Presenter: Neck Disability Index needs to be refined further to be more effective

The NDI is the first patient reported outcome measure for neck pain, but researchers said not enough is known about how well it performs.

NEW ORLEANS — The long-standing instrument used to measure patient reported outcomes for patients with neck pain, the Neck Disability Index or NDI, is the most widely used patient reported outcome measure for these patients, but researchers reported recently it may not be as effective when it is used for patients that are either low functioning or high functioning.

“The NDI is a multidimensional scale, but we have to be careful about how we interpret the data because other dimensions may affect scores. Further studies are needed to investigate improving the NDI, such as a shorter, eight question version, or other potentially better instruments coming,” Darrel S. Brodke, MD, of Salt Lake City, Utah, said at the American Academy of Orthopaedic Surgeons Annual Meeting.

Brodke and colleagues looked at the effectiveness, comparative the psychometric properties, performance and applicability of the NDI in the spine patient population.

Darrel S. Brodke

“The NDI, which is arguably one of the most widely used patient reported outcomes for the neck patient population, is short, only 10 questions, and it is well known and widely used today to assess comparative,” Brodke said. “Interestingly, there are relatively few sufficient psychometric studies with concern to this outcome measure.”

Effectiveness of 10-question form

In his presentation at the meeting, Brodke cited a 2009 study by Van Der Velde and colleagues that found an 8-question version of the normally 10 question NDI that “functioned extremely well.” The study was conducted 5 years ago, however, Brodke said, and the 8-question version of the NDI is not widely used. For their study Brodke and colleagues looked at 865 patients who completed an NDI at a university-based spine clinic between June 2011 to May 2013 with a primary complaint of neck pain, with or without upper extremity symptoms. The majority of the patients were white (94%) with a mean age of 55 years (range, 15 years to 91 years).

All of the patients answered all 10 questions of the NDI.

Poor ceiling and floor effect

Researchers found the NDI had a floor effect of 35.5% and a ceiling effect of 4.6%. The floor effect scores were the most troubling numbers from the NDI results, Brodke said.

“There was a modest ceiling effect, under 5%. It was adequate. Five percent of the patients on the top end of disability were not well covered by this test,” Brodke said. “What is most concerning is that the floor effect was huge, 35%, meaning if you look at those two together, over 40% of patients aren’t well covered by the exam, specifically those at the bottom end of disability. This means that the measure cannot adequately differentiate well-functioning patients. This calls into question results from comparative effectiveness studies that report significant improvement in one group over another where both groups improve, one more than the other, which cannot be accurately discerned.”

Brodke and colleagues noted in the study abstract the raw score multiplied by 2, the common way to report the patient outcome and compare it statistically, should not be used with NDI when interpreting findings because the raw score to measure correlation was poor.

Despite the NDI’s widespread use, Brodke said the analysis his group did and previous research demonstrates the NDI may need to be refined and further investigated so that it is a more effective tool. – by Robert Linnehan

Reference:

Brodke DS. Paper #222. Presented at: American Academy of Orthopaedic Surgeons Annual Meeting; March 11-15, 2014; New Orleans.

For more information:

Darrel S. Brodke, MD, can be reached at Department of Orthopedics, University of Utah, 590 Wakara Way, Salt Lake City, UT 84108; email: darrel.brodke@hsc.utah.edu.
Disclosure: Brodke receives royalties from Amedica, DePuy Synthes and Medtronic. He is also a consultant for Amedica and DePuy Synthes. He receives stock or stock options with Amedica.