A kinase is used in a biological indicator for validation of treatment processes designed to reduce the amount or activity of a biological agent in a sample. The indication can be used for validation of sterilization treatment. The formation of ATP from a substrate comprising ADP is measured via the liciferin/luciferate system in a luminameter. Thermostable adenylate kinase from sulfolobus acidocaldarius is especially suitable for the validation of procedures to inactivate transmissible spongiform encephalopathy agents.