Standard measurement error and minimal detectable change of the Back-PAQ ArgSpan questionnaire: Secondary analysis

Background: Negative attitudes and beliefs about back pain in patients with low back pain (LBP) are associated with high levels of pain and negatively influence clinical outcome. The Back Pain Attitudes Questionnaire (Back-PAQ) was developed to assess back beliefs of patients and healthcare professionals. The minimal detectable change (MDC) is defined as the smallest amount of change that can be detected not due to inherent variation or "noise" in the measure. The MDC values at 68%, 90% and 95% confidence levels of the Back-PAQ ArgSpan are unknown.

Objective: to calculate standard error measurement (SEM) and MDC to confirm the feasibility of Back-PAQ ArgSpan as a reliable outcome measure in clinical and research settings.

Study design: a secondary analysis was carried out using a subgroup of data from the cross-cultural adaptation and validation of the Argentine version of the Back PAQ.

Method: SEM was calculated (SD × √1 - ICC) and MDC as (SEM × z-value × √2). MDC was calculated as percentage as well.

Results: the SEM was 5.16 points. The MDC₆₈, MDC₉₀ and MDC₉₅ of the Back-PAQ were 7.30, 12 and 14.3 points, respectively. The percentages of MDC₆₈, MDC₉₀ and MDC₉₅ of the Back-PAQ were 6.7%, 11.0% and 13.1%, respectively.

Conclusion: The present study demonstrated that the Back-PAQ ArgSpan is a reliable and interpretable measurement tool. When assessing a patient, a change in the score in the Back-PAQ ArgSpan over 15 points shows a true change at 95% confidence level.