Utility of microwave-citrate antigen retrieval in diagnostic immunohistochemistry

Microwave heating of formalin-fixed, paraffin embedded tissue sections in citrate buffer (MHC) has been shown recently to enhance the antigenicity of a number of formalin-sensitive epitopes. To assess the practical utility of this technique in diagnostic immunohistochemistry, we evaluated the effect of MHC on the immunoreactivity of routinely processed tissue from the surgical pathology laboratory with 30 commonly applied, commercially available epithelial, mesenchymal, lymphoid, neuroendocrine, tumor, and prognostic markers. Paired tissue sections from a variety of normal and neoplastic tissues expected to contain the target antigen were immunostained in parallel following both the routine laboratory protocol, including protease digestion if indicated, and microwave heating for 10 min at maximum output in 10 mM citrate buffer without digestion. Slides were scored for intensity (1+ to 3+) and extent (1+, < 1/10; 2+, 1/10 to 1/3; 3+, 1/3 to 2/3; 4+, > 2/3) of immunostaining, and combined staining scores (intensity + extent) were calculated. Enhanced immunoreactivity compared to the routine protocol was observed with MHC for all 30 antibodies tested. The percentage of cases improved ranged from 38% to 100%, with a mean of 82%. The mean increase in staining score was 2.1. We concluded that MHC facilitates the recognition of diagnostically important antigens in formalin-fixed, paraffin-embedded tissue and can be applied routinely without adverse affects.