The past couple of decades have seen growing concern over sick leave in working populations in Western societies as a public health problem. Sick leave has a multi-factorial aetiology [1–3]. Several scientific approaches have therefore been used which may explain the variety of approaches used to measure sick leave within an epidemiological framework: frequency of sick leave spells per individual, the total length of absence during a specified period, incidence rate, cumulative incidence and duration of absence spells . However, in light of the large number of studies on sick leave, it is remarkably that only few validation studies have been performed.
Among the four traditional sources (employer’s personnel files, insurance-based data, national social security registers and self-reported data) from which sick leave data are traditionally retrieved, register-based sick leave data is an option available only in few countries. Even where registers are available, self-reported sick leave data are usually more easily acquired than data from other sources. Company-based data retrieved from employers’ personnel files is considered a golden standard, mainly because these data are also used for calculating earnings [5–8].
To identify sick leave measure validation studies, a systematic literature search was performed in PubMed. The search terms “Sick leave”, “absenteeism”, “presenteeism”, “work”, “registries”, “self report”, “questionnaires”, “reproducibility of results”, “validity/validation”, “sensitivity and specificity”, “predictive value of tests”, and “accuracy” were combined in the search. Twelve validation studies were selected. The validity of sick leave data reported in questionnaires or in interviews was studied and analysed against data retrieved from employers’ personnel files [5, 6, 8–13], insurance companies [9, 14–16] and a national social security register . None of the studies validated insurance-based data either from companies or from national social security registers against employers’ personnel files.
The validation studies found discrepancies between self-reported length of absence and insurer-reported compensation payments. The inconsistencies spoke against self-report and were associated with work status, cause of absence as well as personal characteristics [14, 15].
A study from the Netherlands found poor agreement between workplace-registered sick leave data and data collected in a questionnaire . The ability of this questionnaire to detect frequency of sick leave spells was reported to have a sensitivity of 55% and specificity of 83%. The remaining nine studies concluded that self-reports yielded acceptable validity [5, 6, 8–10, 12, 13, 16, 17]. The total length of absence was the most widely used measure of sick leave [5, 6, 8–10, 12, 13, 16], but also prevalence [5, 12, 17], frequency of spells , incidence and duration of sick leave spells  were used.
Recall periods ranged from 2 weeks to 4 years and were discussed in several papers [6, 9, 10, 17]. These studies were largely unanimous that shorter recall periods could increase the precision of self-reported sick leave. In two papers, the optimum recall period was recommended to be no longer than 2–3 months to obtain valid measures of absence lengths [6, 10].
The Whitehall II study and a Swedish replication of Whitehall II found a worse recollection the longer the absence length [8, 13] which indicates that valid self-reporting may be limited to absence of short duration. Finally, relatively high sensitivities were found in studies where data on absence length, frequency and prevalence of absence were provided as pre-specified categorical questionnaire options [5, 17].
In Denmark opportunities for register-based research are unique . Data on social public transfer payments, like sickness benefits are registered on a weekly basis in a national register called DREAM . Employees’ sickness benefit paid in excess of two weeks is refundable from the municipality according to the Danish Sickness Benefit Act . DREAM data on sickness benefit has been used for follow-up studies, where return to work (RTW) [21–27], sick leave defined as absence > 2–3 weeks [28–32] and long-term sick leave defined as absence >8 weeks [29, 31–40] have been used as endpoints. Study populations have also been defined from the DREAM register [24–26, 29, 41, 42]. One attempt has been made to validate the DREAM register . In this study a random sample of 5,221 Danish citizens were asked about which kind of income they received in a particular week in 2001. According to the DREAM register 82 persons were receiving sickness benefit and of those 38 responded this kind of income; yielding a positive predictive value of 31.7%. The article concluded the DREAM register to be a feasible tool for social and economic research in Denmark. DREAM data on sickness benefit has so far not been validated against workplace-registered sick leave.
1) To validate registered sickness benefit data from DREAM against workplace-registered sick leave spells of at least 15 days. 2) To validate self-reported sick leave days during one year against workplace-registered sick leave.