Subjects, Procedure and Context
This study was embedded within usual care of an outpatient work rehabilitation setting. From January 2011 to January 2012 eligible participants were referred for an interdisciplinary rehabilitation assessment at the rehabilitation clinic in Bellikon (Switzerland) by insurance physicians or case managers of Swiss Accident Insurance Fund (SUVA). Participants were from the German-speaking part of Switzerland. The main reasons for referral included: (1) not regaining full work capacity (WC) within 6–12 weeks after a whiplash injury; (2) exceeding expected healing times; (3) or having plateaued with the provided medical and rehabilitative care. Inclusion criteria were: injured workers with WAD related neck pain and, Grade I or II according the Québec Task Force Classification with reduced working capacity of their actual job. They were within 6–12 weeks after initial injury, and received worker’s compensation benefits.
Ethical approval for this study was granted by the Medical Ethics Committee of the Canton Aargau (EK AG 2010/055). Patients gave consent that their data were used for research purpose.
At base line a review of the medical history and a physical examination was performed by a rehabilitation physician (approximately 60 min), followed by FCE tests administered by a physiotherapist. After determination of eligibility, patients completed questionnaires and carried out FCE tests (60 min). Fitness-for-work certificates or work capacity settlement were explicitly not part of this interdisciplinary assessment.
All participants were insured by SUVA, the largest state owned accident insurance in Switzerland. SUVA covers costs for occupational and non-occupational injuries for employed individuals and unemployed job-seeking persons . Injured persons receive compensation up to a maximum of 80 % of the previous salary, and medical and vocational assistance. Invalidity pensions can also be refunded by SUVA to the injured person.
The SFS was used to measure self-reported functional ability to perform work-related tasks and activities of daily life that involve the spine. The SFS contains 50 drawings with simple descriptions (Item example in the Fig. 1). Patients rated their functional ability for each activity on a 5-point Likert scale: “able” (4), to “restricted” (1, 2, 3) or “unable” (0). The SFS yields a single rating ranging from 0 to 200, with higher scores indicating more or better abilities. The scores can be categorized according the work demands as defined by the Dictionary of Occupational Titles (DOT) . SFS scores have been adapted to the DOT categories previously as follows : SFS score <100 ≈ minimal work demands, 100–124 ≈ sedentary work (<5 kg), 125–164 ≈ light work (5–10 kg), 165–179 ≈ medium heavy work (10–25 kg), 180–194 ≈ heavy work (25–45), >195 ≈ very heavy work (>45 kg) These categories allow a comparison between the self-reported functional ability and work demand. For test–retest reliability of the SFS a sample of patients was tested twice within a week after baseline.
Physician Determined Work Capacity (WC)
To determine the predictive validity for future work status, the WC was used as an estimate of ability of work. The WC was obtained from the accident insurance’s administrative data. WC was determined at 1, 3, 6 and 12 months after baseline by the treating physician, usually a general practitioner, and represents the proportion workability of pre-injury work. Determination of WC was based on proposed WC-forms and recommendations [11, 12]. WC is expressed in a percentage (0–100 %) and is translated in days or hours modified work. For example, if a worker is deemed WC = 50 %, he will work for 2.5 days/week or 5 half days/week modified work. The remaining 50 % is financially compensated. The reliability and validity of the WC determination is unknown. WC in %, is directly related to compensation costs and reflects the proportion of work loss to the employer, the employee and the insurance. Therefore, this method of WC-determination may be less dependent to distortion compared self-reported measures of WC .
FCE is a standardized battery of functional tests that intend to measure a patient’s safe physical ability for work related activity . For the purpose of this study four lifting tests were analyzed: lifting floor to waist, lifting waist to overhead, short two handed carry, long one-handed carry (right). Patients were asked to perform the test to their maximum ability. The tests have good reliability and acceptable agreement in patients with WAD .
Pain intensity was measured with an 11-point numeric rating scale (NRS) ranging from no pain (0) to worst pain (10) . The patient was asked to rate his momentary pain (“pain now”). The NRS is a commonly used scale with proven reliability and validity in patients with neck pain .
Neck pain-related disability was measured with the Neck Disability Index (NDI) . The NDI contains 10 items: pain intensity, personal care, lifting, reading, headaches, concentration, work, driving, sleeping, and recreation. The scale of each item ranges from no disability (0) to total disability (5). A higher score indicates more severe self-reported disability. The NDI is reliable and valid in several languages and settings [18, 19].
