False Negative Results in Fecal Occult Blood Tests - Gastroenterology and Hepatology

Negative occult blood issue

Recently, I had a fecal occult blood test that showed visible blood, but the results came back negative.
1.
Could this be due to hemorrhoids or anal fissures, and how can it be negative if blood is visible? I have also been experiencing frequent left abdominal pain, about 5 cm from the navel (it hurts more when fasting, but feels better after eating), and recurrent gastroenteritis.
I have undergone both colonoscopy and gastroscopy, which only revealed mild ulcers, and I noticed black spots at the corners of my mouth.
2.
Is there a need to perform a small bowel endoscopy? Is it common for polyps or tumors to develop in the small intestine? 3.
If a small bowel endoscopy is performed, can it visualize the entire segment?

K, 30~39 year old female. Ask Date: 2024/03/26

Dr. Lin Minghui reply Gastroenterology and Hepatology

(1) Regarding fecal occult blood sampling, the general purpose of the occult blood test is to detect blood that is not visible to the naked eye.
The most common causes of visible blood after a bowel movement are hemorrhoids and anal fissures.
If there is significant bleeding, it is advisable to follow up with outpatient examinations.

(2) Concerning left abdominal pain, if the pain is near the navel and occurs on an empty stomach, with changes after eating, it may indicate increased gastric acid.
It is recommended to pay attention to dietary habits.
Whether a small bowel endoscopy is necessary may depend on outpatient follow-up.
First, it is important to treat any ulcers found during an upper endoscopy.
While small bowel ulcers, polyps, and tumors can occur, they are less common.
(3) A small bowel endoscopy may not be able to visualize the entire length of the small intestine, as its length varies from person to person.
The small bowel endoscope can be introduced orally or anally, but it may not reach the entire small intestine.
Clinically, small bowel endoscopy is used for unexplained gastrointestinal bleeding, suspected small bowel bleeding, and inflammatory bowel diseases such as Crohn's disease.
Other options for small bowel examination include capsule endoscopy and various imaging studies that can assist in diagnosis.

Wishing you health and safety.

Reply Date: 2024/03/26

More Info

Understanding false negative results in fecal occult blood tests (FOBT) can be quite complex, especially when considering various underlying conditions that may affect the results. The FOBT is designed to detect hidden (occult) blood in the stool, which can be an early indicator of gastrointestinal issues, including colorectal cancer, polyps, or other bleeding disorders. However, there are several reasons why a test might yield a negative result even when blood is visibly present in the stool.

1. Differences in Blood Sources: The blood detected in stool can originate from various sources, such as hemorrhoids (痔瘡) or anal fissures (肛裂). These sources often produce bright red blood, which may not be detected by the FOBT, particularly if the bleeding is intermittent or minimal. The FOBT primarily detects hemoglobin, which is a component of red blood cells. If the blood is from a source that does not release enough hemoglobin into the stool, or if the blood is digested (as may occur with upper gastrointestinal bleeding), it may not be detected. Therefore, while hemorrhoids and anal fissures can cause visible blood, they may not always lead to a positive FOBT result.

2. Recent Symptoms and Gastrointestinal Issues: You mentioned experiencing left abdominal pain near the navel and a history of recurrent gastroenteritis. These symptoms could indicate underlying gastrointestinal issues that may not necessarily correlate with the FOBT results. Since you have undergone both colonoscopy and gastroscopy, and only mild ulcers were found, it is essential to consider other potential causes of your symptoms. The presence of black spots at the corners of your mouth could suggest nutritional deficiencies or other systemic issues, which should be evaluated further.

3. Need for Additional Testing: Given your symptoms and the negative FOBT result despite visible blood, it may be prudent to consider further diagnostic evaluations. A small bowel endoscopy (小腸鏡) could be beneficial, especially if there is a suspicion of small bowel pathology, such as polyps or tumors. While small bowel tumors are less common than colorectal tumors, they can occur, and certain conditions, such as familial adenomatous polyposis (FAP) or Crohn's disease, can increase the risk of small bowel lesions.

4. Scope of Small Bowel Endoscopy: Small bowel endoscopy can visualize a significant portion of the small intestine, but the extent of visualization depends on the specific technique used. Traditional enteroscopy may allow for the examination of the proximal small bowel, while capsule endoscopy can provide a more comprehensive view of the entire small intestine. Capsule endoscopy is particularly useful for detecting small lesions that may not be visible through other means.

In conclusion, while a negative FOBT result can be perplexing, it is essential to consider the source of bleeding, the nature of the symptoms, and the need for further diagnostic testing. Consulting with a gastroenterologist would be advisable to discuss your symptoms, the implications of the FOBT results, and the potential need for additional investigations such as small bowel endoscopy. This approach will help ensure that any underlying conditions are appropriately identified and managed.

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