DNA impurities in a composition comprising a parvoviral virion

A method for identifying and quantifying a nucleic acid impurity in a composition comprising a parvoviral vector, comprising:

• (a) subjecting the composition to nucleic acid sequencing to obtain at least a thousand random reads of nucleotide sequences;
• (b) comparing, using a processor, the at least a thousand random reads from step (a) with a nucleotide sequence of a biological component used in a process for producing the composition, whereby a match between a random read and a nucleotide sequence of a biological component identifies a nucleic acid impurity;
• (c) determining an average number of reads per parvoviral vector;
• (d) determining a distribution of reads by calculating a number of reads per nucleotide of an identified nucleic acid impurity;
• wherein an identified nucleic acid impurity is identified as an overrepresented impurity when the distribution of reads is not random and the number of reads per nucleotide of a nucleic acid impurity is at least 0.001% the average number of reads per parvoviral vector.