False-negative biopsy urease test in bleeding ulcers caused by the buffering effects of blood

A false-negative biopsy urease test (BUT) is common in Helicobacter pylori-associated bleeding peptic ulcers. Although blood in the stomach is thought to interfere with the biopsy urease test, the underlying mechanism remains unknown. This in vitro experiment sought to identify the blood component(s) that interfere with the biopsy urease test, and delineate the mechanism of inhibition.

The modified 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 incubated 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 serum albumin to the urease reagent that contained a fixed quantity of ammonia in the absence of H. pylori.

The color change of the microtiter urease test was significantly reduced by blood (p < 0.0001), regardless of the presence of anti-H. pylori antibody. Electrolytes and serum enzymes did not interfere with the urease test. The color change of the pH indicator was progressively suppressed by higher concentrations of serum albumin.

Blood adversely affects the performance of the BUT. This is mediated by the buffering effect of serum albumin on the pH indicator, rather than by a direct inhibition on the urease activity.