Gastroenterology training and education

Discuss the various tests of malabsorption?

Fat malabsorption

• Stool tests
  • 72 hour stool collection for fat estimation- this was the standard test for malabsorption for decades. However, it is poorly reproducible and unpleasant and thus its use is discouraged. Normal fat excretion is < 6gm/day. Steatorrhea is present, if fat excretion is more than 14 g/day. More than 6gm/day fat is pathologic; however fecal fat excretion can be increased up to 14g/day in diarrheal diseases even without fat malabsorption
  • Single stool analyses- such as faecal fat concentration (g faecal fat/100 g wet stool weight) and semi-quantitative methods such as acid steatocrit or Sudan III staining correlate moderately well with three day faecal fat quantification and offer an alternative method of assessing fat malabsorption but are not readily available in most centres. The stool steatocrit involves separating a faecal homogenate by centrifugation into a lipid, water, and solid phase. Faecal acidification much improves this method.
  • Faecal elastase- is the test most often used. It helps in differentiating between pancreatic and intestinal causes of steatorrhoea. See details on pancreatic function tests.
• Breath test
  • 14 C- triolein breath test- measures 14CO2 in breath after ingestion of a triglyceride that has been labeled with 14C. Fat malabsorption results in decreased pulmonary excretion of 14CO2. It has been reported to have 100% sensitivity and 96% specificity however limitations such as the confounding effects of co-morbidities altering fat metabolism or respiratory function have also been documented. The test assesses both lipolysis and absorption. The test is inappropriate in patients with diabetes, liver disease, or obesity.

Carbohydrate (CH) malabsorption

• D-xylose test- Limited clinical value today. The patient ingests a 25 g dose of D-xylose, and urine is collected for the next five hours. A venous blood sample is also collected after one hour. Normal excretion of D-xylose is 6.0 +/- 1.5 g. Excretion of lesser amounts of D-xylose or a serum D-xylose concentration less than 20 mg/dL suggests abnormal absorption.
  An abnormal D-xylose test suggests mucosal disease. Absorption is usually normal in pancreatic insufficiency. Renal failure and impaired gastric emptying can lead to false positive results.
• Hydrogen breath (lactose tolerance) tests- H2 is produced by bacterial action on unabsorbed CH in the colon. Thus in CH malabsorption, a breath H2 peak will occur 90 minutes after its ingestion when it first arrives in the colon and is broken down by the bacteria. False positive results are possible particularly with small bowel bacterial overgrowth. It can be used to diagnose specific forms of carbohydrate malabsorption (e.g., lactose, fructose, sucrose isomaltase and others).
  Concurrent antibiotic administration will alter the results of H2 breadth test as it relies on bacterial fermentation of carbohydrate.
• Stool pH less than 5.5 is a qualitative indicator of CH malabsorption. (SCFA and lactic acids are the products of CH metabolism)

Protein malabsorption

• Not generally tested. The tests are technically difficult and intestinal protein loss is generally due to either bacterial overgrowth or protein losing gastroenteropathy. Enteral protein loss is demonstrable by measurement of the alpha-1 antitrypsin clearance. In massive enteral protein loss, the exact site of protein leakage may be localized by the infusion of 99mTc-albumin and gamma camera scintigraphy.
• Alpha-I antitrypsin (AIAT) is excreted intact in stool because it is resistant to degradation in the gut lumen. Further it is not secreted or absorbed. Faecal excretion of AIAT is thus used as an indirect measure of enteric albumin loss.
  Limitations:
  • There is poor correlation between random stool concentrations of AIAT and clearance measurements. A 24 hour stool specimen is thus needed.
  • Diarrhoea can increase AIAT clearance. A1AT clearance greater than 24ml/day (4.8mg/g of stool) in patients without diarrhoea and greater than 56ml/day (11.2mg/g of stool) in pts with diarrhoea is considered abnormal.
  • There is an inverse correlation between AIAT plasma clearance and the serum albumin concentration; as serum albumin levels fall below 3 g/dl, the clearance of AIAT exceeds 180 ml/day.
  • AIAT is degraded at a gastric pH below 3 and thus cannot be used to measure gastric protein loss. PPI can be used to avoid this.
  • Intestinal bleeding can also significantly increase the clearance rates. Thus positive OBT make the reading unreliable.
• Technetium 99m-labelled human serum albumin (Tc HSA) scintigraphy is a less sensitive test than AIAT to detect and monitor enteric protein loss. However, it can localise the specific site of gastric or enteric protein loss. It can also be used like AIAT to monitor response to therapy.

Bile Acid Malabsorption
This can cause symptoms of chronic diarrhoea. Three types of bile acid malabsorption are recognised:
Type 1: following ileal disease or resection or bypass surgery.
Type 2: primary idiopathic malabsorption.
Type 3: associated with cholecystectomy, peptic ulcer surgery, chronic pancreatitis, coeliac disease and diabetes mellitus.
Bile acids are cathartic to colonic mucosa, and impair sodium and water absorption. Selenium-75 labeled Homotaurocholic Acid Test (⁷⁵SeHCAT) involves the administration of a selenium75 labeled synthetic bile acid (homotaurocholic acid) orally, followed by measurement of retention of the bile acid by whole body scan or gamma camera at seven days (abnormal is less than 5 percent, normal is >12%, equivocal is between 5 and 12%).