The association between serum nickel (Ni) and oral cancer incidence is unclear and most of the previous studies were observational studies that did not control for confounding factors between groups.

To assess the correlation of serum Ni with oral cancer incidence based on propensity score matching (PSM) and inverse probability of treatment weighting (IPTW).

A cohort of 456 newly diagnosed oral cancer patients was recruited from the First Hospital of Fujian Medical University during November 2011 to May 2019, and residents ordered their health check-up in hospitals or local community health centers over the same period were selected as a control group, which included a total of 1410 participants. Serum Ni was evaluated by inductively coupled plasma mass spectrometry. Case-control pairs were selected using a 1:1 PSM (caliper value of 0.02), and the study subjects in the case group and control group were weighted for subsequent analysis by IPTW. The general characteristics of the study subjects were tested for equilibrium before and after matching by chi-square test and standardized mean difference (SMD). This was followed by exploring the potential nonlinear dose-response relationship between serum Ni and oral cancer using restricted cubic splines as well as analyzing the association between serum Ni and oral cancer incidence by conditional logistic regression and weighted logistic regression.

After controlling for between-group covariates by PSM and IPTW, the dose-response curves demonstrated that the risk of developing oral cancer tended to decline and then increase with the increasing serum Ni level. The outcome of the analysis using PSM demonstrated that as compared to the control group, the risk of developing oral cancer in the 0.09-16.80 μg·L⁻¹ serum Ni group was negatively correlated with serum Ni level (OR=0.36, 95%CI: 0.24-0.54), whereas the risk of developing oral cancer in the >16.80 μg·L⁻¹ serum Ni group was positively correlated with serum Ni level (OR=5.43, 95%CI: 2.76-10.68). After applying IPTW, a negative association was found between the risk of oral cancer and serum Ni concentration within a serum Ni window ranging from 0.09 to 20.55 μg·L⁻¹ (OR=0.39, 95%CI: 0.29-0.52), while a positive association with an OR and 95%CI of 5.54 (3.62-8.49) for the Ni concentration > 20.55 μg·L⁻¹.

In this study, a J-shaped relationship between serum Ni concentration and the risk of developing oral cancer is found, which shows that high serum Ni concentration (>20.55 μg·L⁻¹) may be a risk factor for oral cancer.