Lithium and renal tumors: a critical comment to the report by Zaidan et al.

To the Editor: Zaidan et al.[1] reported a high prevalence of renal tumors in tertiarily referred patients previously treated with lithium and suffering from impaired renal function, as compared to the prevalence in patients who had similar renal impairment but who had not been treated with lithium. The authors concluded that their study provided clear evidence for a potential association between long-term lithium treatment and an increased risk of renal solid tumors. Owing to methodological shortcomings this interpretation is questioned by the members of the International Group for the Study of Lithium (IGSLi), an association of experts on long-term treatment with lithium (http://www.igsli.org).

The study design is a case-series design and not a ‘retrospective cohort study' as claimed by the authors. All patients initiated on lithium (and controls) were not identified and followed over time, implying that time at risk and cumulative events over time were not determined. Therefore, risks or incidence rate ratios as reported cannot be estimated.[2] The observed difference in prevalence of tumors between the two patient samples may be explained by other factors than the effects of lithium, such as differential use of medications and differential referral patterns. Lithium-treated patients are generally under careful observation for renal symptoms, even if not causally related to lithium, and non-lithium-treated patients may suffer from even serious renal problems that never lead to tertiary referral or renal imaging.

In conclusion, we find that no causal relation between lithium and solid renal tumors can be inferred from the study. Even we acknowledge that renal impairment per se occasionally may occur after long-term lithium treatment.[3] We still consider lithium as a safe treatment, provided close monitoring is done, and as the cornerstone in the long-term management of bipolar disorder.[4]