Con·go red (k ng g )n. An acid dye used as an indicator in testing for free hydrochloric acid in gastric contents, as a laboratory aid in the diagnosis of amyloidosis, and as a histologic stain for amyloid. |
Congo red
a synthetic dye, a derivative of benzidine and naphthionic acid. It is used for differential staining of elastic fibers for microscopic examination. Amyloid is stained a light orange-red with Congo red and exhibits apple green birefringence under polarized light. Amyloid in cats stains poorly. Congo red undergoes a change in hue with acidity and thus can be used as an indicator of pH, turning red in the presence of alkalis (bases) and blue when exposed to acids.
Congo red test
a laboratory test used in the diagnosis of amyloidosis, based on measuring the amount of injected dye that is removed by amyloid in the tissues of the human patient. Not now commonly used.
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