Established biodosimetry methods include the determination of dicentric chromosome fragments and the formation of micronuclei. For this purpose, lymphocytes from the peripheral blood count are examined. To date, these cytogenetic changes have been used to estimate individual doses. Acute radiation damage, subsumed under the term acute radiation sickness (ARS), is based on sudden massive cell death, consecutive loss of organ function, and the involvement of many organs, including non-irradiated ones, due to the radiation-induced systemic inflammatory reaction. Four ARS severity levels were defined by a European consortium of experts and clinicians (METREPOL). The severity determines the prognosis and therapy. As the dose increases, ARS severity can be expected to increase. However, other facets of radiation exposure, such as dose rate or radiation quality, as well as differences in inter-individual radiation sensitivity, also contribute significantly to cell death and, thus, the ARS severity. This makes estimating ARS severity based on dose alone problematic and challenging for the clinician to interpret. An interface between radiobiological life-threatening changes and the clinic should be created in this situation. Triage does not require an exact dose estimate but rather a prediction of the ARS severity. It is also sufficient to differentiate between three clinically relevant groups of people, as stated above. In this proposal we want to investigate if established cytogenetic biodosimetry methods can be used to triage clinically relevant groups of people.