guaiac
Meaning of guaiac: A tree resin pressed into service as a chemistry set for finding blood that the eye cannot see.
Definition of guaiac
Guaiac is a resin from the wood of the tropical Guaiacum tree, used as the reagent in an older test for hidden blood in stool. Paper impregnated with the resin turns blue when a developing solution is dripped onto a smear, provided a trace of blood is present. The color comes from the iron-carrying part of red blood cells acting on the resin.
How does the color change work?
Heme, the iron-containing core of hemoglobin, behaves like an enzyme even when the red cell around it has long since broken apart. Given hydrogen peroxide, which is what the developing solution supplies, it strips electrons from a colorless acid in the resin and converts it into a blue pigment. A blue ring appearing within seconds counts as positive. Nothing in the reaction identifies whose blood it is or where along the gut it came from, and that indifference is the source of nearly every difficulty with the test.
Why do results mislead?
| Influence | Effect on the result |
|---|---|
| Recent red meat | Falsely positive, since animal heme reacts identically |
| Raw horseradish, turnip or radish | Falsely positive, as these carry their own peroxidase |
| Large doses of vitamin C | Falsely negative, because it blocks the color change |
| Bleeding that comes and goes | Falsely negative if the sample misses a bleeding day |
How does it compare with the newer stool test?
The immunochemical test uses antibodies raised against human hemoglobin instead of a chemical reaction. That single change removes most of the problems above: animal blood is not recognized, food and vitamins do not interfere, and because the protein part of hemoglobin is digested on its way through the stomach and small bowel, a positive result points to bleeding in the colon rather than anywhere at all. It also needs fewer samples. Guaiac cards required several specimens taken on separate days, since a lesion may bleed only intermittently. Either way a positive result is a reason to look inside the bowel, not a diagnosis, because ulcers, inflammation, hemorrhoids and polyps all bleed as readily as a cancer does.
Also known as
Used in a sentence
The stool sample was tested with a guaiac card and returned a positive result.
Good to know
The guaiac reaction detects heme from any source, so meat in the diet and peroxidase-rich raw vegetables can produce a positive card while vitamin C can mask real bleeding.