A blood test accurately diagnosed a total of 32 patients with a rare form of prion disease, two new analyses report, offering a potentially valuable tool for preventing prion contamination of the blood supply. The approach successfully detected abnormal prion proteins in the blood of two presymptomatic donors before they exhibited any signs of the disease, paving the way to a noninvasive, early diagnostic screen for prion disease and possibly other conditions involving protein misfolding. Prion diseases are a group of fatal, neurodegenerative disorders caused by infectious prions, deformed proteins that form toxic clumps and damage the brain. Variant Creutzfeldt-Jakob disease (vCJD) is a type of prion disease that stems from eating meat infected with bovine spongiform encephalopathy, commonly known as mad cow disease. Because an infected individual may not show symptoms of vCJD for decades, the only way to definitively diagnose the disease is by postmortem examination of the brain. What's more, silent carriers can potentially transmit the disease through blood transfusion, posing a critical public health problem for blood banks, especially in countries like the U.K., where about one in 2000 people are asymptomatic carriers of vCJD. To tackle this issue, Luis Concha-Marambio et al. developed a blood test using an amplification technology to detect abnormal prion proteins in blood from 14 individuals with vCJD and 153 controls. Daisy Bougard et al. tested a similar technique on blood samples from 18 individuals with and 238 without vCJD. Unlike other blood tests in development, which have demonstrated about 70% sensitivity and fall short of the 90% required by the European Commission Directive, the assay used in both studies diagnosed the disease with 100% sensitivity and 100% specificity. Furthermore, Bougard and colleagues were able to detect small amounts of prions in two blood donors more than a year before the onset of symptoms. The researchers say that their diagnostic screen's promising results will need to be confirmed in a larger number of blood samples.