DIAGNOSIS OF BOWEL OBSTRUCTION ON PLAIN ABDOMINAL RADIOGRAPHS - SIGNIFICANCE OF AIR-FLUID LEVELS AT DIFFERENT HEIGHTS IN THE SAME LOOP OF BOWEL

OBJECTIVE. Differential air-fluid levels are two distinct air-fluid in terfaces on horizontal-beam abdominal radiographs that are at differen t heights but within the same loop of bowel. Differential air-fluid le vels have been considered by many to be strong evidence of mechanical bowel obstruction, but others have found this sign unreliable for diff erentiating mechanical from adynamic obstructions. Neither opinion is supported by evidence from large series of patients. Accordingly, we d etermined the efficacy of differential air-fluid levels for distinguis hing mechanical from adynamic bowel obstruction. MATERIALS AND METHODS . We identified patients who had a total of 62 episodes of proved mech anical bowel obstruction and 38 episodes of adynamic obstruction throu gh a computer search of medical records and radiographic files. On hor izontal-beam abdominal radiographs of these patients, the presence and height of intestinal differential air-fluid levels were determined by the consensus of two experienced radiologists. These data were then s tatistically analyzed to determine the usefulness of differential air- fluid levels for distinguishing between mechanical and adynamic bowel obstructions. RESULTS. Plain films showed differential air-fluid level s in 32 (52%) of the 62 episodes of mechanical obstructions compared w ith 11 (29%) of the 38 adynamic obstructions, giving a sensitivity for mechanical obstruction of 0.52 and a specificity of 0.71. As the mini mum significant height of differential air-fluid levels increased, spe cificity increased and sensitivity decreased. The positive predictive value also increased as differential air-fluid level heights increased , reaching a level of 0.86 or greater at 20 mm. CONCLUSION. The presen ce of differential air-fluid levels is an insensitive method of determ ining if a bowel obstruction is mechanical, because only a small propo rtion of mechanical obstructions have differential air-fluid levels. I n our population of patients, however, a differential air-fluid level of 20 mm or greater was moderately suggestive that a bowel obstruction was mechanical in nature.