A random stool collection; sometimes a 72-hour stool collection
For a 72-hour stool collection, follow the instructions from the lab performing the test or your healthcare practitioner. 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 your digestive system is not working properly and/or that you have a condition affecting the digestion of food and absorption of nutrients (malabsorption).
The body digests foods in stages: 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 and cannot be efficiently absorbed. If a condition prevents the intestines from absorbing nutrients, then they are eliminated in the stool. In both cases – improper digestion or absorption – you can experience symptoms associated with malabsorption and, in severe cases, symptoms of malnutrition and vitamin deficiency. If the condition prevents you from digesting and absorbing fats from the diet, then excess fat is present in the stool and you may experience persistent diarrhea with stools that are loose and foul-smelling (steatorrhea).
Fat in stool may be detected with the qualitative fecal fat test, which generally determines the presence or absence of excess fat. This test 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. It should be performed by an experienced professional.
Quantitative measurements of fecal fat, though somewhat more accurate, are less commonly performed. However, they are considered the gold standard. 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 eliminated in stool per day (24 hours).
Another quantitative test is called the acid steatocrit, which provides a rapid but less exact measure of the amount of fat in the stool. It is a less complicated test that can be used on a random stool sample.
How is the sample collected for testing?
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.
Is any test preparation needed to ensure the quality of the sample?
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. You may also be asked to discontinue using exogenous pancreatic supplements if you are using any.
How is the test used?
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:
- Fecal immunochemical test and fecal occult blood test – to detect blood in the stool
- O&P (ova and parasite exam) – to detect parasites
- Fecal white blood cells – to detect white blood cells in the stool; excess white blood cells can be a sign of gut infection or inflammation.
- Fecal elastase – a protein-cleaving enzyme produced and secreted by the pancreas; it is resistant to degradation by other enzymes and so is excreted and can be measured in the stool. The amount of this enzyme is reduced in pancreatic insufficiency.
- Fecal fat, qualitative – usually a suspension of stool placed on a glass slide that is treated with a special stain and examined under the microscope to detect the presence of fat droplets
Blood tests may be ordered to further investigate malabsorption symptoms and may include:
- Complete blood count (CBC) – to evaluate red and white blood cells
- Comprehensive metabolic panel (CMP) – to evaluate liver function, proteins, and electrolytes
- Celiac disease antibody tests – to detect this condition
Tests for cystic fibrosis may be performed as part of the investigation. Pancreatic insufficiency in cystic fibrosis leads to maldigestion and malabsorption:
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 fecal fat is negative and your healthcare provider still suspects excess fecal fat, 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:
- For the quantitative test, you are required to include a moderately high amount of fat in your diet per day prior to and during sample collection so your ability to absorb fat "challenged."
- Fat is not released into the stool at a constant rate so results from collecting stool over 72 hours gives a more accurate picture of average absorption and elimination than a single sample.
When is it ordered?
- Fatty stools that are loose and foul-smelling (steatorrhea)
- Persistent diarrhea
- Abdominal pain, cramps, bloating, and gas
- Weight loss
- Failure to thrive (in children)
What does the test result mean?
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 you may have impaired digestion or malabsorption. (Quantitative results are typically reported as grams per day or g/d.)
Malabsorption is seen with a wide variety of diseases and conditions. Examples include:
- Diseases affecting the intestines such as:
- Pancreatic insufficiency caused by:
- Gallbladder diseases and conditions of the bile ducts
- Narrowing or blockage of the common bile duct, the main tube that carries bile from the liver and gallbladder to the intestines
Other laboratory tests used in conjunction with the fecal fat test are usually required to determine the underlying cause of fat malabsorption.
In a 72-hour fecal fat test, a low level of fecal fat generally indicates that you are digesting and absorbing fats normally and suggests that the symptoms being experienced are likely due to another cause.
If my doctor tells me to eat 100 grams of fat a day, can I vary it between 50 and 150 grams?
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.
Can I just do the screening fecal fat test and not the 72-hour test?
Can the fecal fat test be done in my doctor's office?
Is there anything else I should know?
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 eliminated 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-hour or 48-hour stool sample instead.