Association of Self‐Reported Retained Metallic Objects With Depressive Symptoms in U.S. Adults: Evidence From NHANES and the Exploratory Role of Blood Cobalt Levels

Background The long‐term systemic implications of self‐reported retained metallic objects have received increasing clinical attention, yet their potential relationship with depressive symptoms remains insufficiently understood. This study examined the association between self‐reported retained metallic objects and depressive symptoms and explored whether blood cobalt levels may partially explain this association. Methods This cross‐sectional study included 7211 complete‐case participants from the U.S. National Health and Nutrition Examination Survey (NHANES). Depressive symptoms were assessed using the PHQ‐9, with a score of 10 or higher indicating clinically relevant depressive symptoms. The primary exposure was the self‐reported presence of retained metallic objects in the body, and whole‐blood cobalt levels were used to assess internal metal burden. Multivariable logistic regression models were used to estimate associations, and exploratory mediation analyses were conducted to evaluate the indirect association through cobalt. Results Among the 7211 participants, 622 (8.6%) were classified as having depressive symptoms. Self‐reported retained metallic objects were significantly associated with depressive symptoms in the fully adjusted model (Model 3: OR = 1.28; 95% CI: 1.06–1.55; p = 0.011). When blood cobalt concentrations were categorized into quartiles, the highest quartile was associated with greater odds of depressive symptoms (Model 3: OR = 1.24; 95% CI: 1.01–1.59; p = 0.035), with a significant trend across quartiles ( P for trend = 0.036). Higher cobalt quartiles were also strongly associated with the presence of retained metallic objects (Q4 vs. Q1, Model 3: OR = 1.91; 95% CI: 1.64–2.23; p < 0.001). Exploratory mediation analysis suggested a modest indirect association through cobalt in the overall population (ACME = 0.030; 95% CI: 0.002–0.046; p = 0.013), with similar findings in sex‐stratified analyses. Conclusions Self‐reported retained metallic objects were associated with depressive symptoms, and blood cobalt levels may partially account for this observed association. These findings should be interpreted as associative and require confirmation in longitudinal studies.