The Neck Disability Index (NDI) is a widely used patient-reported outcome measure that provides a standardized way to assess the impact of neck pain on a person’s life. Developed in 1991, the NDI helps clinicians and researchers quantify a patient’s disability and pain intensity. It is a standard tool in physical therapy, chiropractic, and medical settings used to track patient progress and determine treatment effectiveness. The NDI provides an objective measure of a subjective experience, which is important for clinical decision-making and insurance documentation.
Understanding the NDI Structure and Item Scoring
The NDI questionnaire is composed of ten distinct sections, each designed to capture a different dimension of how neck pain interferes with daily life. These sections cover a broad range of activities, including:
- Pain Intensity
- Personal Care
- Lifting
- Reading
- Headaches
- Concentration
- Work
- Driving
- Sleeping
- Recreation
Each of the ten items is scored on a six-point scale. Response options range from the first statement, scored as 0, indicating no difficulty, to the last statement, scored as 5, representing the maximum level of disability. For example, in the “Lifting” section, choosing the first option (0) means lifting heavy weights without increased pain, while selecting the last option (5) means the patient cannot lift anything at all. A patient must choose only one statement that best describes their current situation for each section.
Calculating the Raw NDI Score
The first step is to calculate the raw NDI score, which is the summation of the points assigned to each of the ten answered items. Since each item is scored from 0 to 5, the maximum possible score for a fully completed questionnaire is 50. A raw score of 0 indicates no reported disability, while a score of 50 signifies the most profound level of neck-related disability.
To illustrate, consider a patient who scores 3 points for Pain Intensity, 1 for Personal Care, 4 for Lifting, and 2 for the remaining seven categories. The raw score is calculated by adding these values: 3 + 1 + 4 + (7 x 2) = 22. This raw score provides the necessary numerator for the final percentage calculation.
Converting the Raw Score to a Percentage
The raw score out of 50 is converted into a percentage score ranging from 0% to 100%. The standard formula is to divide the raw score by the maximum possible score (50) and then multiply the result by 100. This calculation is often simplified by multiplying the raw score by two. For instance, a raw score of 25 converts directly to a 50% disability score.
A specific protocol must be followed when a patient fails to answer all ten sections to maintain the validity of the final score. If one or two items are left blank, the score is adjusted based on the number of questions answered. The adjustment involves dividing the sum of the answered items by the maximum score possible for only those answered items, then multiplying by 100. For example, if a patient answers eight questions with a total score of 24, the calculation is (24 / (8 5)) 100, resulting in a 60% disability score. If a patient omits more than two items, the NDI is considered invalid and should not be scored.
Interpreting the Final NDI Score
The final percentage score is used to categorize the severity of a patient’s neck-related functional limitation. The standard classification divides the 0% to 100% range into five categories of disability severity. These categories help clinicians quickly grasp the clinical relevance of the score and guide treatment planning.
NDI Disability Categories
- 0% to 8%: No Disability
- 10% to 28%: Mild Disability
- 30% to 48%: Moderate Disability
- 50% to 68%: Severe Disability
- 70% to 100%: Complete Disability
The concept of a clinically meaningful change is important for evaluating the success of an intervention. A change in the NDI score must exceed a certain threshold to be considered a real clinical improvement. For patients with mechanical neck pain, a change of approximately 10 points (or 10%) is often cited as the Minimum Clinically Important Difference (MCID) needed for a patient to perceive improvement. This threshold can vary slightly depending on the specific condition, such as cervical radiculopathy, where a change of about 13 points may be required.