Malabsorption tests are done to determine if a patient has dietary malabsorption or maldigestion and to help differentiate between these two conditions. Malabsorption occurs when the gastrointestinal (GI) tract cannot take up a dietary compound. This is caused by the loss of function of the cells responsible for absorption. Maldigestion occurs when an important digestive enzyme or tissue is lacking or not functioning correctly. This may be caused by genetic disorders, injury to the tissue that provides the enzyme (i.e., the pancreas ), alterations in pH that make the enzymes inactive, or to surgery. In general, clinicians speak of both disorders as malabsorption disorders since they both result in a lack of absorption of nutrients.
Malabsorption tests are generally used to determine why someone is malnourished or is experiencing gastrointestinal upset. Some malabsorption tests are used as a last resort because the testing procedures are complicated. The physician first needs to rule out other disorders such as ulcers in the stomach and intestine. In the population, the elderly are at the greatest risk for developing malabsorption disorders. Before ordering malabsorption tests, physicians may do a general screen for malnutrition. This can include tests for proteins that reflect nutritional status such as serum albumin and prealbumin (transthyretin); tests for serum calcium, vitamin B12, folate, iron, and vitamin D to detect a deficiency of vitamins or minerals; and a peripheral blood smear to detect anemia, which may have a related cause.
The absorptive capacity of the gastrointestinal tract is staggering. In general, we absorb hundreds of grams of carbohydrates, over 100 grams of fat and 50-100 grams of amino acids per 24-hour period. This is accomplished by the mucosal cells lining the intestine. These surfaces contain many villi, small projections that increase the surface area of the intestinal wall. It is estimated the average adult human intestine has the absorptive surface area of a tennis court. Different parts of the GI tract have different functions and nutrients are broken down and absorbed in different parts.
Carbohydrate digestion begins in the mouth with salivary amylase and continues in the stomach via the action of the stomach and low pH. In the small intestine, pancreatic amylase and intestinal enzymes such as lactase complete carbohydrate hydrolysis, forming simple sugars that are absorbed. Any undigestible carbohydrate (fiber) is excreted in the feces. Fat digestion and absorption is very efficient, with very little fat found in the feces. Pancreatic and gastric lipases are the enzymes most responsible for the breakdown of triacylglycerides (triglycerides) into small glycerides and free fatty acids. The action of lipase requires bile salts and bile acids that are also needed to emulsify the fats. The free fatty acids and small glycerides produced by the hydrolysis of triglycerides are absorbed in the intestine and converted by the mucosal cells into chlyomicrons. Protein digestion begins in the stomach with the action of hydrochloric acid and pepsin, and continues in the intestine via the activity of pancreatic and intestinal proteases such as trypsin, chymotrypsin, and carboxypeptidase. The dipeptides and amino acids produced from protein hydrolysis are absorbed via complex mechanisms by the intestinal epithelial cells.
Malabsorption of nutrients can cause painful GI symptoms and over time cause malnutrition. Patients lose vitamins and minerals along with basic nutrients. Some malabsorption syndromes can cause dehydration since they produce diarrhea. Diagnosing the cause of malabsorption is difficult, and doctors will try many different testing approaches. Malabsorption can be caused by many things including:
- Pancreatic insufficiency, caused by inflammation of the pancreas (pancreatitis ), obstruction of the pancreatic duct, pancreatic cancer, inherited deficiency of pancreatic enzymes.
- Defective stimulation of the stomach due to illness or surgery. Muscles in the stomach need to contract to mix up the food with digestive enzymes and acid.
- Elevated pH of stomach acid (hypoacidity or achlorhydria).
- Lack of bile acids due to obstruction (gallstones or tumors) or due to liver disease.
- Bacterial overgrowth in the lower intestine.
- Food allergies such as celiac disease.
- Inflammation of the intestines or colon, such as colitis, Crohn's disease, inflammatory bowel syndrome.
- Parasitic infections, such as Whipple's disease or tropical sprue.
- Lack of enzymes in the intestine. For example, lactase deficiency causes lactose intolerance, a very common cause of malabsorption in adults.
- Surgical removal of parts of the intestine due to disease, or surgery to decrease the size of the stomach to promote weight loss.
- Diseases such as diabetes, AIDS, cystic fibrosis, thyroid disease, and alcoholism.
