Does Diarrhea Affect the Accuracy of Fecal Occult Blood Tests?

Does diarrhea with loose stools affect the accuracy of a fecal occult blood test?

In the past couple of months, I've been experiencing unexplained pain in my left abdomen, so I underwent a fecal occult blood test.
I did this twice; the first time was around mid-July, and I did a second test in early August because I was still concerned.
Both results came back normal.
However, I suddenly remembered that the fecal occult blood test usually requires a sample of solid stool, and for my second test, I provided a sample of loose stool due to diarrhea.
Could this affect the accuracy of the results? Although both fecal occult blood tests were normal, I still experience intermittent pain in my left abdomen.
What could be the cause of this? Thank you, doctor.

A-Ren, 20~29 year old female. Ask Date: 2022/09/12

Dr. Chen Shidian reply Gastroenterology and Hepatology

It should still maintain accuracy and can be further discussed with a physician.

Reply Date: 2022/09/12

More Info

Diarrhea can indeed affect the accuracy of fecal occult blood tests (FOBT), but it is essential to understand the context and the specifics of the testing method used. Fecal occult blood tests are designed to detect hidden blood in the stool, which can be an indicator of various gastrointestinal issues, including polyps, ulcers, or even colorectal cancer.
When it comes to the accuracy of these tests, the consistency and form of the stool sample play a significant role. Traditional FOBT methods often require solid stool samples because liquid or semi-liquid stools may dilute the blood present, leading to false-negative results. However, modern immunochemical tests (iFOBT) are generally more sensitive and can detect lower levels of blood, even in softer stool samples.
In your case, you mentioned that you had two FOBT tests, both of which returned normal results. The fact that you experienced diarrhea during the second test could raise concerns about the accuracy of that particular sample. However, if the test was an immunochemical test, it is less likely that the liquid stool would significantly affect the results. It is also worth noting that the presence of blood in the stool can sometimes be intermittent, meaning that even if you had blood in a previous sample, it may not be present in subsequent samples.

Regarding your ongoing left abdominal pain, it is essential to consider that this symptom can arise from various causes, including gastrointestinal disorders such as irritable bowel syndrome (IBS), which you mentioned, or even issues unrelated to the digestive system. IBS can cause abdominal pain and changes in bowel habits, including diarrhea or constipation, and is often characterized by a pattern of symptoms rather than a specific underlying disease.
Given your family history of colorectal cancer and your symptoms, it is advisable to continue monitoring your condition and discuss your concerns with a healthcare provider. They may recommend further evaluation, such as a colonoscopy, especially if your symptoms persist or worsen. A colonoscopy can provide a direct view of the colon and rectum and allow for biopsies if necessary.

In summary, while diarrhea can potentially affect the accuracy of fecal occult blood tests, the impact largely depends on the type of test performed. Since both of your tests returned normal results, it is reassuring, but ongoing symptoms warrant further investigation. Always consult with your healthcare provider for personalized advice and to determine the best course of action based on your specific situation.

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