General Tests of Malabsorption

Comments

Qualitative stool fat

Quantitative stool fat

D-xylose test

Lactose tolerance test (LTT)

Hydrogen breath test

Sudan stain of stool; high sensitivity (90%) and specificity (90%) with fat malabsorption >10g/24º. Dependent on adequate fat intake (100g/day).

Gold std for fat malabsorption; requires 3-day stool collection and 100g/day fat diet before and during the collection. Normal stool fat: 7.0g/day.

Distinguishes mucosal malabsorption from that due to pancreatic disease; high false negative (30%) and false positive (20%) rates. Urine D-xylose (25g/500ml water). Blood levels at 1 and 2 hours improve sensitivity.

A 50g oral dose of lactose is given and BS obtained 30 and 120 min. post dose; a rise of >25mg/dl glucose over fasting is normal. LTT has been replaced by the hydrogen breath test which is simpler and more sensitive.

An oral dose of lactose (1g/kg BW) is given following basal breath hydrogen levels. A late peak (within 3-6hrs) of >20ppm of exhaled H₂ following lactose ingestion is suggestive of lactose malabsorption.

Tests Indicating Deficiency State

CBC, Ca ⁺⁺, Mg ⁺⁺, Fe, folate, vitamins B₁₂, A, D, E, albumin, B₂, PT, zinc

Deficiency can be caused by other conditions but often indicates malabsorption.

Tests for Pancreatic Function

Secretin stimulation test

Bentiromide test

Gold std for pancreatic function. Labor intensive, invasive, requires duodenal intubation and collection of pancreatic juice in response to IV secretin or a meal containing fat, CHO, protein (Lundh meal). A sensitive test of pancreatic fct.

N-benzyl-L-tyrosyl-p-aminobenzoic acid (NBT-PABA) is given orally, cleaved in the SB by chymotrypsin to release PABA which is absorbed, conjugated by the liver and excreted in urine. Urine PABA is measured. An indirect measure of pancreatic function and positive only with extensive disease.

Tests for Bacterial Overgrowth

Quantitative cultures of small intestinal aspirate

¹⁴C-D-xylose breath test

Hydrogen breath test

Gold std for SBBO. Greater than 10⁵ CFU/ml suggests bacterial overgrowth. Requires rapid plating and care to avoid contamination.

Diagnostic sensitivity, specificity comparable to that of quantitative culture for dx of SBBO.

Nonabsorbable sugar lactulose (1g/kg BW) or glucose is given orally. When bacterial overgrowth present, increased H⁺ excreted in breath. An early peak (within 2 hrs) of >20ppm exhaled H⁺ suggestive of bacterial overgrowth.

Tests for Mucosal Disease

Small bowel biopsy

Permeability studies

See Table 6

Test of mucosal integrity. Nonabsorbable markers, i.e. mannitol/lactulose or lactulose /⁵¹Cr-EDTA are given and urinary excretion measured.

Tests of Ileal Function

Schilling test

Bile acid breath test

A test of B₁₂ absorption. Performed as Part 1 then 2,3,4 as needed.

Part I: A saturating dose (1mg im) of vitamin B₁₂ is given followed by oral radioactive B₁₂ (0.5-2.0 mcg). Urine collected for 24 hrs; abnormal in all people with B₁₂ deficiency except those with dietary deficiency and food cobalamin malabsorption;

Part II: Test repeated with intrinsic factor (IF).

Part III: Test repeated with pancreatic enzymes

Part IV: Test repeated with antibiotics

¹⁴C-labeled cholylglycine is given orally. Normally this BA is absorbed intact in the ileum. In patients with impaired ileal function, the labeled BAs pass into colon where deconjugation and metabolism of glycine to ¹⁴CO₂ diffuses across tissue and is excreted in breath. Not specific; Abnormal in SBBO.

Tests for Protein Loss

⁵¹Cr-albumin test

Alpha 1-antitrypsin test (alpha 1-AT)

⁵¹Cr-labeled albumin is given IV and 24 stool excretion is measured. Clearance is calculated similar to creatinine.

Alpha 1-AT normally found in plasma but hardly in stool. Poorly absorbed once it enters the GI tract and resistant to proteolytic degradation; thus, can be used to measure stool protein loss. Clearance can be determined by measuring serum and stool alpha 1-AT.