What Factors Affect PET Scan Results?

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  • Written By: Jennifer Voight
  • Edited By: Melissa Wiley
  • Last Modified Date: 14 August 2019
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A positive emission tomography (PET) scan is a diagnostic tool used to detect metabolic changes and glucose uptake in the body’s cells. As it measures chemical processes, certain activities, medications, and medical conditions can affect PET scan results, causing either a false negative or false positive reading. Before patients undergo a PET scan, doctors will advise them to avoid eating or drinking anything other than water, avoid exercise, and discuss medications and health issues to increase accuracy of results.

PET scans are used to detect cancer, heart disease, and brain abnormalities. As PET scan results indicate tissue changes, they can detect disease earlier than either CT scans or MRI. Cancerous tumor cells as well as certain other diseased cells tend to be big consumers of glucose. This causes them to be highly visible during the procedure.

Other tissue changes can cause increased glucose consumption in tissue cells. Exercise within 48 hours of the procedure can show up as a false positive, because exercise causes glucose metabolism to increase temporarily. For that reason, even light exercise should be avoided. Even a massage can cause metabolic changes in cells and should be avoided for about 24 hours prior to the procedure.


The consumption of sugar and caffeine, even the trace amounts found in decaffeinated products and chocolate, can skew PET scan results. Prior to a procedure, a patient should avoid tobacco, caffeine, and limit refined carbohydrates for 24 hours before the procedure. Doctors will tell patients not to eat or drink anything except water for four to six hours before the scan.

Some medications and medical conditions that cause inflammation or infection can cause false positive PET scan results. Doctors may advise refraining from medications, herbal supplements, or vitamins before the scan. Some heart medications and the asthma medication theophylline can interfere with results. Cea/Vac, a vaccine for colorectal cancer, causes false positive readings and should not be administered for several days prior to the procedure.

Some types of tumors and cancers are characterized by lower metabolic activity than others. This makes PET scans less able to see certain types of cancers. Tumors that are less than 1 centimeter in diameter may not show up on a PET scan for the same reason.

The most important way to ensure the most accurate PET scan results is full disclosure of medical history and all medications and supplements currently being taken. Full compliance with a doctor’s recommendations will increase the chances of accurate results. Still, a PET scan is a diagnostic tool to be used in conjunction with other diagnostic information. Like any other diagnostic tool, PET scans are not foolproof and need to be interpreted in conjunction with all other patient medical information.


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