The current invention relates to nucleic acid impurities in a composition comprising a parvoviral vector. In particular, the current invention shows that DNA impurities are not randomly encapsulated within a parvoviral virion. The invention therefore relates to a method for identifying and quantifying a nucleic acid impurity in a composition comprising a parvoviral vector. Finally, the current invention relates to method of determining whether a composition comprising a parvoviral vector is regarded as clinically pure.