The present invention deals with a method of preparing nucleic acids, particularly RNA, from a whole blood sample. The nucleic acids purified by the method of the invention are particularly suited for detection of nucleic acid marker molecules. Preferred are markers for the diagnosis of transmissible spongiform encephalopathy (TSE). Such diagnosis is based on the detection, by means of real-time PCR, of certain mRNAs of the TSE-infected organism. Said mRNAs specifically originate as splicing variants and are isolated from whole blood by the method of the invention.