Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (152 page)

   Obstruction of mesenteric lymphatics (e.g., by lymphoma, carcinoma, intestinal TB)

   Inadequate length of normal absorptive surface (e.g., surgical resection, fistula, shunt)

   Miscellaneous (e.g., “blind loops” of the intestine and diverticula, Z-E syndrome, agammaglobulinemia, endocrine and metabolic disorders)

   Infection (e.g., acute enteritis, tropical sprue, Whipple disease [
Tropheryma whippelii
]; in common variable hypogammaglobulinemia, 50–55% of patients have chronic diarrhea and malabsorption caused by a specific pathogen such as
G. lamblia
or overgrowth of bacteria in the small bowel.)

   Laboratory Findings

Core laboratory
: Serum cholesterol may be decreased. Decreased serum carotene, albumin, and iron; increased stool weight (>300 g/24 hours) and stool fat (>7 g/24 hours).

Hematology
: PT may be prolonged because of malabsorption of vitamin K. Increased ESR.

Anemia is caused by deficiency of iron, folic acid, vitamin B
₁₂
, or various combinations, depending on their decreased absorption.

Other
: Normal
D
-xylose test, low serum trypsinogen, and pancreatic calcification on radiograph of the abdomen establish diagnosis of chronic pancreatitis. If calcification is absent (as occurs in 70–80% of cases), abnormal contents of pancreatic secretion after secretin–cholecystokinin stimulation or abnormal bentiromide tests establish diagnosis of chronic pancreatitis.

   Recommended Tests

Fat absorption indices (steatorrhea)
: Direct qualitative stool examination. ≥2 random stool samples are collected on diet of >80 g of fat daily.

Serum trypsinogen
: <10 ng/mL in 75–85% of patients with severe chronic pancreatitis (those with steatorrhea) and 15–20% of those with mild to moderate disease; occasionally low in cancer of the pancreas; normal (10–75 ng/mL) in nonpancreatic causes of malabsorption.

Carotene tolerance test
: Measure serum carotene following daily oral loading of carotene for 3–7 days. Low values for serum carotene levels are usually associated with steatorrhea. Increase of serum carotene by >35 μg/dL indicates previously low dietary intake of carotene and/or fat. Patients with sprue in remission with normal fecal fat excretion may still show low carotene absorption.

Vitamin A tolerance test (for screening steatorrhea)
: Measure plasma vitamin A level 5 hours after ingestion. Normal rise is 9× fasting level. Flat curve in liver disease. Not useful after gastrectomy. With vitamin A as ester of long-chain fatty acid, flat curve occurs in both pancreatic disease and intestinal mucosal abnormalities; when water-soluble forms of vitamin A are used, the curve becomes normal in patients with pancreatic disease but remains flat in intestinal mucosal abnormalities. An abnormal result indicates a defect in small bowel mucosal absorption function (e.g., sprue, Whipple disease, regional enteritis, TB enteritis, collagen diseases involving the small bowel, extensive resection). Abnormal pancreatic function does not affect the test.

CARBOHYDRATE ABSORPTION INDICES

   Disaccharide malabsorption

   Causes

   Primary malabsorption (congenital or acquired) because of absence of specific disaccharidase in brush border of small intestine mucosa

   Isolated lactase deficiency (also called milk allergy, milk intolerance, congenital familial lactose intolerance, lactase deficiency) (is most common of these defects; occurs in approximately 10% of whites and 60% of blacks; infantile type shows diarrhea, vomiting, failure to thrive, malabsorption, and so on; often appears first in adults; become asymptomatic when lactase is removed from diet)

   Sucrose–isomaltose malabsorption (inherited recessive defect)