Tonsil tissue control is ideal for monitoring estrogen receptor immunohistochemical staining.

Background: Estrogen receptor (ER) testing performed using immunohistochemistry (IHC) is a critical predictive tool for breast cancer treatment. This study aimed to investigate the use of tonsil control for monitoring ER staining and hypothesize that optimal staining would reduce interlaboratory variations.

Methods: A proficiency test for ER IHC was conducted using 21 tissue cores. The staining quality was centrally reviewed based on tonsil ER staining.

Results: We found that 64.9% of participant samples demonstrated optimal or good staining quality. Poor staining quality was significantly associated with the use of Ventana autostainers and concentrated antibodies. Although the concordance rate did not show significant differences across staining quality levels, interparticipant agreement declined as staining quality deteriorated. Among the 19 discordant responses, 63.2% could be attributed to staining problems, whereas 36.8% could be due to misinterpretation. Poor staining quality due to inadequate staining was the primary reason for undercalls, which can lead to false-negative results. Misinterpretations of nonspecific faint staining that was weaker than the staining of the tonsil control were the cause of most overcalls.

Conclusion: Tonsil tissue is an ideal control for monitoring ER staining and can serve as a reference for determining the lower bound for ER positivity. Optimal ER staining and appropriate references for ER positivity can further improve ER IHC quality.