The Psychometric Properties of the Neck Disability Index: a Comprehensive Review

The NDI has been used in a variety of populations, including mechanical neck pain (MNP),1-8 cervical radiculopathy (CR),9,10 whiplash associated disorder (WAD),11,12 and mixed non-specific neck pain (Mixed-NSNP),13-18 with a wide range of symptom durations (acute, subacute, and chronic). Vernon and Mior19,20 suggest the following score interpretation: 0-4 represents no disability; 5-14 represents mild disability; 15-24 represents moderate disability; 25-34 represents severe disability; and greater than 34 represents complete disability. However, Vernon and Mior19 did not describe the process for how these ratings were derived and, to date, no validation of these categories has been performed.21 The NDI has been found to possess excellent test-retest reliability, strong construct validity, strong internal consistency and good responsiveness in assessing disability in patients with mechanical neck pain (MNP),1,2,5-8 cervical radiculopathy (CR),9,10 whiplash associated disorder (WAD),11,19,22 and mixed non-specific neck pain (Mixed-NSNP).13-15,17,18,23

SEM, MDC and MCID

The standard error of measurement (SEM) has been found to be 3.0,6 5.7,10 and 0.6017 – 1.80,18 in patients with MNP, CR, and Mixed-NSNP, respectively. In patients with MNP, the minimum detectable change (MDC) for the NDI has been reported to be 19.6 percentage points,2 8.4,6 10.2,8 4.2,18 and 5.23 In patients with cervical radiculopathy, the MDC has been reported to be 13.410 and 10.2.9 In patients with Mixed-NSNP, the MDC has been found to be 10.5,15 1.66,17 and 4.20.18 In patients with MNP, the minimum clinically important difference (MCID) for the NDI has been found to be 19 percentage points,2 3.5,6 7.5,8 and 5.23 The MCID was found to be 8.510 in patients with CR and 3.515 in patients with Mixed-NSNP. Therefore, clinicians shoulder consider the minimum detectable change (MDC) for the NDI to approximate 4, 7, and 12 points (0-50) in patients with mixed non-specific neck pain, mechanical neck pain, and cervical radiculopathy, respectively. Similarly, the minimum clinically important difference (MCID) for the NDI should be considered to approximate 4, 6, and 9 points (0-50) in patients with Mixed-NSNP, MNP, and CR, respectively.

Responsiveness        

The NDI has been found to possess acceptable or satisfactory responsiveness (AUC > 0.70) in its ability to distinguish improved from stable patients in mechanical neck pain (AUC = 0.832; 0.923; 0.756; 0.798), cervical radiculopathy (AUC = 0.7410; 0.579), and Mixed-NSNP (Responsiveness ratio = 1.8217).

Test-retest reliability         

In patients with MNP, the NDI has demonstrated fair test-retest reliability (ICC = 0.502), moderate test-retest reliability (ICC = 0.648), and excellent test-retest reliability (ICC = 0.866, r = 0.8919, r = 0.81 – 0.991, r = 0.9423, r = 0.967). In patients with CR, the NDI has demonstrated fair test-retest reliability (ICC = 0.5510; ICC = 0.689). In patients with Mixed-NSNP, the NDI has shown excellent test-retest reliability (ICC = 0.9314; ICC = 0.9017; ICC = 0.8918).

Internal consistency

The NDI has demonstrated high internal consistency (Cronbach’s alpha > 0.7) in patients with MNP (Cronbach’s alpha = 0.925, 0.897, 0.724, 0.774), WAD (Cronbach’s alpha = 0.819), and Mixed-NSNP (Cronbach’s alpha = 0.86414).

Construct validity    

The NDI possesses strong/excellent construct validity in patients with MNP when compared with the Global Rating of Change (r = 0.528; r = 0.817), the Neck Bournemouth Questionnaire (r = 0.804 pretreatment; 0.774 posttreatment), the Neck Pain and Disability Scale (r = 0.863), and Problem Elicitation Technique (r = 0.623). However, in patients with CR, the NDI has demonstrated poor construct validity.9 In patients with WAD, the NDI possesses strong construct validity when compared with the Northwick Park Questionnaire (r = 0.8811) and only adequate construct validity when compared with Problem Elicitation Technique (r = 0.5711). In patients with Mixed-NSNP, the NDI has been found to possess adequate construct validity when compared with the Mental Component Summary (MCS) score of the SF-36 (r = 0.4716) or the Physical Component Summary (PCS) score of the SF-36 (r = 0.5316), and strong/excellent construct validity when compared with the Patient-Specific Functional Scale (r = 0.7318 at admission; r = 0.8118 at discharge) or the Disabilities of the Arm, Shoulder & Hand (DASH) scale (r = 0.7513).

