Several diagnostic standards for measurement of transit time have emerged.
Time based assessment
Hinton et al (1969), originators of this methodology described two evaluation metrics.
- time taken to pass the first pellet
- time taken to pass 80% or markers
The choice of 80% of markers (total 20) was based on the observation that “the last 1 or 2 pellets tend to be disproportionally delayed”.
All subjects (25 adult males) passed the first marker within 66 hours (3 days). Majority passed 80% of markers within 114 hours (5 days).
Proportion passed
Evans et al (1992) reported passage in healthy volunteers. Importantly, they included 25 male and 18 female subjects at different menstrual stages. These data are therefore more broadly generalizable. Their validation studies identified that:
- Normal subjects retain more than 20% of markers within 12 hours
- Normal subjects retain less than 80% of markers after 120 hours
Sectoral assessment
In this approach, the bowel is divided into three sectors i.e. right colon, left colon and rectosigmoid. Transit of markers through each segment is assessed. Segmental transit times were derived from the distribution of markers at different time points, assessed through serial X-Ray.
Arhan et al (1981) contrasted sectoral transit times in adults and children. Mean transit times for children were :
- Right colon = 7.7 hours
- Left colon = 8.7 hours
- Recto-sigmoid = 12.4 hours
The limitation of this method in children is the need for serial X-Ray.
Gestalt assessment
This is the most common method used in practicality. An experienced clinician assesses the distribution of markers.
- Slow transit corresponds to an even distribution of majority of markers throughout the colon
- Rectal evacuation disorder corresponds to a retention of majority of markers in the rect0-sigmoid segment
This methodology yields a qualitative assessment that may be poorly reproducible.
Density methods / Scintigraphy
Scintigraphy was first described as a colonic transit tool Krevsky et al (1986). A radioactive tracer e.g. DTPA is ingested. A gamma-sensitive camera is used to image the body at defined time intervals. The percentage of tracer in the remaining in colonic sectors or regions of interest (ROI). A correction is made for decay of the radio-isotope and attenuation of emission by the body wall.
The relative density of tracer in defined regions of the colon is then quantified using geometric centre analysis. Simply stated, this is the peak where the tracer is most dense. A curve of geometric centre over time after ingestion is generated.
- Subjects with a prolonged phase when the geometric centre lies between the caecum to hepatic flexure are considered to have right-sided delayed transit.
- Subjects with a prolonged phase in splenic flexure onwards are considered to have left-sided delay or evacuation disorder.
Transit index
This method combines sectoral and density methods. A benefits of this method are :
- use of a single radiograph at the end of the study period
- quantitative assessment that is reproducible
- avoidance of radio-isotope ingestion