To test the performance characteristics of 69 primary immunohistochemistry antibodies after expiration and compare with fresh primary antibodies wherever possible.
A total of 69 expired primary antibodies were evaluated for specificity, background staining, and intensity. An optimal staining result corresponded to a semiquantitatively scored 2+ or 3+ intensity, with intact specificity devoid of moderate or strong background staining. Any deviation from a normal staining pattern was also considered to be a suboptimal result.
Nearly half of the antibodies studied showed an optimally positive staining result after expiration (34/69, 49.2%). Overall, 10.1% (7/69) of antibodies could be compared with fresh primary antibodies of the same clone with equivalent results. Eight of 69 (11.6%) expired antibodies showed splotchy or granular staining.
Evidence from this study and previous work point to maintained functionality of a fair number of primary immunohistochemical antibodies after expiration. Decisions about the use of such reagents should be guided by a thorough assessment of functionality by the pathologist rather than a manufacturer-specified deadline. Quality maintenance should imply a sensible balance between histopathologic performance and economics.

© American Society for Clinical Pathology, 2020. All rights reserved. For permissions, please e-mail: