Background: The cellular protein p16INK4a is used as a tissue bio-marker for HPV induced high grade cervical preneoplasia and cancer. It specifically highlights cells with transforming HPV-infections. Due to its abundant expression in cervical pre-cancer and cancer cells it is likely that p16INK4a is released in the circulation and could serve as a diagnostic or prognostic biomarker. So far p16INK4a protein levels have not been measured in blood or serum samples.

Methods: We used a sandwich ELISA system based on two monoclonal p16INK4a antibodies for the detection of p16INK4a from serum samples. Optimal conditions for measuring serum p16INK4a were determined by evaluating the recovery of spiked external p16INK4a in sera. Using these conditions, preoperative and first year follow-up serum p16INK4a levels from 36 consecutive cervical cancer patients were measured and correlated to patient's age, tumor volume and tumor stage.

Results: Preoperative serum p16INK4a levels were associated with tumor stage and were significantly higher in patients with FIGO stage III (n=5, Mann-Whitney p=0.007) and IV (n=6, p<0.001) as compared to stage I (n=60). Further an association of p16INK4a serum levels with tumor volume was observed. Serum p16INK4a levels in preoperative sera tended to be higher than six and nine months after surgery. No correlation with the age of the patient was observed.

Conclusions: For the first time we could show that p16INK4a is detectable in serum samples. The observed association with tumor burden suggests that measuring serum p16INK4a might be a tool for cancer patient's surveillance during follow up or for tumor early detection.