Content validity       

Of the eleven problems identified by most patients with MNP during problem elicitation technique (PET) interviews,3 six of these were found to not be included as items in the NDI; more specifically, En et al3 found the dimension of frustration, as well as other emotional and social functions, are not addressed in the NDI,3 suggesting limitations or gaps for the content validity of this outcome measure. In patients with WAD, and following PET interviews, the nine most commonly identified problems were ‘work for wages’, ‘fatigued during the day’, ‘participation in sports’, ‘depression’, ‘drive a car’, ‘socialize with friends’, ‘sleep through the night’, ‘frustration’, and ‘anger’; however, only three of these problems are specifically included in the NDI (work, driving, and sleeping).11 Additionally, in patients with WAD, exploratory factor analysis found a 2-factor instrument: (1) pain intensity and interference with level of cognitive functioning, and (2) interference with level of physical functioning.12 Likewise, Ailliet et al13 concluded the unidimensionality of the NDI could not be confirmed by clinicians and researchers with expertise in neck pain; that is, the NDI likely measures more than physical function. Moreover, expert clinicians and researchers suggested to fully capture the “burden of disease”, a number of items were likely missing from the NDI including: computer work, participation in sports, mobility, mental issues, dizziness, symptom radiation, the distinction between household work, and professional activities.13 Ailliet et al13 also suggested that the NDI does not take into account the use of over-the-counter or prescription medication and its potential effect on modulating neck pain and disability.13

Over-the-counter or prescription medication use, participation in sports, and personal computer work are not addressed by the NDI suggesting the instrument may not completely capture important aspects of the modern disablement process in patients with neck pain.13 Missing data frequently occurs for item 8 (i.e. driving) in cultures that restrict females from driving;7 therefore, like the Neck Pain and Disability Scale (NPDS), it has been suggested that this item be answered by either a driver or passenger.7 The NDI has no “response set bias” suggesting subjects are responding to the content, not the item order or the sequencing of the responses format.5 The NDI does not have any serious floor or ceiling effects;7,16 therefore, the NDI is able to assess the full range of disability in relation to neck pain. However, a clear differential item functioning (DIF) for the “headaches” item has been observed; moreover, women appear to score significantly worse than men with similar levels of disability.13

Unidimensionality  

The NDI has traditionally been accepted to be unidimensional20,21 and tends to capture more of the physical aspects of pain and disability rather than the emotional and social aspects. However, although exploratory factor analyses had originally identified a 1-factor model that accounted for 59% to 65% of the variance,5 a 2-factor structure (1 subscale describing ADLs and the other subscale representing pain and concentration) recently explained 67.58% of the total variance.7 Additionally, another recent study found two factors to explain 53.8% of the variance within the NDI.13 Nevertheless, although the NDI adequately captures the physical aspects of neck pain, it may not capture the broader emotional (frustration, depression, and anger) and social function dimensions, which are frequently reported by patients with neck pain.3,11

Summary of the psychometric properties of the NDI     

The NDI is the oldest and most widely used instrument for self-reporting of disability in patients with neck pain.20,21,24 Its psychometric properties have been well established in a large number of independent studies;20,21,24 furthermore, the NDI is the scale most widely validated among different populations and settings of patients with neck pain.21 The NDI has been found to possess fair,2,9,10 moderate8 and excellent1,6,7,14,17-19,23 test-retest reliability, strong3,4,7,11,13,18 construct validity, strong4,5,7,14,19 internal consistency and acceptable/satisfactory2,6,8,10,17,23 responsiveness in patients with mechanical neck pain, whiplash associated disorder, or mixed non-specific neck pain; additionally, in the main, it has been found to possess a 1-factor structure (i.e. unidimensional) for “physical disability”.20,21

AUTHOR:

James Dunning, DPT, MSc (Manip Ther), MMACP (UK), FAAOMPT
Director, AAMT Fellowship in Orthopaedic Manual Physical Therapy
President, Alabama Physical Therapy & Acupuncture, Montgomery, AL

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