Colon Transit Study: Purpose and Interpretation
Primary Purpose
A colon transit study is performed to distinguish between slow transit constipation (STC) and normal transit constipation (NTC), but only after defecatory disorders have been excluded, as patients with pelvic floor dysfunction frequently have secondary slow transit that improves once the primary disorder is treated. 1, 2
Critical Pre-Test Requirements
Before ordering a colon transit study, you must:
Perform a digital rectal examination to assess for paradoxical pelvic floor contraction, high anal resting tone, and inability to expel soft stool or enema fluid—these findings indicate a defecatory disorder that must be addressed first 1, 2, 3
Exclude secondary causes including metabolic disturbances (hypercalcemia, hypothyroidism, diabetes), neurologic disorders (Parkinsonism, spinal cord lesions), colonic diseases (stricture, cancer, anal fissure, proctitis), and constipating medications (opiates, anticholinergics, calcium channel blockers) 1, 2, 3
Obtain anorectal testing first (manometry and balloon expulsion test) to identify defecatory disorders before proceeding to transit studies 3
When to Order the Study
The colon transit study should be evaluated:
Only if anorectal tests do not show defecatory disorder OR if symptoms persist despite treatment of the identified defecatory disorder 3
After failure to respond to over-the-counter laxatives and fiber supplementation for 1-2 weeks 3
Never as a first-line test in the constipation workup 3
Interpretation of Results
Normal Transit (74.8% of patients)
- Geometric center (GC) at 24 hours ≥1.3 and GC at 48 hours ≥1.9 4
- These patients have normal transit constipation (NTC), often associated with irritable bowel syndrome features 1, 3
- No further colonic motility evaluation is needed 5
- Symptoms likely reflect altered bowel habits, hard stool consistency, or coexisting IBS rather than true motility disorder 1
Slow Transit Constipation (20.8% based on 24-hour, 11.2% based on 48-hour criteria)
- GC at 24 hours <1.3 or GC at 48 hours <1.9 4
- An additional 9.2% can be identified using the difference between GC48 and GC24 (Δ48-24): abnormal if <0.38 for females or <0.29 for males 4
- These patients may have reduced colonic propulsive activity or increased uncoordinated motor activity in the distal colon 1
- Resected specimens from STC patients show marked reduction in colonic intrinsic nerves and interstitial cells of Cajal 1
Key Clinical Pitfalls to Avoid
Never assume infrequent bowel movements equal slow transit—bowel movement frequency correlates poorly with actual colonic transit time 2
Repeat the transit study before considering colectomy in patients with apparent colonic inertia, as reproducibility is poor (correlation coefficient only 0.12) and surgical outcomes are significantly better when a second study confirms the diagnosis 6
Do not proceed with colectomy without confirming normal anorectal function—this is the most critical error in management 2
Recognize combination disorders—some patients have both STC and defecatory disorders, or STC with IBS features, requiring tailored management 1, 2
Therapeutic Implications Based on Results
For Normal Transit Constipation
- Focus on fiber supplementation (20-25g daily, prioritizing soluble fiber like psyllium) 2
- Osmotic laxatives (polyethylene glycol) as first-line pharmacotherapy 2
- Address coexisting IBS symptoms if present 1
For Slow Transit Constipation
- Stimulant laxatives (bisacodyl or senna) to achieve non-straining evacuation every 1-2 days 2
- Prucalopride 2 mg once daily as the most evidence-based prokinetic option for patients failing standard laxatives 2
- Consider colectomy with ileorectal anastomosis only in severe, refractory cases after confirming diagnosis with repeat transit study and excluding defecatory disorders 5, 6