The lidA study is a cohort study on work, age, health, and work participation. It aims at examining the long-term effect of work on the health and work participation of older working employees based on a sample from two age cohorts born in 1959 and 1965, respectively [28]. They present the beginning and the end of Germany’s so-called baby boomers who will make up a significant share of the potential older work force in the coming years [29]. The sample was drawn from registry data pertaining to the integrated employment biographies (IEB) of the German Federal Labour Office (Bundesagentur für Arbeit (BA)). It is based on all gainfully employed people covered by social insurance in Germany on December 31, 2009. Public servants and self-employed persons are not part of the study population. The utilization rate is 27.3% [28]. During the computer-assisted interviews (CAPI) of the first two waves in 2011 and 2014, participants were asked for their informed consent to have their interview data linked individual with their claims data. During the second survey in 2014, those participants who had changed statutory health insurance funds were asked again for their consent, as well as those who had not given their consent during the first survey or if other mismatches had occurred [30,31]. A total of 63 percent of all respondents who participated in both survey waves agreed to have their survey and claims data linked [31]. Cooperation agreements with eleven statutory health insurance funds were signed [32]. Finally, data on outpatient and inpatient treatment as well as sick leave and outpatient drug prescription provided by a total of ten statutory health insurance funds[1] were used for the linkage [33-35].
A total of 4,244 persons participated in the follow-up [31]. Of these only those participants were included in the validation study for whom linked survey and claims data was available (n=1,031) (Fig. 1).
During both waves of the primary survey (first wave: 2011, second wave: 2014), study participants were asked exhaustively about their work situation. As regards back pain, we used the questionnaire for the analysis of musculoskeletal symptoms (Nordic questionnaire) [36]. To this end, participants had to answer the following question: “Did you experience any symptoms or pain in the following parts of your body during the past 12 months?” Possible answers included “upper back or thoracic spine” and “lower back or lumbar region”. The analysis included all respondents, who affirmed the question of back pain during the past twelve months (self-reported prevalence) in both waves.
The claims data comprised both in- and outpatient data as well as data on sick leave for the period from 2009-2013. Due to unspecific medication and a lack of means to assign it to specific diagnoses, we decided not to use outpatient drug prescription data. Besides, the data would have comprised subscription pain medication only. Data on self-medication using over-the-counter (OTC-)analgetics is not documented in claims data. Back pain is coded using the ICD-10-Code M54 (“back pain”) when it is diagnosed either as a principal or secondary diagnosis made by hospitals, as an outpatient diagnosis (classified as ‘confirmed’ or ‘condition after recovery’), and in medical certificates of sick leave. Two different definitions are used to define administrative prevalence in claims data:
- Definition 1 (Def1): A person is considered to suffer from back pain if “M54” was stated at least once in one of the three sectors between 2009 and 2013.
- Definition 2 (Def2): A person is considered to suffer from back pain if the person received two “M54” diagnoses in at least two quarters within four consecutive quarters (M2Q criterion) [37] across (all three) sectors between 2009 and 2013.
There seems to be a gap of three years, because claims data were used for the whole years 2009 – 2013 but the two questionnaires conducted in 2011 and 2014 respectively. Since we use all respondents, who reported back pain in both waves, the period of claims data is almost covered. With the definition of back pain in the CAPI data (back pain in both waves) and the definitions of back pain in claims data, especially in Def2, the study looks less at actually back pain than at chronic back pain.
Statistical analyses
First, self-reported and administrative prevalence of back pain was determined descriptively. The agreement of survey and claims data was determined using Cohen’s Kappa. A Cohen’s Kappa < 0.40 indicates a low level of agreement, a Kappa between 0.41 – 0.60 means a moderate level of agreement and a Kappa between 0.61 – 1 means a high level of agreement [38]. The overall prevalence of back pain was calculated as the sum of self-reported and administrative prevalence. In order to examine possible differences between the two cohorts as well as between women and men, Cohen‘s Kappa was then analyzed carefully according to cohort affiliation and gender. Sensitivity and specificity as well as the positive predictive value and the negative predictive value were not calculated because of the missing gold standard and deviations in both directions.
The analyses were performed using IBM SPSS 24 ©.
[1] The data oft he following health insurance funds are part oft the cumulative data set: AOK Bremen/Bremerhaven – Die Gesundheitskasse, AOK – Die Gesundheitskasse für Niedersachsen, AOK Nordost – Die Gesundheitskasse, AOK NORDWEST – Die Gesundheitskasse, AOK Rheinland/Hamburg – Die Gesundheitskasse, AOK Rheinland-Pfalz/Saarland – Die Gesundheitskasse, AOK Sachsen-Anhalt – Die Gesundheitskasse, BARMER GEK, IKK gesund plus, Techniker Krankenkasse [33].