 overall health of patients with depression can be improved when patients receive appropriate therapies [7,8]. Screening alone does not improve the health of patients with undiagnosed depressive disorders [9][10][11][12] but screening combined with patient-support programs, such as regular nurse follow-ups and close monitoring of adherence to therapy, seems to be useful [13]. Therefore, the U.S. Preventive Services Task Force recommends screening for depressive disorders in the general adult population when there are staff-assisted depression care supports in place to assure accurate diagnosis, effective treatment, and follow-up [14]. Additionally, screening for depressive disorders is recommended in populations at risk such as those with a family or personal history of depressive disorders, multiple medical problems, unexplained physical symptoms, chronic pain, or use of medical services that is more frequent than expected even if no depression care supports are available [15].
For screening purposes, different instruments exist [16]. Administering and evaluating comparatively long screening instruments can be time-consuming and it may thus be difficult to implement them in busy clinical settings [17]. Simple tests focusing explicitly on depressive disorders and without the need for additional computation on the clinician's side seem to have the highest probability that this information is integrated into the clinical decision-making process [18]. In the context of comprehensive research evaluations long instruments may increase respondent burden [19]. This is why research teams searching for the shortest possible measure proposed and evaluated screeners consisting of one or two items [20]. Williams et al. presented a simple and easy-to-administer single item question ("Have you felt depressed or sad much of the time in the past year?") and reported good sensitivity and less specificity as compared to the Center for Epidemiologic Studies Depression Screen (CES-D) [21] using a diagnostic SCID interview (Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders) as the criterion (85% vs 88% and 66% vs 75%, respectively) [11]. In contrast, Corson et al. found this single item to be specific (88%) but less sensitive (78%) when using the 9-item Patient Health Questionnaire (PHQ-9) [22] algorithm for major depression as the reference standard [20].
Given these discrepancies and given the fact that the previous studies were conducted in very specific study populations (predominantly female Hispanics or veterans in the USA), this study evaluates measures of test performance of the Williams et al. single-item screener in comparison to PHQ-9 in a population-based sample of adults from Germany. The aim of this study is to conclude on the utility of this single item screener to screen for depression in the general population.

Study design and subjects
The data stem from the city of Augsburg (Bavaria, Germany) and from surrounding districts covering about 600,000 inhabitants drawn from mixed urban and rural areas whose demographic and socioeconomic characteristics roughly reflect those of the average middle European population in general. The present analysis investigates data from the population-based KORA F3 survey conducted in 2004/05 within the framework of the ongoing KORA project (Cooperative Health Research in the Augsburg Region, Germany), a research platform for population-based health research [23]. The KORA F3 survey is a follow-up survey to the MONICA S3 survey conducted in 1994/95-at that time one cooperative centre within the worldwide WHO MONICA (Monitoring Trends and Determinants on Cardiovascular Diseases) project investigating the general and cardiovascular health of diverse populations. For the MON-ICA S3 survey, a stratified random representative sample of 6481 eligible subjects was drawn in 1994/95 from the population, of whom a total of 4856 subjects (response rate: 74.9%) participated in the S3 baseline survey. By the F3 follow-up study one decade later (2004/05), a total of 405 (8%) subjects had died. Furthermore, subjects were considered ineligible for inclusion in the F3 follow-up survey if they lived too far outside the study region or were completely lost to follow-up (n = 222, 5%), or had demanded deletion of their address data (n = 270, 6%). Of the remaining 3959 eligible subjects, 161 could not be contacted, 295 were unable to come because they were too ill, and 497 were not willing to participate, resulting in an interim total of 3006 participants in the F3 follow-up survey (response rate: 76% of S3 participants). Furthermore, additional efforts were made to reach those 1300 eligible subjects from the original S3 sampling frame who had not participated in the S3 baseline survey. Thus, another 178 (14%) participated in the present KORA F3 study, for a total sample size of 3184 (overall response rate: 49.12%). Written informed consent was obtained from each study participant and the study was approved by the ethics committee of the Bavarian Medical Association.

