Herpes simplex is ubiquitously spread within the human population. Seroprevalence of anti-HSV1 IgG in adult populations is reported at approximately 80% [
1‐
3]. Infection occurs through direct person-to-person contact, and susceptible persons may be infected anytime throughout life [
3]. Several studies from Europe and the U.S.A. have reported a decreasing age-adjusted prevalence, indicative of a decreasing childhood and adolescent risk of HSV1 infection in the population [
1,
3,
4]. This trend might be accompanied with a higher incidence of primary infections among adults, but this is not well studied. Neonatal transmission from mothers with genital HSV infection is a substantial risk, especially when the mother is infected for the first time close to birth [
5]. The infection cycle typically involves a primary infection, often located at the mucosal surface of the oral cavity or the genital tract, followed by infection of sensory nerve endings and retrograde spread to neural ganglia where the virus establishes life-long latency. Reactivation may occur, resulting in viral shedding and sometimes extensive mucosal viral replication. This process is often subclinical, but may also cause symptoms such as cold sores. Antibody avidity, i.e. the compound value for binding strength for a pool of multivalent antigens and antibodies, normally increases during the maturation of an immune response. Avidity testing has diagnostic value, especially in identifying cases of genital HSV with risk for neonatal infections [
5‐
7]. Recently, as HSV1 infection suggests an increase in the risk of developing Alzheimer’s disease [
8‐
13], avidity testing might provide further predictive value [
14‐
16]. ELISA kits, used to assess serum for presence of anti-HSV IgG, are provided by several manufacturers, including kits that distinguish anti-HSV1 from anti-HSV2. Through the addition of a washing step including a chaotropic salt, some HSV2-kits have been validated to also provide avidity data. HSV1 type specific kits have been experimentally modified in a similar manner by several research groups [
5,
6,
14,
15], but no validated kit is, to the best of our knowledge, currently commercially available. For use in future studies, we established the current protocol, HSV1 avidity index (HSV1 AI) with the aim of reproducible determination of anti-HSV1 IgG avidity.