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To detect and measure excess fat in the stool; to help diagnose conditions causing malabsorption
When you have symptoms of malabsorption, such as persistent diarrhea and fatty stools
A random stool collection; sometimes a 72-hour stool collection
For a 72-hour stool collection, follow your healthcare practitioner's instructions; this may include consuming 50-150 grams of fat a day in your diet for 2-3 days prior to and during the stool collection period.
This test measures the amount of fat in a stool sample. Excess fecal fat (termed steatorrhea) may be an indication that a person's digestive system is not working properly and/or that the individual has a condition affecting the digestion of food and absorption of nutrients (malabsorption).
The body digests foods in three stages: first proteins, fats, and carbohydrates are broken down, in the stomach by acid and enzymes and in the small intestines by enzymes produced by the pancreas and bile from the liver, into their component parts. They are then absorbed, primarily in the small intestines. Finally, the nutrients are transported throughout the body and used or stored.
If there are not enough pancreatic enzymes or bile available, then fat and other foods cannot be properly digested. If a condition prevents the intestines from absorbing nutrients, then they are "lost" by excretion in the stool. In both cases – improper digestion or absorption – the affected person can experience symptoms associated with malabsorption and, in severe cases, symptoms of malnutrition and vitamin deficiency. If the condition prevents the digestion and/or absorption of fats from the diet, then excess fat is present in the stool and the person may experience prolonged diarrhea with stomach pain, cramps, bloating, gas, and weight loss.
Fat in stool may be detected with the qualitative fecal fat test, which generally determines the presence or absence of excess fat. This is the simplest test for fecal fat and is performed by placing a suspension of treated or untreated stool onto a glass slide, adding a fat stain, and observing the number and size of fat globules that are present.
Quantitative measurements of fecal fat, though somewhat more precise, are less commonly performed. They require a timed stool collection and a dietary sheet to help calculate the total fat intake during the collection period, usually a 72-hour stool collection. Results are reported as the amount of fat excreted per 24 hours. A variation of the test is called the acid steatocrit, which provides a rapid but less exact measure of the amount of fat in the stool.
For a 72-hour stool collection, save all stool during the collection time period in the container(s) provided. It may be collected in a variety of ways, such as on plastic wrap, but should not be contaminated by urine, toilet water, or toilet paper. For a random sample, collect a single specimen in the same fashion.
For a 72-hour stool collection, follow your healthcare provider's instructions and dietary recommendations. This may include consuming a fat-controlled diet containing 50-150 grams of fat a day for 2-3 days prior to and during the stool collection period. You will also be asked to avoid certain oils and fat substitutes during the collection as these can invalidate the test result.
Fecal fat testing is typically ordered along with or as a follow up to other stool tests and blood tests to investigate the cause of chronic diarrhea and loose, fatty, foul-smelling stools (steatorrhea).
Stool tests may include:
Blood tests may be ordered to further investigate malabsorption symptoms and may include:
Tests for cystic fibrosis may be performed as part of the investigation:
If the qualitative fecal fat is negative, then a 72-hour quantitative fecal fat test may be ordered. This is a better evaluation of fat digestion and absorption. There are two reasons for this:
A fecal fat test is primarily ordered when a person has signs and symptoms of malabsorption such as:
These symptoms may or may not be accompanied by other indicators such as fatigue, anemia and/or specific nutritional deficiencies in, for example, iron or vitamin B12.
A qualitative fecal fat may be ordered as a screening test. If it is positive for excess fecal fat, then a quantitative test is generally not necessary.
If the qualitative test is negative and the healthcare provider still suspects excess fecal fat, then a quantitative 72-hour fecal fat may be ordered.
A positive qualitative fecal fat test or an increased amount of fat in a 72-hour quantitative fecal fat test indicates that fat is likely not being absorbed normally and that the person may have impaired digestion or malabsorption.
Malabsorption is seen with a wide variety of diseases and conditions. Some causes of malabsorption include:
Other laboratory tests used in conjunction with the fecal fat test are usually required to determine the underlying cause of fat malabsorption.
A negative qualitative fecal fat test does not necessarily rule out malabsorption so it may be followed up with a quantitative test.
In a 72-hour fecal fat test, a low level of fecal fat generally indicates that the person tested is digesting and absorbing fats normally and suggests that the symptoms being experienced are likely due to another cause.
You should try to follow your healthcare provider's recommendations as closely as possible. Eating a similar amount of fat each day will help ensure that the interpretation of your results is accurate. For more information on the fat content of various foods, visit the page on Food Exchange Lists by the National Heart, Lung and Blood Institute.
If the screening test is positive, then the 72-hour test is not generally necessary. However, if it is negative, then your healthcare provider may want the additional information that the 72-hour sample provides.
The qualitative fecal fat test may be done in a healthcare provider's office if it has the proper stain and equipment. The quantitative test requires specialized equipment; it needs to be performed by a laboratory and may need to be sent to a reference laboratory.
Laxatives, enemas, barium, mineral oil, fat-blocking supplements, psyllium fiber, and fat substitutes may affect test results.
Children cannot ingest as much daily fat as adults. Their test preparation will be adjusted and their 72-hour fecal fat test results will typically be reported as a percentage. This result is a "coefficient" that compares the amount of fat eaten to the amount excreted in order to evaluate the quantity of fat absorbed.
Although 72 hours is the typical sample collected for a quantitative fecal fat, a healthcare provider may sometimes ask for a 24- or 48-hour stool sample instead.
Sources Used in Current Review
Lehrer, J. (2014 August 19 Updated). Fecal fat. MedlinePlus Medical Encyclopedia [On-line information]. Available online at https://www.nlm.nih.gov/medlineplus/ency/article/003588.htm. Accessed on 10/08/15.
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Goebel, S. (2014 December 16 Updated). Malabsorption. Medscape Drugs & Diseases [On-line information]. Available online at http://emedicine.medscape.com/article/180785-overview. Accessed on 10/08/15.
Stefano Guandalini, S. et. al. (2015 July 17 Updated). Pediatric Malabsorption. Medscape Drugs & Diseases [On-line information]. Available online at http://emedicine.medscape.com/article/931041-overview. Accessed on 10/08/15.
Delgado, J. and Grenache, D. (2015 September Updated). Malabsorption. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/Malabsorption.html?client_ID=LTD. Accessed on 10/08/15.
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Sources Used in Previous Reviews
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Delgado, J. and Grenache, D. (Updated 2010 November). Malabsorption. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/Malabsorption.html?client_ID=LTD. Accessed June 2011.
Ruiz, A. (Revised 2008 January). Malabsorption. Merck Manual for Healthcare Professionals [On-line information]. Available online at http://www.merckmanuals.com/professional/sec02/ch017/ch017a.html?qt=malabsorption&alt=sh. Accessed June 2011.
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