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The diagnosis of squamous intraepithelial lesions in cervical tissue specimens is subject to substantial variability. Adjunctive immunohistochemical (IHC) staining for p16 has been shown to add objective biomarker information to morphologic interpretation of hematoxylin and eosin (H&E)–stained tissues. In the CERvical Tissue AdjunctIve aNalysis (CERTAIN) study, we systematically analyzed the impact of adjunctive p16 IHC on the accuracy (agreement with reference pathology results) of diagnosing cervical intraepithelial neoplasia of grade 2 or worse (CIN2+) in the United States. Eleven hundred cervical biopsies were divided into 4 sets of 275 cases by stratified randomization. All H&E slides from each set were interpreted by 17 to 18 individual surgical pathologists, for a total of 19,250 reads by 70 surgical pathologists. After a wash-out period and blinding to original results, cases were re-read by the same pathologists using H&E+p16-stained slides. Using expert consensus diagnoses on H&E+p16 as reference, adjunctive p16 IHC use significantly improved diagnostic agreement of surgical pathologists by 4.7% (95% confidence interval [CI], 3.9, 5.4; P<0.0001). This improvement was driven by an increase of 11.5% (95% CI, 9.3, 13.5; P<0.0001) in sensitivity and an increase of 3.0% (95% CI, 2.2, 3.7; P<0.0001) in specificity. Diagnostic performance was significantly increased as well when expert consensus diagnoses established on H&E only was used as reference. Furthermore, interobserver reliability improved significantly from moderate (H&E: κ=0.58) to substantial (H&E+p16: κ=0.73; P<0.0001). Adjunctive use of p16 IHC provides more accurate and reproducible diagnostic results in the interpretation of cervical biopsies, ensuring that more patients are treated correctly without treating more patients.