Pitfalls in Detecting Amyloid in a Congo Red Stain

Finding

Pitfall

Amyloid can be focal and/or limited in a biopsy, making a false negative result possible.

The clinician assumes that a negative Congo red stain excludes amyloidosis.

Amyloid can be focal and/or limited in a biopsy.

Failure to completely and carefully examine the entire slide.

Detection of amyloid in a Congo red stained slide requires a polarized microscope.

Use of a standard clinical microscope rather than a metallurgical microscope.

Plastic cover slips may interfere with the ability to perform a crossed polarized light examination.

Using plastic cover slip instead of glass.

Collagen and other matrix proteins may have a blue-green hue that can result in a false positive interpretation.

Using a polarizing filter with built-in compensator.

Sensitivity can be increased in borderline cases by use of polar mounting media or omitting the alcohol differentiation step.

Failure to detect small amounts of amyloid because processing or staining has not been optimized.

Amyloid can be detected by additional tests such as electron microscopy.

Failure to perform specialized tests in a borderline case.