Abdominal X-Ray (KUB)

The plain abdominal radiograph (KUB - kidneys, ureters, bladder) is a low-cost bedside test that surveys the bowel gas pattern, calcifications, soft tissues and lower thoracic and pelvic bones. Although CT has largely replaced it for most acute abdominal pathology, the KUB retains a role in suspected bowel obstruction, assessment of foreign bodies and enteric/urinary tube position, and evaluation of constipation. Its diagnostic yield is limited, so appropriate patient selection is essential.

Indications

Suspected small or large bowel obstruction
Suspected bowel perforation (usually combined with an erect chest film for free air)
Assessment of severe constipation or faecal loading
Localisation and follow-up of radiopaque foreign bodies and ingested objects
Suspected renal or ureteric calculi (limited sensitivity; CT KUB preferred)
Confirmation of nasogastric, gastrostomy or ureteric stent position
Suspected toxic megacolon in inflammatory bowel disease
Suspected sigmoid or caecal volvulus

Contraindications & Cautions

No absolute contraindications, but low diagnostic yield means requests should be justified
Pregnancy: avoid where possible; use ultrasound or MRI alternatives and justify if performed
Effective dose is moderate (~0.7 mSv, roughly 3-4 months of background) - higher than a chest film
Avoid as a routine screen for non-specific abdominal pain where CT or ultrasound is more appropriate
Apply ALARA; do not repeat unnecessarily

Patient Preparation

Generally no specific preparation for the acute setting
Remove metallic objects, belts and clothing with radiopaque fasteners from the field
Confirm pregnancy status in women of reproductive age
Ensure the bladder is not deliberately emptied when assessing lower urinary calculi
Explain the breath-hold (expiration) to reduce motion and diaphragmatic obscuration
For suspected obstruction, plan for an additional erect or decubitus view

Technique & Parameters

Standard projection: supine AP with the patient breath-holding in expiration
Coverage from the diaphragms superiorly to the symphysis pubis inferiorly, including both flanks
Centre at the level of the iliac crests (L4)
Moderate kVp (70-80 kVp) with a grid for soft-tissue and calcification contrast
Erect abdominal or left lateral decubitus view when obstruction or perforation is suspected to demonstrate air-fluid levels and free air
Erect chest radiograph is the most sensitive plain film for subdiaphragmatic free gas
Ensure both properitoneal fat lines and the symphysis are included

Systematic Review

Assess adequacy: coverage from diaphragm to symphysis, both flanks, penetration
Bowel gas pattern: small bowel (central, valvulae conniventes spanning the lumen) versus large bowel (peripheral, haustra)
Calibre: small bowel >3 cm, large bowel >6 cm, caecum >9 cm suggest dilatation
Look for gas where it should not be: pneumoperitoneum, pneumobilia, portal venous gas, intramural gas
Calcifications: renal tract, gallstones, pancreas, vascular, phleboliths, appendicoliths
Solid organs and soft tissues: liver, spleen, renal outlines, psoas margins, bladder
Bones: lower ribs, spine, pelvis, sacroiliac joints and hips
Foreign bodies, tubes, lines and stents

Key Findings & Significance

Dilated central loops with valvulae conniventes and multiple air-fluid levels indicate small bowel obstruction
Dilated peripheral colon with haustra and a collapsed distal segment indicates large bowel obstruction
A 'coffee bean' or inverted-U loop suggests sigmoid volvulus; a distended caecum displaced to the left upper quadrant suggests caecal volvulus
Rigler sign (gas on both sides of the bowel wall) and free subdiaphragmatic gas indicate perforation
Continuous diaphragm sign and the falciform ligament outlined by gas confirm pneumoperitoneum
Ureteric calculi appear as calcific densities along the expected ureteric course
Mural thickening with 'thumbprinting' suggests colitis or ischaemia; loss of haustra with dilatation suggests toxic megacolon
Faecal loading throughout the colon supports constipation

Differential Considerations

Dilated bowel: mechanical obstruction, adynamic ileus, pseudo-obstruction (Ogilvie), volvulus
Abnormal gas: pneumoperitoneum, pneumatosis intestinalis, emphysematous cholecystitis/pyelonephritis, abscess
Right upper quadrant calcification: gallstones, renal calculus, costal cartilage, hepatic lesion
Pelvic calcification: phleboliths, bladder calculus, uterine fibroid, dermoid, appendicolith
Curvilinear calcification: aortic aneurysm, splenic artery, vas deferens
Absent psoas outline: retroperitoneal fluid, mass or haemorrhage (non-specific)
Multiple air-fluid levels: obstruction, ileus, gastroenteritis, or normal in small numbers

Pearls & Pitfalls

The erect chest radiograph detects as little as 1-2 mL of free intraperitoneal gas - more sensitive than the abdominal film
Only about 10-15% of gallstones and a variable proportion of renal stones are radiopaque; a normal KUB does not exclude them
Distinguish small from large bowel by mucosal folds and distribution, not just position
Phleboliths (rounded with lucent centres) are commonly mistaken for distal ureteric stones
A left lateral decubitus film is useful when the patient cannot stand for free-air assessment
Beware overcalling ileus versus mechanical obstruction; a transition point favours mechanical obstruction
Retained surgical items, patches and external artefacts can mimic pathology - correlate clinically

Structured Report

State projection(s) and adequacy of coverage
Describe the bowel gas pattern, calibre and any transition point or dilatation
Report presence or absence of free intraperitoneal or abnormal gas
Describe calcifications, soft tissues, solid organ outlines and bones
Note tubes, stents, lines and foreign bodies with their positions
Impression: state whether findings support obstruction, perforation or other pathology and recommend CT where appropriate

References

ACR-SPR Practice Parameter for the Performance of Abdominal Radiography
Royal College of Radiologists iRefer guidelines
ACR Appropriateness Criteria: Suspected Small-Bowel Obstruction
Begg JD. Abdominal X-Rays Made Easy, Elsevier
Gore RM, Levine MS. Textbook of Gastrointestinal Radiology, Elsevier

Educational clinical decision support only. Protocols vary by institution and equipment; always confirm with a qualified radiologist and local guidelines before clinical use.

Abdominal X-Ray (KUB)

📋 Indications

⛔ Contraindications & Cautions

🧑‍⚕️ Patient Preparation

⚙️ Technique & Parameters

🔎 Systematic Review

🩻 Key Findings & Significance

🧠 Differential Considerations

💡 Pearls & Pitfalls

📝 Structured Report

📚 References