Fecal Fat

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 occult blood test (FOBT) – to detect blood in the stool
• O&P (Ova and Parasite) – to detect parasites
• Fecal white blood cells – to detect white blood cells in the stool
• 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:

• Trypsin and chymotrypsin
• Trypsinogen
• Sweat test
• CF mutation panel

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:

• For the quantitative test, the person being tested is required to ingest a moderately high amount of fat per day prior to and during sample collection so their absorption ability is being "challenged."
• Fat is not released into the stool at a constant rate so the combination and mixing of the stool from a 72-hour collection gives a more accurate picture of average absorption and elimination than a single sample.

A fecal fat test is primarily ordered when a person has signs and symptoms of malabsorption such as:

• Fatty stools that are loose and foul-smelling (steatorrhea)
• Persistent diarrhea
• Abdominal pain, cramps, bloating, and gas
• Weight loss
• Failure to thrive (in children)

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:

• Diseases affecting the intestines such as:
  • Infections, including parasitic, bacterial or viral
  • Celiac disease
  • Inflammatory bowel disease (Crohn disease, ulcerative colitis)
• Pancreatic insufficiency caused by:
  • Chronic pancreatitis
  • Pancreatic cancer
  • Cystic fibrosis (affects the function of the pancreas)
  • Shwachman-Diamond Syndrome
• Diseases and conditions of the bile ducts and/or gallbladder
  • Cancer
  • 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.

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.

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. 

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.