Instruments
All participants underwent a standardized face-to face interview including the Patient Health Questionnaire and the single item screener and an extensive medical examination. The interviews were performed by experienced study nurses at the KORA Study Centre, Augsburg. Before start of the study, they received an extended training program and were certified thereafter. All interviews were taped and subjected to a routine quality assessment in the KORA data centre to avoid bias. At study halftime, all interviewers were recertified. Depression was assessed in an interview version of the 9 item depression module of the Patient Health Questionnaire (PHQ-9) [24]. Patients rate the frequency of symptoms of depression over the past 2 weeks on an ordinal scale (0 = not at all, 1 = several days, 2 = more than half the days, 3 = nearly every day). The 9 items are based on the 9 DSM-IV criteria for the diagnosis of depression. The total score ranges from 0 to 27. In order to be congruent with the DSM-IV criteria, the algorithm developed and validated by Spitzer et al. was used for classification: "Major depressive disorder" was defined as having at least five questions answered with "more than half the time in the past two weeks", of which at least one of the first two questions (little interest in doing things, feeling depressed) had to be included. Participants were labelled to have "depressive mood" when 2 to 4 questions were answered with "more than half the time in the past two weeks", also including one of the first two questions of the PHQ-9 questionnaire [24]. PHQ-9 was used as reference standard in this study because it has been shown to have a sensitivity of 88% and a specificity of 88% for major depression compared with diagnostic SCID interviews [24] as well as concurrent validity, high internal consistency, and test-retest reliability [25].
The single item screener "Have you been depressed or sad most of the past year?" uses a yes/no response format [11]. Based on a frequently used question for medical history taking, this single item question has been developed in the context of a randomised controlled trial of case finding for depression. The sample was predominantly female and Hispanic and was recruited at family and internal medicine clinics in the United States. Consecutive patients were randomly assigned to be asked the single item screener, to fill out the 20-item (CES-D), or to usual care. Corson et al. reported a LR + of the single item screener of 6.77 and an area under the ROC of.83 (95% confidence interval (CI).79,.87) [20]. The single item screener was administered directly in advance to the PHQ-9.

Statistical analyses
Firstly, the distribution of socio-demographic and clinical characteristics across the study sample was calculated for description of the study population. Secondly, we calculated several measures of test performance of the single item screener in comparison to the reference standard PHQ-9. This was done for the PHQ-9 "depressive mood" definition as well as for the "major depressive disorder" definition based on a 2×2 table (see Table 1). Specifically, we calculated the prevalence of persons with "depressive mood" and of "major depressive disorder". Sensitivity (the proportion of persons having depression according to the PHQ-9 who test positive in the single item screener), specificity (the proportion of persons without the disease according to the PHQ-9 who test negative in the single item screener), receiver operating characteristic (ROC) area, the positive likelihood ratio (LR+, the probability of a person who has the disease according to the PHQ-9 and tests positive in the single item screener divided by the probability of a person who does not have the disease and tests positive), the negative likelihood ratio (LR-, the probability of a person who has the disease and tests negative divided by the probability of a person who does not have the disease and tests negative), the positive predictive value (PPV, the proportion of persons testing positive in the single item screener who have the disease), and the negative predictive value (NPV, the proportion of persons testing negative in the single item screener who do not have the disease) of the single item screener in comparison with either PHQ-9 depressive disorder definition were calculated, including 95% confidence intervals. The ROC is a graphical plot of the fraction of true positives out of the total actual positives (sensitivity) vs. the fraction of false positives out of the total actual negatives (1-specificity), at various threshold settings. The area under the ROC is a measure for test accuracy with a value of 1 representing a perfect test and an area of 0.5 representing a worthless test. These analyses were repeated stratified for age group (34-44, 45-54, 55-64, 65-74, 75-85 years) and gender (female, male), two variables known to be linked with