Cross-cultural adaption and validation of the Swedish version of the Core Outcome Measures Index for low-back pain

The COMI covers five different domains, with seven individual items; pain intensity (two separate items measuring back pain and leg/buttock pain), back function in everyday life (one item), symptom-specific well-being (one item), general quality of life (one item) and disability (two separate items measuring social disability and work disability).

The composite COMI score (range 0–10) is calculated using the average score of the five domains. Higher COMI scores indicate worse status. For the domain pain intensity, the data are collected using 0–10 graphic rating scales, with the higher of the two values for back pain and leg/buttock pain being used to represent the "pain" domain. Five-point scales (1–5) are used for the remaining domains, with the scores being rescaled into a 0 to 10-point range (score (1–5) minus 1, multiplied by 2.5). The values for the two disability items are averaged to represent the "disability" domain [5]. Averages of the five domains form the COMI (0–10, best–worst).

The translation and cross-cultural adaption was carried out according to recommended guidelines for health status measures [22]. Two native-speaking Swedish persons (one expert T1 and one naïve T2) who were fluent in English completed a forward translation from English to Swedish. All translators and the expert group consisting of three physiotherapists (two researchers/clinicians and one clinician) synthesised the results of the translation resulting in a common translation (T-12). In the process, it was considered important that consensus resolved the issues and not one person’s opinion. The T-12 version was then back-translated from Swedish to English by two translators (BT1 and BT2) fluent in Swedish but with English as their mother tongue. Back translation highlights inconsistencies or conceptual errors in the translation. The same expert group as before, with addition of the PROMs developer, consolidated all versions (the original, T1, T2, T-12, BT1 and BT2) and developed the pre-final version of the Swedish COMI for field-testing.

Patients with a primary complaint of LBP, with or without leg pain > 3 months, and with a good understanding of the Swedish language were consecutively recruited at a physiotherapy primary health care clinic with direct access to physiotherapists, and an orthopaedic surgery clinic with patients being considered for spinal surgery (decompression for spinal stenosis, surgery for disc hernia or spinal fusions). The exclusion criteria were pregnancy and “red flags” such as fracture, cancer and inflammatory disease. Written informed consent was obtained from all participants. One-hundred-two participants were included in the validity study and 49 of those were included in the test–retest analysis (Table 1). The study sample size was determined according to the recommendations [23].

At their first visit to either of the clinics, all participants completed the COMI, the reference scales and questions concerning socio-demographic variables. The chosen reference scales were Oswestry Disability Index (ODI) [24] and the EuroQol-5 Dimensions Index (EQ5D) [25]. The ODI is a spine specific PROM assessing LBP-related disability using 10 questions rated from 1–5 and transformed to a score of 0 (no disability) to maximum 100 for worst disability [24]. The EQ5D is a generic instrument measuring Health Related Quality of Life and comprises 5 items concerning mobility, self-care, usual activities, pain/discomfort and anxiety/depression [25]. We used the 3-level version of the EQ5D in which each item is rated on a 3-point adjectival scale. To calculate the EQ5D Index, the time trade-off valuation technique (TTO) was used (https://​euroqol.​org/​docs/​EQ-5D-3L-User-Guide.​pdf).

The Swedish version of the COMI is presented in Appendix 1 in ESM. COMI was successfully forward- and back-translated into Swedish with neither semantic nor language ambiguities, and all items were approved by the expert group. Pre-testing of the preliminary Swedish COMI revealed no ambiguities.

Our results on floor and ceiling effects showed somewhat diverse effects compared to previous validation studies. There was a notable ceiling effect (best status) for the item leg pain (16%) and work disability (54%), and a floor effect (worst status) for the item symptom-specific well-being (48%). For the composite score no floor or ceiling effects were seen, which concurs with the findings of previous studies. A floor effect for symptom-specific well-being has previously been reported [8‐10, 12, 13]. The Hungarian study showed a floor effect of 75.2% for this item, which might be considered above the critical value of 70% but might be explained by their evaluation of a surgical population [12]. Even if consistently showing a higher ceiling effect, this specific question is considered to add valuable information and should therefore continue to be part of the COMI [4, 5]. The ceiling effect of 54% for work disability is in keeping with the fact that our cohort was only moderately disabled (ODI 30, SD 17); more than 50% were still at work and only 12% were sick listed.

