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Laboratory Diagnosis

The diagnosis of tropical sprue requires the demonstration of symptomatic intestinal malabsorption of at least two unrelated substances such as fat, xylose and vitamin B12 and the exclusion of other diseases that can cause secondary malabsorption.

Biochemical tests used to differentiate the various types of malabsorption include measurement of the absorption of fat (fecal fat content, serum carotene level, iodine-131 labeled triolein absorption test) and carbohydrates (e.g., xylose, glucose). Three tests, the D-xylose excretion test, the vitamin A absorption test, and the 24 hour fecal fat measurement provide accurate diagnostic methods within the scope of most laboratories.

1. D-xylose excretion test: The urinary excretion of xylose is measured 5 hours after the ingestion of an oral dose of 25 gm of D-xylose. Normal subjects excrete about 4 to 6 gm in 5 hours. Excretion of less than 4 gm of D-xylose in 5 hours is considered abnormal.

2. Vitamin A absorption test: Following ingestion of 300,000 International Units of vitamin A (in the form of fish liver oil concentrate) the serum vitamin A level should exceed 85 mg at 5 hours and 125 mg at 7 hours.

3. Fecal fat: The measurement of 24 hour fecal fat is not difficult but imposes the problem of stool collection and measurement of fat intake. A 3 to 4 day stool collection can be used. The normal fecal fat is about 5 gm/day and the ratio of free fatty acid (FFA) to triglyceride should be 2:1. Most patients with sprue exceed 10-gm/day and show a ratio of FFA to triglyceride of 4:1.

Assay of the absorption of protein (glycine absorption, blood tryptophan products, etc.) and excretion of protein (Iodine-131 PVP or albumin labeled with Chromium-51 or Iodine-131) may be useful. Tests for the absorption of iron, vitamin B12 (Schilling test), and folic acid are also used.

Because of variations in gastric emptying time and glucose utilization, and the flat curves seen in normal subjects, the oral glucose tolerance test is nonspecific in the diagnosis of sprue, although it is still used in many laboratories. Intraduodenal administration of glucose overcomes some of these variables, but is too difficult for routine use.

There is usually a macrocytic anemia present in patients with tropical sprue which may be severe in advanced cases. The red cell count may be 2 to 4 million per mm³; the individual cell hemoglobin content may be high.

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