This prospective observational study aims to develop and validate an Italian version of the Waddell Disability Index (WDI-I), a self-administered questionnaire designed to assess disability related to chronic low back pain. Low back pain is a leading cause of disability worldwide, and patient-reported outcome measures are increasingly recognized as essential tools for comprehensive clinical assessment and rehabilitation planning.

The study involves a standardized cross-cultural translation process, including forward and backward translations by bilingual translators and review by a panel of expert clinicians, followed by pilot testing. After consensus, the finalized Italian version (WDI-I) will undergo psychometric validation.

A total of 115 adult patients with chronic low back pain (≥3 months) will be recruited from two rehabilitation centers. At baseline, participants will complete the WDI-I along with the Numeric Pain Rating Scale (NPRS), the Italian Oswestry Disability Index (ODI-I), and the Italian Pain Catastrophizing Scale (PCS-I). The WDI-I will be re-administered after 7-10 days to assess test-retest reliability.

The primary endpoints are internal consistency, reliability, and construct validity of the WDI-I. Statistical analyses will include Cronbach's alpha and item-total correlations for internal consistency, intraclass correlation coefficients (ICC), standard error of measurement (SEM), and minimum detectable change (MDC) for reliability, and Spearman correlation coefficients to evaluate concurrent validity with established pain and disability measures.

The study seeks to provide a reliable and valid Italian instrument for assessing disability in individuals with chronic low back pain, facilitating both clinical practice and research

Rationale Low back pain is a very common condition and represents the leading cause of disability in industrialized countries, with a substantial economic and social impact.

The assessment of individuals with low back pain can be performed using physical tests (e.g., joint mobility, strength, gait) and outcome measures that evaluate pain intensity, functional limitations during daily activities, and quality of life. Over time, these latter measures have demonstrated strong reliability and validity, in some cases even surpassing the reliability and validity of physical tests. There is a growing consensus in the literature that such outcome measures, often self-completed by patients, represent an important complement to the overall assessment of individuals with low back pain and support the planning of rehabilitation programs. However, these measures are still viewed with some reluctance by healthcare professionals, despite not requiring additional assessment time and instead providing valuable supplementary information.

In this context, an outcome measure known as the Waddell Disability Index (WDI) exists. It was published in 1984 in the international journal Spine by Gordon Waddell and Chris Main and consists of a self-administered questionnaire including several activities of daily living, excluding work- and sport-related activities. The questionnaire is simple to administer and requires approximately five minutes to complete.

Considering the epidemiological background, the need for appropriate assessment tools, and the absence of a culturally adapted Italian version of the WDI, it is considered useful to develop an Italian adaptation of the WDI in individuals with low back pain and to analyze its reliability and validity properties.

Objectives The objective of this study is to produce an Italian-language version of the WDI through a cross-cultural translation procedure and to validate it in terms of internal consistency, reliability, and validity in a population of patients with chronic low back pain.

Endpoints

To demonstrate that the Italian version of the WDI is internally consistent, reliable, and correlated with the following scales:

• Numeric Pain Rating Scale (NPRS)
• Oswestry Disability Index - Italian version (ODI-I)
• Pain Catastrophizing Scale - Italian version (PCS-I)

Population Patients A total of 115 patients referred to the Istituti Clinici Scientifici Maugeri for outpatient medical evaluation for chronic low back pain will be enrolled.

65 patients will be recruited from the Neuromotor Rehabilitation Unit of the Scientific Institute of Tradate 50 patients will be recruited from the Neuromotor Rehabilitation Unit of the Scientific Institute of Lissone

Inclusion Criteria

• Age between 18 and 90 years
• Ability to speak and understand Italian fluently
• Low back pain lasting at least 3 months

Exclusion Criteria

• Cognitive impairment (Mini Mental State Examination < 24)
• Central or peripheral neurological disorders
• Systemic diseases or severe comorbidities (acute cardiac events, acute neurological events, chronic liver or kidney disease)
• Uncorrected visual impairment
• Individuals receiving compensation related to work-related disability

Translators Two native English-speaking translators One Italian language teacher (voluntary contribution)

Study Design An Italian-language version of the questionnaire will be developed through a standardized procedure involving translators and a group of five experienced clinicians.

