Immunostaining for p16-INK4A (henceforth p16) is a sensitive and specific method for detection of high-grade cervical intraepithelial neoplasia (CIN) in women infected with human papillomavirus (HPV), but longitudinal data have not been obtained. We investigated the relation between p16 status and risk of CIN during 3 years of follow-up.

Women aged 25–60 years were enrolled between June 10, 2003, and Dec 31, 2004, in a multicentre randomised trial comparing HPV testing with cytology. HPV-positive women were referred for colposcopy and, in seven of nine centres, were tested for p16 overexpression by immunostaining. If no CIN was detected, these women were followed up at yearly intervals until clearance of HPV infection. The primary endpoint was histologically confirmed CIN of grade 2 or worse (CIN of grade 2 [CIN2], CIN of grade 3 [CIN3], or invasive cervical cancer) at recruitment or during follow-up. We calculated the absolute and relative risks by p16 status at recruitment. We also calculated the longitudinal sensitivity of p16 testing. Additionally, we assessed the relative sensitivity of an alternative strategy (referral to colposcopy and follow-up of only HPV-positive, p16-positive women) versus conventional cytology in two age groups. Percentages were weighted by the inverse of the tested fraction. The trial in which this study is nested is registered, number ISRCTN81678807.

Of 1042 HPV-positive women who were tested for p16 with no CIN detected during the first round of screening, 944 (91%) had further HPV tests. 793 (84%) of these 944 were followed up until detection of CIN2 or worse, HPV infection clearance, or for at least 3 years. CIN2 or worse was detected during follow-up in more p16-positive women (31 of 365, 8·8% [95% CI 5·8–11·8]) than in p16-negative women (17 of 579, 3·7% [1·9–5·4]; relative risk [RR] 2·61 [95% CI 1·49–4·59]). RR was higher in women aged 35–60 years at recruitment (3·37 [1·39–8·15]) than in those aged 25–34 years (2·15 [1·00–4·61]), but age was not a significant modifier. CIN3 or worse was detected during follow-up in more p16-positive women (16 of 365, 4·4% [2·3–6·6]) than in p16-negative women (six of 579, 1·3% [0·2–2·3]; RR 3·90 [95% CI 1·57–9·68]). Longitudinal sensitivity of p16 testing for detection of CIN3 or worse during follow-up at all ages was 77·8% (95% CI 63·9–91·6). The relative sensitivity of the alternative strategy compared with conventional cytology was 2·08 (1·13–3·56) in women aged 35–60 years and 2·86 (1·28–5·36) in those aged 25–34 years. HPV-positive, p16-negative women aged 35–60 years had a higher cumulative risk of CIN3 or worse during recruitment or follow-up (2·0%, 95% CI 0·3–3·7) than did HPV-negative women (0·01%, 0–0·04) or those who were cytologically normal (0·04%, 0·02–0·09) at recruitment.

p16 overexpression is a marker for CIN2 or worse or for development of CIN2 or worse within 3 years in HPV-positive women, especially those aged 35–60 years. HPV-positive, p16-positive women need immediate colposcopy and, if the assessment is negative, annual follow-up. Immediate colposcopy can be avoided in HPV-positive, p16-negative women, who can be safely managed with repeat screening after 2–3 year intervals.

European Union; Italian Ministry of Health; Regional Health Administrations of Piemonte, Tuscany, Veneto and Emilia Romagna; and Public Health Agency of Lazio Region.