Symptoms of malabsorption are varied because the disorder effects so many systems. General symptoms may include loss of appetite (anorexia), weight loss, fatigue, shortness of breath, dehydration, low blood pressure, and swelling (edema ). Nutritional disorders may cause anemia (lack of iron, folate, and vitamin B12), bleeding tendency (lack of vitamin K ), or bone disease (lack of vitamin D). Gastrointestinal symptoms include flatulence, stomach distention, borborygmi (rumbling in the bowels), discomfort, diarrhea, steatorrhea (excessive fat in stool), and frequent bowel movements.
Most malabsorption tests require a blood sample collected by venipuncture. The nurse or phlebotomist must follow universal precautions for the prevention of transmission of bloodborne pathogens. Most of the tests done to measure malabsorption are relatively safe but do require some effort on the patient's part. Many require an overnight fast. Some patients who need malabsorption tests may be malnurished or dehydrated. Clinicians should watch for low blood pressure, weakness, thirst, concentrated urine (dehydration), and dizziness. Some tests require ingestion of highly concentrated nutrients that may be difficult for the patient to digest. Asking patients to collect urine or feces over a long time period can cause problems with compliance. It is important to make sure the patient understands the test and why he or she must comply. Clinical laboratories will reject any samples that appear to have been collected or stored incorrectly.
Breath hydrogen test
The breath hydrogen test is used to measure two things, carbohydrate malabsorption such as lactose intolerance and bacterial overgrowth. Hydrogen is produced by bacterial fermentation of unabsorbed carbohydrates in the intestines. Bacterial overgrowth can occur in this situation because there is a large food supply. The hydrogen produced goes into the blood stream and is excreted through the lungs. The test is done using a gas chromatograph, an apparatus that can separate compounds from one another based on their chemical composition. The patient is asked to fast overnight, and his or her breath is collected in a plastic syringe at the start of the test. The patient is then given something to eat depending on what is being evaluated. If the doctor suspects that the patient has trouble absorbing carbohydrates, then he or she may be given rice, glucose or fructose. If lactose intolerance is suspected, the patient is given a food containing lactose such as milk. For general bacterial overgrowth tests, the patient will be given glucose. The patient's breath will be collected in a plastic syringe every thirty minutes for the next two to five hours, depending on the test. The syringe will be capped and sent to the laboratory for analysis. The test is simple and non-invasive and while not diagnostic, it gives the doctor an idea of what may be wrong.
D-xylose absorption test
D-xylose is a sugar that is not normally found in the blood. It can be easily absorbed by healthy intestinal cells without the aid of pancreatic enzymes, and is poorly metabolized so that at least 50% of the dose is excreted in the urine within 24 hours. This test is a good general screen for malfunction of absorption, and helps to differentiate intestinal malabsorption syndromes (reduced D-xylose absorption) from pancreatitis (normal D-xylose absorption). Adults are given an oral dose (usually 25 grams) of D-xylose. A five-hour timed urine sample is collected, and a blood sample is collected two hours after the dose is given. Children are given a 5 gram dose of D-xylose, and a blood sample is collected one hour after the dose is given. Adults should excrete at least 25% of the dose in the five-hour urine sample, and have a two-hour blood level of at least 25 mg/dL. Children should have a one-hour blood level of at least 20 mg/dL. The D-xylose test will be normal if the patient has normal absorptive capacity in the intestine, or if the patient has malabsorption that is caused by a pancreatic problem. It will be low if the patient has celiac disease, tropical sprue, Crohn's disease, advanced AIDs, or pellegra (niacin deficiency).
Tests for celiac disease
Celiac disease is a disorder characterized by antibodies to gluten, a protein found in wheat. The disease produces lesions in the intestine and decreases the tissue's ability to absorb many different nutrients. Patients have diarrhea and lose weight over time. The lesions will improve when foods containing gluten are removed from the diet. Tests for this disease involve drawing the patient's blood and testing for the presence of three antibodies, antigliadin, antiendomysium, and antireticulin antibodies. Patients with celiac disease are followed closely by their doctors, even after dietary changes, because they are more prone to developing intestinal cancers and intestinal ulcerations.
