Definition

An occult blood test is a diagnostic tool used to detect microscopic or hidden blood in a stool sample. Occult blood is not visible to the naked eye and is often present in very small amounts, detectable only through chemical testing.

The presence of occult blood in stool typically indicates bleeding within the digestive tract, which may stem from various conditions. This test can confirm the presence of hidden blood but cannot determine the source or cause of the bleeding. Therefore, additional tests are often necessary to identify the underlying issue.

There are three main types of occult blood tests:

• Guaiac-based Fecal Occult Blood Test (gFOBT)

This test uses guaiac chemicals to detect blood in stool samples collected over 2–3 bowel movements on separate days.

• Fecal Immunochemical Test (iFOBT/FIT)

This test employs antibodies to identify occult blood in the stool and is considered more sensitive than gFOBT.

• FIT-DNA

Combines FIT with DNA screening to detect genetic changes linked to colorectal cancer.

Indications

Doctors may recommend an occult blood test for individuals experiencing symptoms of gastrointestinal bleeding, such as:

• Bloody or black stools
• Persistent abdominal pain
• Discomfort around the anus
• Bleeding from the anus
• Fatigue

Gastrointestinal bleeding often results in stools appearing bloody or black. However, when blood loss is minimal, it may not alter the stool's appearance. An occult blood test can detect this hidden blood.

This test is also widely used for colorectal cancer screening, particularly in developed countries. Colorectal cancer is one of the most common cancers globally among men and women. Early detection is crucial, as identifying cancer in its initial stages can lead to timely treatment and significantly improve survival rates.

Healthcare providers strongly advocate for regular colorectal cancer screenings, especially for individuals aged 45–50 and those with an elevated risk. Routine screenings are generally advised until the age of 75. Consult your doctor to evaluate your risk of colorectal cancer and determine the appropriate screening schedule.

Contraindications

There are no specific contraindications or restrictions for stool-based occult blood tests. Additionally, no serious complications associated with this test have been reported.

Preparation Before the Test

While the fecal immunochemical test (FIT) does not require special preparation, the guaiac-based test (gFOBT) necessitates certain dietary and medication adjustments in the days leading up to the test. Patients may be advised to avoid:

• Certain vegetables, including broccoli and turnips
• Red meat, such as beef, lamb, and pork, as residual blood from the meat may interfere with test results
• Foods or supplements containing vitamin C
• Non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin, naproxen, and ibuprofen

Test Procedure

The procedure typically involves collecting a stool sample for laboratory analysis. To ensure the sample's accuracy, avoid contaminating the stool with urine or toilet water during collection. Place the sample in a clean, securely sealed container. Deliver the container to the laboratory for analysis.

Stool samples should not be collected under certain conditions, such as menstruation, active hemorrhoids, urinary tract infections, or when visible blood is present in the stool.

Laboratory analysis will determine whether occult blood is present, guiding further diagnostic or treatment steps as necessary.

Normal and Abnormal Values

Occult blood test results are considered normal when no microscopic blood is detected in the stool sample. A negative result suggests the absence of bleeding in the gastrointestinal tract. Conversely, a positive result indicates the presence of hidden blood in the stool, which suggests gastrointestinal bleeding and may point to various underlying conditions. 

Results and Recommendations (Further Testing)

Several medical conditions can cause the appearance of microscopic blood in stool samples, including:

• Hemorrhoids
• Peptic ulcers
• Colorectal cancer
• Iron deficiency anemia associated with suspected gastrointestinal bleeding

If the test result is positive, the doctor may recommend additional diagnostic tests, such as a colonoscopy. A colonoscopy involves inserting a thin tube equipped with a camera into the rectum to examine the colon, pinpoint the exact location of bleeding, and determine the underlying cause.

Consult the Right Doctor

Should your occult blood test yield positive results, consult a general practitioner or an internal medicine specialist to evaluate your condition further and discuss the next steps.

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