From the Guidelines
Blood mixed into the stool is more likely to indicate a proximal source of bleeding, whereas blood on the surface of the stool is more likely to indicate a distal source of bleeding, such as an anorectal source.
Key Differences
- Blood mixed into the stool suggests a more proximal source of bleeding, which may require further investigation, such as colonoscopy or CT angiography (CTA) 1.
- Blood on the surface of the stool, particularly bright red blood, is more likely to indicate a distal source of bleeding, such as hemorrhoids or an anal fissure 2.
Clinical Implications
- Patients with bright red rectal bleeding should undergo direct anorectal inspection and may require further evaluation with anoscopy and flexible sigmoidoscopy 2.
- Patients with hemodynamic instability and suspected lower gastrointestinal bleeding (LGIB) should undergo CTA as the first-line investigation 1.
- The presence of blood in the stool should not be attributed to hemorrhoids until the colon has been adequately evaluated 2.
Diagnostic Approach
- A precise patient history and careful physical examination are essential for accurate diagnosis 2.
- Further investigation, such as colonoscopy or CTA, may be warranted to determine the source of bleeding [(1, 2)].
From the Research
Clinical Significance of Blood in Stool
The clinical significance of blood mixed into the stool versus blood on the surface of the stool can be understood through various studies.
- A study published in 1998 3 found that patients with substantial lesions were more likely to give a history of blood mixed within their stool.
- Another study from 1995 4 used an objective test of stool color to predict the location of gastrointestinal bleeding, and found that patients pointing to a black color on the test card were more likely to have an upper bleeding source, while those pointing to bright red colors were more likely to have a coloanorectal bleeding source.
Diagnostic Approaches
Different diagnostic approaches have been suggested for detecting blood in the stool.
- A 1979 study 5 recommended a rectal examination that includes a Hemoccult slide test for all patients over 40.
- However, a more recent study from 2021 6 suggested that guaiac-based tests (Hemoccult) are obsolete and have been replaced by faecal immunochemical tests (FITs), which are more reliable and have better performance.
- Another study from 1993 7 found that a Hemoccult-positive rectal examination is clinically significant, and that testing stool obtained during rectal examination for occult blood does not increase the rate of false-positive results.
Predictive Value of Stool Color
The predictive value of stool color in determining the location of gastrointestinal bleeding has been evaluated in several studies.
- The 1995 study 4 found that the positive predictive value of a black color for an upper bleeding source was very high, while the positive predictive value of bright red colors for a lower bleeding source was also high.
- The study suggested that an objective test of stool color can improve upon subjective descriptions in predicting bleeding locations.