p16 Immunohistochemistry in Oropharyngeal Squamous Cell Carcinoma Using the E6H4 Antibody Clone: A Technical Method Study for Optimal Dilution

Routine testing for p16 immunohistochemistry (with selective HPV-specific test use) has been recommended for clinical practice in oropharyngeal squamous cell carcinoma (OPSCC). Data suggests that the E6H4 clone performs best for this purpose, yet no studies have evaluated the optimal antibody concentration for OPSCC testing. We evaluated three concentrations (undiluted, 1:5, and 1:10) of the primary antibody solution for E6H4 using tissue microarrays from a cohort of 199 OPSCC patients with a > 70% staining cutoff for positivity. Concordance was evaluated using percent agreement and Cohen’s kappa. The concentrations were evaluated for sensitivity and specificity using high risk HPV RNA in situ hybridization (RNA-ISH) and also correlated with Kaplan–Meier overall survival analysis. Inter-rater agreement was very high between p16 results at each concentration and also with RNA in situ hybridization (p < 0.0001 for all). Agreement between p16 undiluted and 1:5 dilution (agreement 98.2%; Kappa 0.943; p < 0.0001) was very high and between p16 undiluted and 1:10 dilution (agreement 79.2%; Kappa 0.512; p < 0.0001) much lower. Intensity of the staining did decrease with the 1:5 and 1:10 dilutions compared to undiluted, but not in a manner that obviously would change test interpretation or performance. Results suggest that the E6H4 antibody performs well at dilutions of up to 1:5 fold with a minor decrease in staining intensity, minimum loss of sensitivity, and no loss of specificity in OPSCC patients. This could result in reagent and cost savings.