Significant variations in the seroprevalence of C6 ELISA antibodies in a highly endemic area for Lyme borreliosis: evaluation of age, sex and seasonal differences.

The purpose of this study was to assess the seroprevalence of C6 ELISA antibodies in healthy blood donors in Kalmar County, Sweden, in relation to age, sex and time of year (peak season vs off season). In addition, we wanted to assess serological status over time in a group of C6 ELISA seropositive blood donors. Sera were collected from 273 (131 women, 142 men) blood donors in autumn 2011 and 300 (144 women, 156 men) in winter 2014. All sera were analysed in the C6 ELISA and the results were interpreted according to the manufacturer's instructions. The seroprevalence was 22% (females 16%, males 28%) in 2011 and 24% (females 15%, males 33%) in 2014. The seroprevalence was significantly higher in males and increased with age. The highest seroprevalence was observed among elderly men, 60-70 years old (46% in 2011 and 52% in 2014). No significant difference was detected in seropositivity between the samples collected in winter and autumn. All (34/34) seropositive blood donors followed over time remained seropositive at follow-up after 22-29 months. C6 ELISA seroprevalence in healthy blood donors is high in Kalmar County, thereby reducing the specificity of a positive test result regarding the clinical diagnosis of Lyme borreliosis (LB). Although C6 seroprevalence appears not to be affected by seasonal sample time, it varies greatly with age and sex. A careful evaluation of pre-test probability is therefore of the utmost importance in the clinical diagnosis of LB, especially in elderly men. We suggest that colleagues in other endemic regions also consider initiating similar evaluations to optimize the laboratory and clinical diagnosis of LB in relation to age and sex.