While formalin fixation and paraffin embedding has become a universal mechanism of tissue preservation and a gold standard for immunohistochemistry, fixation and processing variables that may confound assay effectiveness have received little attention from the scientific community.

To identify discrete steps in specimen fixation and processing that may impact immunostaining, assess the magnitude of reported effects in the literature, and highlight preanalytical variables that require further investigation.

Thirty-nine primary research articles that investigated immunohistochemical effects of 1 or more preanalytical variables were identified by our literature survey. Thresholds identified in the literature were then compared with published immunohistochemistry guidelines for formalin-fixed, paraffin-embedded specimens.

Of the 62 preanalytical variables identified, 27 were examined in published research. Meta-analysis revealed 15 preanalytical variables that were capable of impacting immunohistochemistry (including fixation delay; fixative type; time in fixative; reagents and conditions of dehydration, clearing, and paraffin impregnation; and conditions of slide drying and storage) and 12 variables with no reported influence (including the type of processor used; the number and position of specimens during dehydration, clearing, and paraffin impregnation; and the duration of paraffin block storage). Variables with antigen-dependent or inconsistent effects were highlighted. Comparison of literature-supported thresholds with published recommendations revealed (1) strong agreement among preanalytical variables for optimal immunostaining, (2) discrepancies among thresholds for adequate immunostaining, and (3) the continued need for rigorous research and comprehensive guidelines on specimen fixation, processing, and storage.