The data collection and methods are described in detail in a previous article [18], but will be presented briefly here.
In Denmark, all patient contacts with hospitals, both inpatient and outpatient, are recorded in the National Patient Registry with a primary diagnosis and any optional secondary diagnoses [19]. It includes data from all patient contacts, and thus, it is representative of all patients diagnosed with ‘neck injuries’ seen in a hospital. More specifically, data containing the ICD diagnostic code of ‘cervical-column distortion’ (S134) were extracted from the registry for this study. Data were available throughout the 12-year observation period from 1998–2009. The pre- and post periods covered up to 3 years before and after the diagnosis year, but patients were only included in the calculations if they had at least 1 year prior to or post the diagnosis year in the observation period. That is, the patients included in the co-morbidity analysis were diagnosed between 1999 and 2008. For the recording of mortality, all available years within the 12-year period after the individual injuries were included. Thus, all individual data on hospital co-diagnoses can be traced retrospectively and/or prospectively.
All patients with a first-time diagnosis of a neck injury were included, irrespective of which type of accident had led to that diagnosis, and irrespective of whether or not the diagnosis was a primary or secondary diagnosis. There is some underestimation of the national number of patients with neck injuries, because those who contact the primary care sector, in contrast to the hospital sector, are recorded as having had contact, but without necessarily a diagnosis being recorded.
To assess what neck trauma this diagnosis actually covered, we sampled all patient records with this primary diagnosis from the emergency department of Glostrup University Hospital from 2 Jan 2013 to 10 June 2014. We then selected all those born on the first day of any month (irrespective of the month or year of their birth), then added all those born on the second day of any month, and so on, until 100 consecutive patients had been selected. We then categorised the trauma mechanism of the injuries of these 100 people into: whiplash from rear-end collision/whiplash from frontal collision/whiplash with direction not indicated/whiplash-like, not due to a car accident but neck injury sustained through a head trauma/other trauma.
By examining the National Patient Registry, we identified all patients whose first diagnosis of neck injury was in the period from 1998 to 2009. This was chosen because the first year that valid hospital cost data were available was 1998 and the last was 2009, and because we had used this interval for the other studies assessing health care system costs [2, 19]. Then, using data from the Civil Registration System Statistics Denmark Database (which includes information about social factors, marital and cohabiting status, incomes, pensions), we randomly selected matched control subjects with the same age, sex, and marital status as each individual patient, but without a registered diagnosis of neck injury. Parity of socio-economic status was achieved by selecting controls from each patient’s county of residence. A patient-to-control ratio of 1:4 was used to optimise the representativeness of the controls. Data from patients and control subjects who could not be identified in the Coherent Social Statistics database were excluded from the sample. All of these characteristics in both groups were successfully matched.
The patients and control subjects were followed over the entire study period. Specifically, we recorded health data for up to 3 years before, and up to 3 years after, the year of injury. Thus, for individuals with an injury e.g. in year 4 (=2001) in the 12-year period, both the injured and the matched controls were tested for co-diagnoses for the years 1–3 (i.e. before the injury) as well as the years 5–7 (after the injury). Those with a recorded injury in the years 2–3 as well as 10–11 contributed less before and after data respectively. Those with injuries that occurred at the beginning of the 12-year period and who could therefore not contribute ‘before data’, and those with injuries that occurred at the end of the 12-year period who could not contribute ‘after data’, were not included in the calculation of co-morbidity. However, all mortality data from 2009 were included.
Evaluation of morbidity by recorded diagnoses before, and morbidity/mortality after a neck-injury diagnosis
Information before and after each neck injury diagnosis was extracted from the database for the years 1998–2009. Pregnancy, childbirth and post-partum periods were considered irrelevant for our study, and not included in our calculations. However, for completeness, they are included in Tables 3 and 4. Likewise, comorbidities occurring in <1 % of patients or controls were not included. Morbidity data were extracted as primary and secondary diagnoses and further classified into main disease groups, in accordance with the ICD-10 criteria from the World Health Organisation (WHO). A conditional logit model was used to estimate odds ratios (ORs) with 95 % confidence intervals (CIs) for each disease group separately.
Statistical analysis
Statistical analyses were performed using SAS 9.1.3 (SAS, Inc., Cary, NC, USA). These took the form of conditional logit models. The dependent variable was the binary variable for case-control groups, and the independent variables were dummy variables for each of the 21 co-morbid diagnosis groups listed in Tables 3 and 4.
The results are presented as ORs with their associated 95 % CIs and p-values. ORs are estimated in the conditional logistic regression where case/control is on one side and co-morbidities are on the other. Extreme values were manually validated, and no errors were identified.
Ethics
The data extraction from the National Patient Registry was approved by the department of Research Assistance (Forskerservice) – under the Danish Ministry of Health.
The records held in The National Patient Registry (LPR) and Civil Registration System Statistics Denmark (CPR) are not publicly available, and permission to get data is only given by written permission from in accordance with the Data Protection Act § 10, stk. 3 (our approval nr. 2012-54-0271) [20].
Data were analyzed on Statistic Denmark secure server. All data were anonymized by means of a project-specific key by Statistics Denmark before data were entered into the researcher computer (all identifying variables such as CPR numbers, addresses etc. are replaced by project specific random numbers). Researcher are not allowed to download any datasets and all output are aggregated to an extent that eliminates any risk of direct or indirect identification of persons before results are downloaded.
In relation to this Study and the data used for the Article in question, we applied for and were given a permission by the Research Assistance, and data was supplied anonymously. The consent of this public authority by law negates the need for the consents of the patients, which thus does not have to be individually obtained in accordance with the Data Protection Act § 10, stk.3 [21].
Ethical approval in Denmark under the Committee Act is only relevant for studies involving intervention or biological material. The Regional Research Ethics Committee is an Institutional Review Board (IRB), and thus there was no need to obtain permission from them, because our study involved register data only.
Also approval from the National Board of Health to review 100 neck injury patient records was given (nr: 3-3013-436/1).