Test–retest reliability of an infectious disease questionnaire and evaluation of self-assessed vulnerability to infections: Findings of Pretest 2 of the German National Cohort

INTRODUCTION/OBJECTIVES: Large scale population-based studies focusing on infectious diseases are scarce. This may be explained by methodological obstacles concerning ascertainment of data on infectious diseases requiring, e.g. collection of data on relatively short-termed symptoms and/or collection of biosamples for pathogen identification during a narrow time window. In the German National Cohort (GNC), a novel self-administered questionnaire will be used in addition to biosampling to collect data on selected infectious diseases and symptoms. The aim of this study was to evaluate in Pretest 2 of the GNC newly added items on self-assessed vulnerability to several infectious diseases and to assess test–retest reliability of the questionnaire. METHODS: The study was conducted in two study centres (Hamburg and Hanover) during Pretest 2 of the GNC. A self-administered paper questionnaire was applied. In Hamburg, participants were asked to fill in the questionnaire during their regular visit at the study centre. For test–retest reliability, participants in Hanover filled in the same questionnaire at home twice. To evaluate agreement, item-related percentage agreement and kappa (κ) were calculated. In addition, we computed Bennet’s S and Krippendorf’s alpha (α). Items on self-assessed vulnerability to infections were evaluated by comparing them with the corresponding self-reported frequency of infections. An explanatory factor analysis was applied to construct the scores of self-reported infection frequency and self-assessed vulnerability to infections. RESULTS: The evaluation of the internal consistency of the five-item instrument of self-assessed vulnerability to infections resulted in a Cronbach’s α of 0.78. The factor analysis yielded evidence of one factor. The factor was divided into three groups (lowest quintile classified as “less prone to infections” compared to peers; second, middle and fourth quintiles classified as “similarly prone to infections” and highest quintile classified as “more prone to infections”). Participants classified as “less prone to infections” reported fewer infections than participants classified as “more prone to infections”. Spearman’s correlation of the two scores (self-reported infection frequency and self-assessed vulnerability to infection) was 0.50 (p < 0.0001). For quantifying reliability, 88 participants with a median time of 8 days between filling in both questionnaires could be included in the analysis; for items sensitive to disease occurrence between both questionnaires only participants with no relevant disease in this time interval were included (n = 75). The weighted κ ranged between 0.65 and 0.87 for the items on infectious disease frequency in the last 12 months, for items on symptom frequency in the past 12 months between 0.77 and 0.90, and for items on vulnerability compared to peers between 0.68 and 0.76. CONCLUSION: A five-item instrument on self-assessed vulnerability to infections seems to be promising, but requires further evaluation. Overall, the questionnaire on self-reported infectious diseases used in Pretest 2 of the GNC is a moderately reliable instrument and, thus, can be applied in future studies on infectious diseases. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi: 10.1007/s00103-014-2045-x) contains supplementary material, which is available to authorized users.