The Hospital Anxiety and Depression Scale (HADS) was used to assess the symptom severity of anxiety disorders and depression in non-psychiatric populations . The HADS consists of two scales, one for anxiety and one for depression (A- and D-scale respectively). Each scale contains 7 items, with each item rated from 0 (best) to 3 (worst). The scale scores are calculated by summing the responses to the items up to a maximum score of 21 points (severe case) per scale. A higher score indicates more severe anxiety or depression. Good reliability, validity have been reported for the use of the HADS in the general and various clinical populations [20, 21].
Normal distribution was visually assessed using P–P plots and tested with the Kolmogorov–Smirnov and the Shapiro–Wilk tests. Floor and ceiling effects for the SFS were considered to be present if more than 15 % of participants achieved the lowest or highest possible score of the items .
Internal consistency was assessed by item-to-total correlations and Cronbach’s alpha. Optimal consistency for measurements at group level was considered when alpha value is between 0.7 and 0.9. Values <0.7 may be indicative for items measuring different traits, values >0.9 may be indicative for item redundancy .
The unidimensionality of the 50 SFS items was measured with principal component analysis (PCA) with Kaiser normalization and Varimax rotation. An Eigenvalue criterion of 1.0 was used for the factor analysis. Unidimensionality was assumed when ratio of the first to the second factor was 3:1 .
Test–Retest Reliability and Agreement
Test–retest reliability was expressed as an Intraclass Correlation Coefficient (model 1; one-way random) (ICC). ICC was interpreted as follows: ICC ≥ 0.90 is excellent; good when ICC was between 0.75 and 0.90; moderate when ICC was between 0.50 and 0.75; and poor when ICC ≤ 0.50. ICCs were acceptable when ICC ≥ 0.75, and the lower boundary of the 95 % confidence interval of the ICC ≥ 0.50 . Agreement was expressed in limits of agreement (LoA) (mean difference ± 1.96 × SD of mean difference) .
Construct Validation: Hypothesis Testing
Eight predefined hypothesis on the strength of the association of SFS and four FCE lifting tests, NDI, Pain NRS, and HADS A + D are displayed in Text Box A. The strength of the association is expressed in the absolute value of the correlation coefficient. Associations were calculated using Spearman rank correlation coefficient and interpreted as follows: 0.00–0.25 little if any (“not correlated”); 0.26–0.49 low or weak; 0.50–0.69 moderate; 0.70–0.89 high or strong; 0.90–1.00 very strong correlation . The SFS was considered valid, when 7 out of 8 hypotheses (≥80 %) of the a priori hypotheses were not rejected .
Predictive Validity for Work Status at 1, 3, 6 and 12 months
Sensitivity and specificity, positive predictive value as well as likelihood ratio of a positive test were calculated to evaluate the predictive validity of the SFS items at baseline for work capacity at 1, 3, 6 and 12 months after baseline assessment. In a setting of injured workers, who are in a transition phase from acute to chronic disorder, the aim is to identify those patients with a high probability of not returning to work (N-RTW) in order to target specific rehabilitation interventions to those patients. We used two cut-off points to measure N-RTW i.e. WC < 50 %, or WC < 100 %. These two cut-off points were determined based on distribution-plots of WC. The index test was the SFS. Sensitivity was defined as the proportion of patients, identified for different DOT categories based on the SFS score, not have N-RTW. Specificity was defined as the proportion of patients, identified for different DOT-categories based on the SFS score, who did return to work. The positive predictive value for N-RTW was calculated as the percentage of patients within a DOT category that were correctly identified not to have regained full work capacity. Likelihood ratio was calculated as Sensitivity/1 − Specificity. Based on a previous study, it was expected that “minimal”, perceived ability (SFS score <100, less than sedentary work) score would have a high positive predictive value in identifying those patients who would N-RTW at follow-up times . Receiver operating characteristic (ROC) curves were drawn and area under the curve (AUC) was calculated. The AUC has a maximum value of 1.0, indicating a perfect predictive validity which is reached if the curve lies in the upper-left corner; a value of 0.5, represented by the diagonal, means that the measurement instrument cannot distinguish between patients N-RTW or RTW. An AUC of at least 0.70 is considered “appropriate” . As a cut off indicating statistical significance p < 0.05 was used. All analyses were performed using SPSS (Statistical Package for Social Sciences, Version 21).