Initially, each clinician independently produces a translated version of the scale together with the Italian-speaking translator. A synthesis process then leads to a single translated version, which is submitted to the two native English-speaking translators, each of whom performs a back-translation. The equivalence between the back-translations and the original version is then verified.

The resulting version is pilot-tested in an initial group of 15 patients (three patients per clinician).

After confirming applicability and reaching consensus, the final version (WDI-I) is defined.

At the two institutes (Tradate and Lissone), 50 patients each will be included in the final validation study, accounting for an expected dropout rate of 10%.

At time point T0, the following assessments are collected in a single session:

• WDI-I
• Numeric Pain Rating Scale (NPRS)
• Oswestry Disability Index - Italian version (ODI-I)
• Pain Catastrophizing Scale - Italian version (PCS-I) After 7-10 days, the WDI-I will be administered again to assess test-retest reliability.

Type of Study Prospective observational study

Sample Size The sample size was determined based on standard practice, as no formal sample size calculation exists for validation studies. Ten patients per item were recruited, with an additional 10% to account for possible dropouts. An extra 15 patients were included for the pilot study, resulting in a total of 115 patients.

Collected Variables

• WDI-I: Italian version of the WDI obtained through the translation procedure. The WDI is a self-administered questionnaire consisting of 9 items related to the impact of low back pain on activities of daily living. Each item is scored as positive (1 point) or negative (0 points). The total score ranges from 0 (no disability) to 9 (maximum disability).
• NPRS: An 11-point unidimensional scale assessing pain intensity in adults, ranging from 0 (no pain) to 10 (worst imaginable pain).
• ODI-I: A self-administered questionnaire considered the current gold standard for assessing disability in patients with chronic low back pain. It consists of 10 items, each with 6 response options scored from 0 to 5. The total score is converted into a percentage, with 0% indicating no disability and 100% indicating complete disability.
• PCS-I: A self-administered questionnaire assessing pain catastrophizing. It consists of 13 items scored from 0 (never) to 4 (always). The maximum total score is 52 points and includes three subscales: Rumination (items 8, 9, 10, 11); Magnification (items 6, 7, 13); Helplessness (items 1, 2, 3, 4, 5, 12)

Data Monitoring and Collection A dedicated data collection form will be prepared for each patient, including demographic and clinical data as well as the results of the clinical scales specified in the protocol.

Patient data will be pseudonymized and processed in accordance with Article 13 of EU Regulation 2016/679 (GDPR).

Statistical Analysis Methods

The statistical methods used to analyze the psychometric properties of the WDI-I include:

-Internal Consistency Internal consistency (the extent to which items measure the same construct) will be calculated using Cronbach's alpha and item-total correlations.

α > 0.90 is considered excellent (values > 0.95 may indicate item redundancy), α between 0.80-0.89 is considered good, and α between 0.70-0.79 is acceptable.

Item-total correlation assesses how well each item correlates with the total score, excluding that item. A minimum acceptable value of 0.40 is used.

Reliability Analysis Test-retest reliability will be evaluated to assess the stability of WDI-I scores across two administrations. The Intraclass Correlation Coefficient (ICC; two-way random, absolute agreement) with a 95% confidence interval will be calculated.

Agreement analysis, reflecting absolute measurement error, will include:

• Standard Error of Measurement (SEM) calculated as SD × √(1 - ICC);
• Minimum Detectable Change at 95% confidence (MDC95) MDC95 = 1.96 × SEM × √2

Validity Analysis Construct validity will be assessed by examining correlations with other concurrently administered measures. Concurrent validity will be quantified using Spearman's rank correlation coefficient between WDI-I scores and NPRS, ODI-I, and PCS-I scores.

Correlation coefficients will be interpreted as: 0.75: excellent; 0.50-0.75: moderate to good; 0.25-0.50: fair.

Inclusion Criteria:

• Ability to speak and understand Italian fluently
• Low back pain lasting at least 3 months

Exclusion Criteria:

• Cognitive impairment (Mini-Mental State Examination < 24)
• Central or peripheral neurological disorders
• Systemic diseases or severe comorbidities (acute cardiac events, acute neurological events, chronic liver or kidney disease)
• Uncorrected visual impairment
• Individuals receiving compensation related to work-related disability

Patients referred to the Istituti Clinici Scientifici Maugeri for outpatient medical evaluation for chronic low back pain will be enrolled