Studies for Assessing Fecal Transit Time
The most effective study for assessing fecal transit time is radioisotope scintigraphy, which allows for comprehensive measurement of transit through multiple regions of the gastrointestinal tract. 1
Primary Methods for Measuring Fecal Transit Time
1. Radioisotope Scintigraphy
- Uses gamma scintigraphy to obtain serial images of labeled meals
- Can track transit through stomach, small bowel, and colon
- Advantages:
- Provides comprehensive assessment of entire GI tract
- Can distinguish regional transit times (gastric, small bowel, colonic)
- If isotope has long half-life, can determine orocaecal and colonic transit
- Best for:
- Determining whether transit disorders are localized or generalized
- Distinguishing between different types of motility disorders
2. Radiopaque Marker (ROM) Studies
- Simple and widely available method
- Protocol options:
- Single abdominal X-ray after 7 days of marker ingestion 2
- Multiple X-rays after single marker dose
- Advantages:
- Inexpensive and widely available
- Can differentiate between rapid, normal, and prolonged transit
- Can identify segmental colonic dysfunction
- Limitations:
- Requires radiation exposure
- Less standardized protocols
- Less detailed information than newer methods
3. Wireless Motility Capsule (SmartPill)
- Ambulatory, minimally invasive diagnostic modality
- Measures intraluminal pH, temperature, and pressure
- Advantages:
- No radiation exposure
- Provides regional transit times (gastric, small bowel, colonic)
- Ambulatory testing with standardized protocols
- Comparable diagnostic accuracy to radiopaque markers (ROC 0.73 vs 0.71) 3
- Reveals gender differences and upper-gut dysfunction
- Limitations:
- Risk of capsule retention (though can be mitigated with patency testing)
- More expensive than ROM studies
4. Delayed-Release Capsules with Radio-Opaque Markers
- Uses special capsules that release markers in the colon
- Advantages:
- Highly correlated with scintigraphic methods (r = 0.994) 4
- Good reproducibility and responsiveness
- Simpler than traditional ROM methods
- Limitations:
- Less widely available than standard ROM
Clinical Considerations
When to Order Transit Studies
- Order transit studies for patients with refractory constipation after:
- Ruling out secondary causes (metabolic, neurologic, medication-induced)
- Failure of initial therapeutic interventions
- Need to categorize into normal transit constipation (NTC) or slow transit constipation (STC) 1
Interpretation Considerations
- Women typically have longer transit times than men 3
- Stool form (using Bristol Stool Scale) correlates well with transit time (r = -0.54) and can be used as a simple clinical assessment tool 5
- Consider defecatory disorders, which may coexist with slow transit and require separate evaluation
Pitfalls and Caveats
- Breath hydrogen tests (lactulose) are not recommended for transit assessment in patients with propulsive failure due to high false results from bacterial overgrowth 1
- Liquid meals may not clearly demonstrate transit abnormalities; solid meals are preferred 1
- Capsule studies should be used cautiously in patients at risk for obstruction; patency capsules can mitigate this risk 1
- Consider that some patients may have combination disorders (e.g., slow transit with defecatory disorders) 1
In summary, while radioisotope scintigraphy provides the most comprehensive assessment, the choice between wireless motility capsule and radiopaque marker studies should be based on availability, cost considerations, and the need for regional versus whole-gut transit information.