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A fecal fat test measures the amount of fat in your feces or stool. The concentration of fat in your stool can tell doctors how much fat your body absorbs during digestion. Changes in stool consistency and odor can indicate that your body isn’t absorbing as much as it should.
Fecal fat testing usually spans 24 hours, but it can sometimes last for three days. During the testing period, you’ll need to collect each stool sample with a special testing kit. Your local laboratory will provide you with the testing kit and specific instructions about usage. Some fecal test kits require you to collect the samples with plastic wrap. Others include special toilet paper or plastic cups.
Fecal fat testing might be performed if your doctor suspects that your digestive system isn’t functioning correctly. In a normal person, the absorption of fat is based on a variety of factors:
If any of these organs are not working properly, your body might not be able to absorb as much fat as you need to remain healthy and nourished. Decreased absorption of fat can be a sign of many different illnesses, including:
People who have decreased absorption of fat often notice changes in their bowel habits. This is because the fat that is not digested is excreted in the feces. You might notice your stool is looser, almost diarrhea-like in consistency. Stool with a high fat content also emits a fouler-than-normal odor and is likely to float.
Everyone who undergoes fecal fat testing is required to follow a high-fat diet for three days prior to the test. This allows for accurate measurement of the fat concentration in the stool. You will be asked to eat 100 grams of fat each day for three days before taking the fecal fat test. This is not as difficult as one might think: Two cups of whole milk, for example, contain 20 grams of fat, and 8 ounces of lean meat contain approximately 24 grams of fat.
Your doctor or dietitian may help you determine how to eat the required fat each day. You might be given a list of suggested foods to help you plan your meals. Whole milk, full-fat yogurt, and cheese can boost your fat intake. Beef, eggs, peanut butter, nuts, and baked goods are also good sources of fat. Reading the nutrition labels of the foods in your pantry gives you an idea of how much fat you consume in each meal or snack. If you tend to eat more than 100 grams of fat each day, the dietitian will teach you how to cut fat out of your diet and make healthier choices.
After following the high-fat diet for three days, you’ll return to a normal diet and begin the stool collection process. Have the collection kit ready at home for the first day of testing.
You need to collect stool each time you have a bowel movement during your testing period. You might be given a plastic "hat" to place over the toilet bowl, or be directed to loosely cover the bowl with plastic wrap. Urinate before you place the hat or plastic over the toilet bowl. Urine, water, and regular toilet paper can contaminate your sample and render the test results inaccurate.
After the collection apparatus is in place, collect your stool sample. You may be given additional tools, like a wooden or plastic scoop, to transfer the sample into a special container. Cover the container tightly and place either in the refrigerator, freezer, or in a separate cooler that is insulated and filled with ice. Repeat the process each time you have a bowel movement during the 24- or 72-hour testing period.
To carry out fecal fat testing in children, line the diaper of babies and toddlers with plastic wrap. Try to place the plastic in the back portion of the diaper to prevent the mixing of feces and urine.
When you have completed the fecal fat testing, write your (or child’s) name, date, and time on the container. Return the sample container to the lab.
The normal range for fecal fat testing is 7 grams over a 24-hour period. Normal results for a 72-hour test would be 21 grams. Your doctor will review results that are lower than normal. You might undergo further testing based on your medical history and symptoms, to determine why your fecal fat concentration is low.
Written by: Erica Roth
Medically reviewed on: Jul 09, 2015: Steven Kim, MD
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