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Substances that interfere with guaiac card tests: implications for gastric aspirate testing
H K Gogel1, D Tandberg, R G Strickland
1Department of Medicine, University of New Mexico School of Medicine, Albuquerque 87131.
Insights
Many foods, medications, and acidic conditions can interfere with guaiac tests for gastrointestinal bleeding. Physicians should use caution when interpreting these test results, especially for outpatients.
Area of Science:
- Gastroenterology
- Clinical Chemistry
Background:
- Guaiac-based fecal occult blood tests are common diagnostic tools.
- Previous research indicates that certain substances can interfere with guaiac test accuracy.
Purpose of the Study:
- To evaluate the impact of common foods, beverages, and drugs on the sensitivity and specificity of guaiac tests for gastric aspirates.
- To identify specific substances that cause false-positive or false-negative results.
Main Methods:
- An in vitro study was conducted using over 100 ingested substances.
- Substances were diluted to concentrations relevant to outpatient gastric aspirates.
- Diluted substances were mixed with known blood concentrations and tested using various guaiac card brands.
Main Results:
- Decreased guaiac test sensitivity was observed with activated charcoal, N-acetylcysteine, red chile, certain medications (e.g., rifampin, Pepto-Bismol), and foods/beverages (e.g., red Jell-O, orange juice, spaghetti sauce, red wines).
- False-positive results were associated with chlorophyll and methylene blue, while false-negative results were confirmed for vitamin C, bile, and antacids.
- False-positive results were also noted with iodide, bromide, cupric sulfate, iron salts, and hypochlorite.
Conclusions:
- Numerous common ingestible substances can significantly interfere with guaiac card tests for gastric aspirates.
- Physicians must exercise caution when interpreting guaiac test results, particularly in outpatient settings, due to potential false positives and negatives.
Abstract:
Previous studies have shown that acidic pH and several ingestible substances can cause misleading guaiac tests of gastric aspirates. In this in vitro study, over 100 foods, beverages, and drugs were diluted to concentrations potentially present in the stomachs of outpatients being evaluated for gastrointestinal bleeding. These were mixed with known concentrations of blood and tested with different brands of guaiac cards. Decreased guaiac test sensitivity was associated with activated charcoal, dimethylaminoethanol, red chile, N-acetylcysteine, rifampin, red Jell-O (General Foods Corp, White Plains, NY), orange juice, Pepto-Bismol (Norwich Eaton Pharmaceuticals, Norwich, NY), simethicone, spaghetti sauce, and several red wines. Chlorophyll and methylene blue-containing tablets produced false-positive results, but other blue and blue-green colored tablets did not, except at high concentrations. Previously described false-negative results with vitamin C, bile, and certain antacids were confirmed, as were false-positive results with iodide, bromide, cupric sulfate, iron salts, and hypochlorite. Physicians should exercise caution when interpreting guaiac card tests of gastric aspirates, especially in the outpatient setting.