To the Editor Jackson et al1 reported significant advances in the detection of abnormal prion protein in blood. These could prove helpful in developing strategies to mitigate the risk for variant Creutzfeldt-Jakob disease (vCJD) being transmitted via blood transfusion, as happened in the United Kingdom in 3 known instances in or before 1999. However, care is needed in interpreting test performance, even assuming that a true-positive result necessarily indicates vCJD infection (differing views on which are reflected in the Food and Drug Administration assessment of vCJD risk to US blood recipients2). The results reported by Jackson et al1 suggest a test sensitivity of about 70% and a possible specificity of 99.97% (a false-positive rate of about 1 in 3000) once the test is repeated. The latter is a theoretical figure derived from finding no repeat reactives in a relatively small control study. The article then turns to the key question of positive predictive value (PPV), considering what a repeat reactive result would mean if true prevalence of vCJD were 1 in 2000, as suggested by a recent survey of UK appendix samples.3 It refers to a PPV of about 78% (where 78% of positive results would be true) and an unwary reader could understand this to refer to a test with the performance figures just given. However, this is not the case.
Bennett P. Blood Test for Variant Creutzfeldt-Jakob Disease. JAMA Neurol. 2014;71(8):1054. doi:10.1001/jamaneurol.2014.944