In line with previous validation studies, the composite COMI score as well as the scores for each of the separate COMI items correlated to at least some extent with the full scores of the reference scales, the ODI [24] and the EQ5D [25]. The COMI composite score showed a strong correlation with the full scores for the reference scales (ODI, r = 0.72; EQ5D, r = − 0.73). The correlation with ODI concurs with the findings of some previous studies, including those evaluating Brazilian Portuguese (0.64) [8], Chinese (0.69) [9], Hungarian (0.83) [12], and Korean (0.83) [15] versions of the COMI. The correlation with EQ5D also concurs with some previous studies [11, 13‐15]

Our hypotheses of a moderate correlation between each of the COMI items and the composite reference scales were confirmed, with correlation coefficients ranging from 0.41 to 0.73, with the exception of that for back pain, which had a lower correlation with ODI (r = 0.33). This might be considered odd as the ODI is considered a back pain related disability instrument. However, this was not evident when the higher of the pain score was used; then the relationship was stronger (r = 0.60). Further, the item Symptom-specific well-being showed lower correlations with the ODI full scale (r = 0.41) and with the EQ5D (r = − 0.46) in the present study. This concurs with previous studies also showing lower correlations for this specific item either measured with the ODI (Polish 0.43 [17], Hungarian 0.44 [12], Chinese 0.45 [9]) or the EQ5D (French 0.36 [11], Norwegian 0.43 [16].

For analyzing test–retest reproducibility, 49 participants from the primary care cohort filled in the COMI twice with 7 days in-between. We considered this a reasonable period as 7–10 days previously has been recommended [23]. We reached a fair to good reliability for the specific COMI items (ICC 0.41–0.78, Kappa W 0.41–0.77). The COMI composite score showed good reliability (ICC 0.63) but was somewhat lower than that reported in previous studies (French 0.85 [11]; Italian 0.92 [13], Norwegian 0.89 [16]; Brazilian-Portuguese; 0.91 [8] and Chinese 0.91 [9]). The ICC is dependent on the variance between and within the subjects. Our data, compared with previous studies [8, 15, 17], showed a lower COMI score (mean) at both T1 and T2 and a slightly narrower variance, which might have affected the ICC value (Table 3).

For a test retest design the item measured over time should be stable, to avoid bias [23]. We did not use a transition question between the tests as the time interval was only 7 days and no treatment was scheduled during this week. Even so, we cannot rule out the possibility that patients' symptoms were not sufficiently stable between the tests, and that there was indeed real change between them, which would have influenced the reproducibility statistics. However, looking into the differences between the tests in response to each domain on the COMI, only back function fell short of the recommended 90% of ± 1 category (6 out of 49, 12%) [30].

Our participants filled in the COMI and the reference scales at the clinic on the first occasion and at home on the second, prompted by a text-message. This methodology has successfully been used before [31]. The different environments in which the COMI was completed might however have influenced the stability of the data. Encouragingly, there were no significant differences in the mean scores on the two test occasions for back pain, leg pain, the other specific COMI domains and the COMI score. Our SEM (1.0), and hence MDC (2.8), was somewhat higher than reported in some of the previous COMI studies [5, 9, 17]. Our results are still in line with the Norwegian (MDC 2.2) [16] and not far from the French (MDC 2.0) [11] studies. Our MDC for the COMI score indicates that a change of 2.8 points or more gives a 95% likelihood that it is a result of "real change" in the patient's condition instead of a measurement error. Based on previous COMI studies, the estimated minimal clinically important difference for the COMI summary score is between 2 and 3 [32].

A strength of our study is that we followed strict guidelines in the cross-cultural adaptation and validation processes. Another strength is that we included participants from both primary and secondary care to increase the validity of the COMI in a Swedish context. Some limitations however need to be considered. As our patients were not highly disabled compared with those included in some of the previous validation studies, our results can only be generalized to a population like ours. Even so, our study included a diverse sample of patients with and without leg pain (51% without leg pain) and from both primary (60%) and secondary care.

The COMI covers several domains in one short, easy to complete instrument. Short instruments are warranted and useful for clinical and research purposes. A recent study by Osthols et al. [33] who surveyed physiotherapists in Sweden concluded that few PROMs are currently used in the everyday work of primary care physiotherapists, and at a low frequency, and one reason given for this is lack of time. We therefore find that to present the COMI in a Swedish context is important.

The Swedish COMI shows acceptable psychometric properties and may thus be suitable to use as a short instrument, measuring important domains in patients suffering from low-back pain with and without leg pain. Future studies should further evaluate its sensitivity to measure change in response to treatment.