Stool fat testing
Stool fat testing, measuring fats in the feces, is a sensitive way to determine if the patient has fat malabsorption but the test does not differentiate between pancreatic and intestinal causes. Fat is normally absorbed very efficiently by the intestinal cells. High levels of fat in the feces causes steatorrhea, a type of feces that appears pale in color and greasy. Before the test, the patient is put on a high fat diet, consuming between 50-150 g/day of fat for three days. The patient must collect their feces over the next 72 hours using a 1-gallon paint can that can be well sealed. The fecal sample must be refrigerated to prevent any bacterial action. Fecal fat analysis is performed by first weighing the sample and then extracting the lipids with an organic solvent. The extraction solvent is evaporated and the dry weight of the fat that remains is measured. Normal absorption of fat is indicated by a fecal fat level of less than or equal to 7 grams per day.
A more simple but less accurate way to measure fat absorption is to count the fat droplets in a well mixed sample of the stool specimen using a microscope and a neutral fat stain. Another simplified screening test is the fat tolerance test called the butterfat or the fatty meal test. The patient is asked to fast overnight and is given 1 gram of fat per kg of body weight. This is a substantial amount of fat and usually is given as 1 gram of butter per kg spread on a piece of toast or as 6 ounces of corn oil. Blood is drawn before the dose and again three and six hours afterwards. The fasting, three-hour and six-hour plasma samples are analyzed for triglyceride concentration. Normal absorption is indicated by at least a 50% increase in triglycerides over the fasting level.
The patient should be advised about the test he or she is taking and what the testing procedure involves. It is important with all tests, except the celiac antibody tests, that the patient fasts overnight. This allows the clinician to determine a "baseline" or starting value. Patients who are required to collect a 72-hour fecal sample must seal the sample well and keep it refrigerated to prevent any degradation.
Some patients may feel sick after the procedures since they are being exposed to compounds that they may have trouble absorbing. Nurses should be careful to discuss any side effects with the patient beforehand, and the patient should be given the smallest amount of substance possible to avoid problems. In addition, patients may be malnourished and need something to eat and drink once the procedure is over.
The hydrogen breath test may not be accurate if the patient is a smoker, has pulmonary disease, is not fasting or is hyperventilating. Patients being measured for bacterial overgrowth must not only fast overnight, but avoid fiber-rich foods for three days before the test. Patients taking the fecal fat test must remain on a high fat diet before and during the test.
Falsely low results with the D-xylose test will be seen if the pateint has been vomiting, has gastric stasis, fluid build up (ascites), fluid retention (edema) or bacterial overgrowth. There is a decrease in urinary excretion of D-xylose with aspirin, colchicine, digitalis, MAO inhibitors, food consumption, neomycin and opiates. In addition, excretion is lower in those with impaired renal function and in elderly patients.
D-xylose absorption should be greater than 1.2 g/5 hours with a 5 g dose of D-xylose and 4.0 g/5 hours in an adult given a 25 g dose of D-xylose. Fecal fat should be less than 7 g/day in adults and less than 2 g/day in a child. Hydrogen breath test results are based on the baseline value, which in most people should be extremely low. The results are measured in parts per million and the actual numbers depend on the sugar or carbohydrate given. Abnormal results with the celiac tests are any antibodies measured since healthy people do not generate antibodies to gluten protein.
Health care team roles
Malabsorption tests are ordered by a physician. The malabsorption tests call for a skilled team of nurses and dieticians who administer the tests. Clinical laboratory scientists/medical technologists perform malabsorption tests, and the physician interprets the result.
Absorption— Process of taking up digested food substances into the cells and blood stream of the body.
Carbohydrate— A macromolecule (large molecule) that is made of sugars or starch.
Diarrhea— Excessive production of feces. This is usually due to a large amount of water in the stool.
Fats— A macromolecule that is not soluble in water. Fats are also called lipids.
Gastrointestinal— Having to do with the stomach and intestines.
Gluten— One of the proteins found in wheat products.
Lactase— An enzyme that breaks down lactose.
Lipase— An enzyme that breaks down fats.
Mucosa— The cells lining the digestive tract. These cells secrete mucus to protect themselves from digestion.
Protease— An enzyme that breaks down proteins.
Protein— A macromolecule that is made of amino acids.
Steatorrhea— Feces that contain a high level of fat due to poor fat absorption in the GI tract.
Villi— Projections of tissue into the intestine that increase the surface area.
Burtis, Carl A. and Edward R. Ashwood. Tietz Textbook of Clinical Chemistry. Philadelphia: W.B. Saunders Company, 1999.
Kaplan, Lawrence A. and Amadeo J. Pesce. Clinical Chemistry Theory, Analysis and Correlation. St. Louis: Mosby Publishing, 1996.