The in vitro effect of cimetidine concentration and pH on two brands of guaiac-slide tests was studied. Samples of simulated gastric fluid at pH 1.2, 2.8, and 4.0 were prepared. Cimetidine was added to each of these solutions, and subsequent dilutions of the three samples were made to produce cimetidine concentrations of 10, 5, 2.5, 2.0, 1.5, 1.25, and 0.625 mg/ml. Samples of each pH-concentration combination were placed on two test areas of two Hemoccult and two Fe-Cult slides and tested in accordance with manufacturers' instructions. To determine more specifically the effect of pH, seven aliquots of a cimetidine solution of 1.5 mg/ml in distilled water were prepared (initial pH 8.8). Hydrochloric acid was added to six aliquots to adjust the pH to 7.3, 6.5, 4.6, 3.4, 2.6, and 1.5. A sample of each aliquot was applied to two test areas of four Hemoccult and four Fe-Cult slides. In general, at a given concentration, fewer positive results were obtained with samples prepared from the solution lowest in pH. All results were negative at cimetidine concentrations less than 1.5 mg/ml. When the cimetidine concentration was constant at 1.5 mg/ml, pH values greater than 4.6 for Hemoccult and greater than 7.3 for Fe-Cult were associated with positive results. Both tests appeared to become less sensitive to the effect of cimetidine at lower pH values. Positive Hemoccult and Fe-Cult guaiac-slide test reactions occur in vitro at cimetidine concentrations greater than or equal to 1.5 mg/ml. Because it is doubtful that in vivo concentrations of cimetidine in gastric fluid exceed 1.5 mg/ml, the clinical importance of this interaction is probably minor.

 

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