FALSE-NEGATIVE BIOPSY UREASE TEST IN BLEEDING ULCERS CAUSED BY THE BUFFERING EFFECTS OF BLOOD

Objectives: A false-negative biopsy urease test (BUT) is common in Hel icobacter pylori-associated bleeding peptic ulcers. Although blood in the stomach is thought to interfere with the biopsy urease test, the u nderlying mechanism remains unknown. This in vitro experiment sought t o identify the blood component(s) that interfere with the biopsy ureas e test, and delineate the mechanism of inhibition. Methods: The modifi ed Hazell's microtiter test was used to detect the urease activity of H. pylori. A positive result was indicated by a color change of the pH indicator, bromothymol blue, at 630 nm. Human whole blood, sera with and without anti-H. pylori antibody, electrolytes, and enzymes were in cubated with H. pylori to identify the blood component(s) causing the inhibition of urease activity. In addition, any interference of the pH color indicator was tested by adding different concentrations of seru m albumin to the urease reagent that contained a fixed quantity of amm onia in the absence of H. pylori. Results: The color change of the mic rotiter urease test was significantly reduced by blood (p < 0.0001), r egardless of the presence of anti-H. pylori antibody. Electrolytes and serum enzymes did not interfere with the urease test. The color chang e of the pH indicator was progressively suppressed by higher concentra tions of serum albumin. Conclusions: Blood adversely affects the perfo rmance of the BUT. This is mediated by the buffering effect of serum a lbumin on the pH indicator, rather than by a direct inhibition on the urease activity. (Am J Gastroenterol 1998;93:1914-1918. (C) 1998 by Am . Coll. of Gastroenterology)