The invention is related to diagnostic methods for detecting transmissible spongiform encephalopathies (TSEs) such as BSE and scrapie and related disease in humans. The invention provides use of guanidine thiocyanate (gdnSCN) or a functional equivalent thereof for treating at least one sample derived from a mammal, including humans for reducing the risk of scoring a false-positive test result in testing said sample for the presence of aberrant prion protein.