Background: Recent studies have used self-administered screening scales in community and clinical samples to identify individuals who probably or likely had the disorder of interest. A better understanding of the statistics of screening, specifically positive predictive value, would indicate that the conclusions drawn from these studies are not justified.Methods: The principles and statistics of diagnostic screening and how screening is distinguished from case-finding are reviewed, followed by a review of studies that have failed to consider the positive predictive value of the screening scales in the samples studied.Results: Multiple studies of both clinical and general population samples have used screening measures as case-finding instruments. For example, two recent studies of response to electroconvulsive therapy in depressed patients used a screening scale for borderline personality disorder (BPD) and concluded that the patients with and without BPD responded equally well to treatment. However, the positive predictive value of the screening scale in these studies was less than 50%, meaning the majority of patients considered to have BPD would not have been so diagnosed if interviewed. A similar problem has also been observed in studies using screening scales for bipolar disorder in general population and primary care settings.Conclusions: When studying a disorder with a relatively low prevalence, it is near impossible for a screening test to have sufficient positive predictive value to be used to validly compare the individuals who do and do not screen positive. Researchers using screening measures as diagnostic proxies need to discuss the issue of positive predictive value.