values for xylose renal clearance and elimination rate constant display a similar dependence upon age. The ratio of urinary recoveries after oral compared with intravenous dosing remains constant up to age 65 years but decreases after 70 years. Age appears to explain most of the variation associated with the above parameters. This analysis supports the suggestion made previously by several investigators that the xylose test as currently used to assess absorption in the elderly has not been properly interpreted.