The Sitz Marker Test, also known as the Radiopaque Marker Study or Colonic Transit Study, is a diagnostic procedure used to evaluate the movement of material through the large intestine (colon). It measures the rate at which food residue travels through the digestive tract. The test is primarily performed on individuals experiencing chronic constipation that has not responded well to initial treatments, helping physicians gain insight into potential motility issues.
Conditions Diagnosed by the Test
The Sitz Marker Test classifies the specific type of functional constipation a patient has, which is important because treatment strategies differ significantly. Constipation not caused by a structural blockage or underlying disease is categorized into different motility disorders. The test is highly effective at distinguishing between two major subtypes: slow transit constipation and functional outlet obstruction.
Slow transit constipation, also known as colonic inertia, means the colon muscles are not contracting effectively to move stool forward. This results in a prolonged transit time, causing residue to remain in the colon too long, leading to excessive water absorption and hard stools. The test also identifies functional outlet obstruction, which is difficulty in the final stage of defecation, often due to the failure of pelvic floor muscles to relax properly.
A third category, normal transit constipation, is diagnosed when a patient has symptoms but the Sitz Marker Test shows material movement is happening at a normal speed. Differentiating these types is paramount; for instance, surgery for slow transit constipation would be inappropriate for a patient with an outlet obstruction. Physicians use the results to tailor therapy, directing patients with slow transit to specialized medications or those with an outlet obstruction to pelvic floor physical therapy.
Executing the Sitz Marker Test
Performing the Sitz Marker Test is a straightforward, non-invasive process that typically spans five to seven days. Preparation involves temporarily stopping the use of laxatives, enemas, and suppositories, as these could interfere with the natural movement of the colon. Patients must maintain their normal diet and physical activity levels throughout the testing period.
The test begins with the patient swallowing a capsule containing a set number of radiopaque markers, which are small, plastic rings or pellets visible on an X-ray. This capsule usually holds 20 to 24 markers. The capsule quickly dissolves, releasing the markers to travel with the digestive contents.
After the markers are ingested, an abdominal X-ray is scheduled, most commonly five days later. Some protocols involve taking a capsule for three consecutive days with X-rays taken on day four or day seven. The X-ray image taken at the end of the period reveals how many markers remain in the colon and their location, providing the data needed for diagnosis.
Interpreting Marker Retention Patterns
A physician interprets the results by counting the number of markers remaining on the final X-ray and observing their location. Retention of a large number of markers indicates a delay in overall colonic transit time. Retaining more than 20% of the original markers (typically five or more out of 24) after five days is considered an abnormal result consistent with delayed transit.
The specific visual pattern of the retained markers helps differentiate the subtypes of constipation. If the retained markers are scattered uniformly throughout all segments of the colon, from the ascending to the descending colon, it suggests slow transit constipation. This scattered pattern confirms that the colon as a whole is struggling to propel its contents forward.
In contrast, if the majority of the retained markers are clustered almost exclusively in the rectosigmoid area (the section just above the rectum), it often points to a functional outlet obstruction. This clustering suggests that the material successfully traveled through the main part of the colon but is being held up at the final exit point. The location of the markers is just as important as the total count for guiding patient care.