Background: Long-term deleterious effects of repeated blood donations may be masked by the donors' healthy lifestyle. To investigate possible effects of blood donation and iron loss through blood donation on cancer incidence while minimizing "healthy donor effects," we made dose-response comparisons within a cohort of Swedish and Danish blood donors.
Methods: We used a nested case-control study design, in which case patients were defined as all donors who were diagnosed with a malignancy between their first recorded blood donation and study termination (n = 10866). Control subjects (n = 107140) were individually matched on sex, age, and county of residence. Using conditional logistic regression, we estimated relative risks of cancer according to number of blood donations made or estimated iron loss 3-12 years before a case patient was diagnosed with cancer. All statistical tests were two-sided.
Results: No clear association was observed between number of donations and risk of cancer overall. However, between the lowest (< or = median, < 0.75 g) and highest (> 90th percentile, > 2.7 g) categories of estimated iron loss, there was a trend (P(trend) < .001) of decreasing risk for cancers of the liver, lung, colon, stomach, and esophagus, which are thought to be promoted by iron overload (combined odds ratio [OR] = 0.70, 95% confidence interval [CI] = 0.58 to 0.84), but only among men and only with a latency of 3-7 years. The risk of non-Hodgkin lymphoma was higher among frequent plasma donors (> 25 vs 0 donations, OR = 2.14, 95% CI = 1.22 to 3.74).
Conclusions: Repeated blood donation was not associated with increased or decreased risk of cancer overall. The lack of consistency across latency periods casts doubt on an apparent association between reduced cancer risk and iron loss in men. The positive association between frequent plasma donation and risk of non-Hodgkin lymphoma deserves further exploration.