Relatively short term test re-test reliability of Neck Disability Index by long term test re-retest reliability method of Oswestry Disability Index in healthy office workers

1.1 Scoring system and interpretation intervals

NDI and ODI have the same scoring system but disability interpretation intervals are not the same. This issue may be the only important point that can cause mistakes in formulation and interpretation of combine assessment results. In the original NDI study, suggested interpretation intervals was different from ODI; the total score from 0–4 reflects no disability, 5–14 reflects mild disability, 15–24 reflects moderate disability, 25–34 reflects severe disability and 35 and the higher scores reflects complete disability [9] (see Table 1). There are also differed suggestions for interpretation intervals in NDI literature [10, 11].

Also, the minimally clinical important change (MCIC) is different. MCIC for ODI is 9% and conventionally 3 to 5 point is used for NDI but there are studies that suggest higher change values like 10.5 and 10.2 points [12–14]. Also, there is a smaller suggestion for NDI as 1.66 for clinically stable patients [15].

In 2000, Fairbank clearly stated the need for new studies on healthy participants for ODI [16]. Fairbank may have had doubts about the control group response to ODI for both research and clinical applications [17–19].

In our previous study, we focused on long-term test re-test reliability of ODI in healthy male office workers according to the Fairbank recommendations [17, 20]. We have assessed healthy male participants on 1st, 2nd, 4th, 8th, 15th, 30th days to determine the stability of ODI scores over time. We have found that the difference between median score of the days with 1st day was neither statistically (χ= 6.482, p > 0.05) nor clinically significant. We have shown that ODI has long-term test re-test reliability in healthy subjects (n = 36) over a one month time interval [17].

The categories of the NDI studies reported by Vernon in his review is as below; 21 studies on psychometric properties of NDI, 14 studies where NDI is used as a follow up instrument and, 2–14 clinical trials where NDI was used as an outcome measure for different treatment modalities [21]. In psychometric properties of NDI group, only one study has a control group [21]. The minimum baseline NDI score was 2.4 in studies to follow up prognosis [16, 21]. In clinical trials group studies have 1–3 NDI follow up scores and baseline NDI score is not lower than 13 except only one study [15, 21].

There are relatively very long-term studies in the NDI literature (2-3 years to 17 years) when compared with ODI [20, 22]. However, NDI is generally used as a follow-up instrument in these studies. Improvement in health status is calculated according to the base line scores instead of healthy controls as in ODI studies [21]. For this reason, the test re-test reliability of NDI for healthy control groups may be an open issue in the literature.

In our previous study we have clarified, the normal values of ODI in healthy subjects and test re-test reliability in one-month time interval. This result can be used in clinics for cases where the main disability is in cervical spine. When treating the neck, the clinician can use our previous findings, to identify if is there a significant elevation in disability status of lumbar spine. For inverse cases where the main problem is in the lumbar region, there is a lack of clinical decision support data for the cervical region.

We would like to investigate the stability of NDI in the same time intervals as ODI for the following reasons; the interaction between low back disability and neck disability, the use of NDI and ODI together for disability assessment of the whole spine and common structural origin of ODI and NDI.

The purpose of this study is to investigate relatively short-term test re-test reliability of NDI by long term test re-retest reliability method of ODI on healthy office workers.

2.1 Population

Participants who have no chronic neck pain history were included in this study. The incidence of current neck pain was evaluated via visual analogue scale (VAS) prior to the study to ensure that participants met the inclusion criteria.

Participants who have following conditions were excluded from the study; chronic neck pain history, higher NDI scores showing more than a mild disability level, acute neck pain during the study, previous surgical operations related to neck pain, previous physical therapy innervations related to neck pain, inability to speak Turkish fluently, inability to complete questionnaires by themselves, or request not to participate the study [17].

2.2 How were the subjects recruited?

Subjects were invited to join the study by invitation e-mails. E-mail invitations were sent to administrative staff members and engineering faculty students. The study was described in details to the candidates who responded to the invitation mail. The candidates who accepted to participate in the study voluntarily and meet the study acceptance criteria were included in the study. All subjects gave their written consent.

The study was approved by the authorized local ethical committee for human researches (35877407/2015).

2.3 Data collection

Data collection was performed using a web-based NDI database. Participants were invited to complete web-based NDI forms by reminder e-mails. Reminder e-mails were sent to participants on Days 1, 2, 4, 8, 15, 30 [17]. NDI forms were requested to be submitted by the participants within 24 hours. Late submissions are not processed and participants were excluded from the study [17].

Most of the subjects were administrative staff members who worked 8 hours per day. The rest of the participants are engineering faculty students who have at least 5 hours of computer use per day [23–25].

2.4 Data analysis

A Kolmogorov-Smirnov test was used to check whether the data were normally distributed or not. A Friedman test was used to investigate the statistical significance of the differences between the NDI scores on the 1st day through the 30th day [17].

All tests were performed by using SPSS 11.

3.1 Subjects

When the study had completed; 49 (20 female, 29 male) out of 106 (57 female, 49 male) were left for a response rate of 42%. The number of excluded participants and the reasons are as follows; 21 subjects whose first assessment scores were higher than the healthy population, 22 subjects did not answer the first NDI assessment properly and 14 subjects did not complete and submit the re-test questionnaires in 24 hours.

The mean age of the participants were 26.46±6.19 years old. The educational status of participants were bachelor degree (administrative staff members) or bachelor degree candidates (engineering faculty students).

NDI responses were not normally distributed. There is a floor and ceiling effect and data were evaluated by a non-parametric approach. The mean score of 294 questionnaires (6×49 = 294) over highest possible raw score (50) was 4.64±2.83 (median 5, IQR 0–4) (see Table 2). The mean score of all questionnaires was over the highest limit of no disability range ([0–4]) and under the lower limit of mild disability range ([5–14]). The scores of all subjects were between 0 and 12.

NDI daily scores were not normally distributed (KS = 0.078 p < 0.05). There was no statistically significant difference among the median scores of each day (χ= 9.275, p > 0.05). The median score difference from day1 is presented in Table 3 and Fig. 1.

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Fig. 1

(a) Median NDI scores at different time intervals. (b) Mean NDI scores at different time intervals. Solid line represents the mean NDI scores. Dotted line represents the ±1 standard deviation.

Number of subjects in 3-point score interval is presented in Table 4.