Should a one-view or two-view KUB (Kidney, Ureter, Bladder) X-ray be used to assess constipation?

KUB for Constipation Assessment: One View is Standard

A single-view supine abdominal radiograph (KUB) is the standard approach when plain radiography is used to assess constipation, as no evidence supports the routine use of two views, and guidelines consistently reference "plain abdominal X-ray" without specifying multiple projections. 1

Guideline Recommendations on Plain Radiography

The ESMO Clinical Practice Guidelines explicitly state that plain abdominal X-ray may be useful to image the extent of fecal loading and exclude bowel obstruction, but emphasize it is "limited as a tool in itself" 1. The American Gastroenterological Association similarly notes that structural evaluation may be appropriate in certain circumstances, but does not recommend routine imaging for uncomplicated constipation 1.

Key point: Guidelines universally refer to "plain abdominal X-ray" in singular form, indicating standard single-view imaging is the accepted practice. 1

Evidence Supporting Single-View Approach

Research demonstrates that a single supine KUB provides adequate assessment when radiography is indicated:

• A prospective study of 30 patients found that plain abdominal radiograph scores (single supine view) correlated significantly with objective measures including stool weight and transit time 2
• The left colon (descending colon and rectosigmoid) on single-view radiography provided the most clinically relevant information for constipation assessment 2
• Objective measurements from single-view KUB (cecal diameter >3.7 cm, total stool length >33.4 cm, rectal stool length >5.9 cm) achieved excellent diagnostic accuracy (AUC 0.87) for identifying constipation 3

Critical Limitations of KUB for Constipation

The American College of Radiology does not recommend KUB as a primary diagnostic tool for constipation, and clinical diagnosis based on history and physical examination is more reliable than radiographic assessment. 4

Important caveats include:

• Plain radiography in ED patients with suspected constipation frequently did not affect management, with treatment often contradicting radiographic findings 5
• Fecal loading on radiography does not exclude more serious diagnoses 5
• In one study, 42% of patients with moderate-to-large stool burden received no ED treatment, while 55% with minimal stool burden were diagnosed with constipation 5

When Radiography May Be Considered

Plain abdominal X-ray (single view) has limited utility in specific scenarios:

• To exclude bowel obstruction when clinically suspected 1
• In patients with severe symptoms, sudden changes in bowel habits, or blood in stool 1
• When clinical assessment is unreliable or objective documentation is needed 6, 3

Rectal ultrasound measuring rectal diameter (≥3.4 cm) is a non-radiating alternative that correlates with stool burden and should be considered instead of KUB when objective assessment is needed. 7

Practical Algorithm

For constipation assessment:

• First-line: Clinical diagnosis using Rome IV criteria, Bristol Stool Form Score, and digital rectal examination 1
• If imaging needed: Consider rectal ultrasound over KUB to avoid radiation 7
• If KUB ordered: Single supine view is standard; focus interpretation on left colon findings 2
• Never order KUB: As routine screening or when clinical diagnosis is straightforward 4, 5

Bottom line: When plain radiography is deemed necessary for constipation (which should be rare), a single-view supine KUB is the standard approach—there is no evidence supporting routine two-view imaging for this indication.