Urticaria and the risk of cancer: a Danish population-based cohort study

Background Urticaria has been tentatively linked to cancer, but epidemiological evidence supporting this link is sparse and conflicting. We conducted a population-based cohort study using healthcare databases covering the Danish population (January 1980-December 2022). We followed 87 507 people for a median of 10.1 years after their first hospital contact for urticaria.Objectives To examine associations of a hospital diagnosis of urticaria with incident cancer.Methods We computed the absolute risk of cancer and standardized incidence ratios (SIRs) with 95% confidence intervals (CIs) standardized to Danish national cancer rates. In a cross-sectional analysis, we examined whether the extent of cancer spread differed between people with vs. without a previous urticaria diagnosis.Results The overall SIR for all types of cancer was 1.09 (95% CI 1.06-1.11) based on 7788 observed vs. 7161 expected cases. The risk for any cancer was 0.7% (95% CI 0.6-0.7) for the first year of follow-up. Cancer was diagnosed in 588 people with urticaria during the first year of follow-up (SIR 1.49, 95% CI 1.38-1.62) and in 7200 people thereafter (SIR 1.06, 95% CI 1.04-1.09). During the first year of follow-up, we found strong associations with haematological cancers (e.g. non-Hodgkin lymphoma; SIR 2.91, 95% CI 1.92-4.23). Cancer stage was similar in people with vs. without a previous urticaria diagnosis.Conclusions At the time of urticaria diagnosis, or in the first year afterward, we found a large increase in the risk of cancer. In subsequent years, a persistent 6% increase in risk remained. Diagnostic efforts may partly explain the elevated short-term risk, but occult cancer may promote urticaria, or cancer and urticaria share common risk factors. Urticaria (also known as hives) is a common skin condition that causes wheals and/or angioedema (swelling). Approximately 9% of people will experience it in their lifetime. Urticaria is classified according to how long the symptoms last and is considered to be 'chronic' when the wheals and/or angioedema last for 6 weeks or more. Case studies have suggested that urticaria may be linked to cancers where the primary origin is not known. Only two other studies have compared different groups of people to estimate the risk and association between urticaria and cancer.Using healtcare information from Danish databases, we looked at the risk of cancer in people with urticaria and compared it with the general population. To do this, we followed 87,507 people for around 10 years after they first attended hospital for urticaria.We found that the risk of getting any cancer was 0.7% for the first year of follow-up. Cancer was diagnosed in 588 people with urticaria during the first year of follow-up and in 7,200 people after the first year. In the first year of follow-up, we found there was a strong association with blood cancers like non-Hodgkin lymphoma. Cancer stage was similar in people with a diagnosis of urticaria versus those without it.Our findings suggest that urticaria could be associated with a higher short-term risk of cancer, but the overall risk is low. Cancer of an unknown origin could promote urticaria, or cancer and urticaria share common risk factors. Several case reports and studies have indicated that urticaria may be linked to occult cancers. We found that urticaria may be a marker of occult cancer, but extensive diagnostic cancer screening in people with urticaria is unlikely to be cost-effective owing to the low absolute risk.