Congo red is a special stain pathologists use to detect amyloid, an abnormal protein that can build up in tissues and organs. Amyloid can damage the heart, kidneys, nerves, and other organs. The Congo red stain is the standard test for confirming amyloid in a tissue sample.
Congo red is a test, not a diagnosis. A positive result shows that amyloid is present, and the condition caused by amyloid buildup is called amyloidosis. This article explains how the Congo red stain works, when it is used, and what the results in your pathology report mean.
Amyloid forms when certain proteins fold into the wrong shape. These misfolded proteins stick together and form tiny fibers that the body cannot easily remove. The fibers build up between cells and interfere with how an organ works.
Many different proteins can form amyloid. In one common type, called AL amyloidosis, the amyloid is made from light chains produced by abnormal plasma cells. In another common type, called ATTR amyloidosis, it is made from a protein called transthyretin.
The Congo red stain is applied to a thin slice of tissue on a glass slide. The dye attaches to amyloid fibers and colors them orange-red under a regular microscope.
The pathologist then examines the slide under polarized light, a special type of light filtered in a single direction. Under polarized light, amyloid stained with Congo red appears green or yellow-green. Your report may describe this as “apple-green birefringence.” This color change confirms amyloid because other materials can look orange-red but do not change color the same way.
The Congo red stain can be used on almost any tissue sample. Common examples include:
A positive result means amyloid is present in the tissue. Your report may say the stain is “positive” or describe “apple-green birefringence under polarized light.”
A positive result does not tell the pathologist which protein formed the amyloid. Because treatment depends on the amyloid type, further testing is usually done. The most accurate method is mass spectrometry, which identifies the exact protein in the amyloid. Immunohistochemistry is sometimes used as well.
A negative result means no amyloid was seen in the sample. Because amyloid can be patchy, a negative result does not always rule out amyloidosis. If your doctor still suspects amyloidosis, they may test a sample from another site.