Determining the Minimal Clinical Important Difference for Medication Quantification Scale III and Morphine Milligram Equivalents in Patients with Failed Back Surgery Syndrome

Abstract

The Medication Quantification Scale III (MQS) is a tool to represent the negative impact of medication. A reduction in medication can serve as an indicator to evaluate treatment success. However, no cut-off value has yet been determined to evaluate whether a decrease in medication is clinically relevant. Therefore, the objective is to estimate the thresholds for the MQS and morphine milligram equivalents (MMEs) that best identify a clinically relevant important improvement for patients. Data from the Discover registry, in which patients with failed back surgery syndrome were treated with high-dose spinal cord stimulation, were used. Patient satisfaction was utilized to evaluate a clinically important outcome 12 months after stimulation. Anchor-based and distribution-based methods were applied to determine the minimal clinical important difference (MCID). Distribution-based methods revealed a value of 4.28 for the MQS and 33.61 for the MME as MCID. Anchor-based methods indicated a percentage change score of 41.2% for the MQS and 28.2% for the MME or an absolute change score of 4.72 for the MQS and 22.65 for the MME. For assessing a treatment outcome, we recommend using the percentage change score, which better reflects a clinically important outcome and is not severely influenced by high medication intake at baseline.

Keywords: clinical importance; minimal clinically important difference; responsiveness.

Figure 1

Boxplot of Medication Quantification Scale III (MQS) absolute change score between baseline and 12-month follow-up. On the y-axis, baseline MQS score minus follow-up MQS score is presented. On the x-axis, “better” and “worse/neutral” levels are presented based on patient satisfaction scores. Within the boxex, the median is allocated with a black line and the mean with a black circle. The white circles on the plot are presenting more extreme data observations. Within-patient change score is the mean absolute change score for the “better” group. The difference in the absolute change score of the patients classified as “better” and the group classified as “worse/neutral” is the between-patient change score.

Figure 2

Scatterplot of MQS (A} and morphine milligram equivalent (MME) (B) percentage change score between baseline and 12-month follow-up, separated according to patient satisfaction. The colours represent the different levels of patient satisfaction. Levels on the right side of the solid black line are considered clinically successful and categorised as “better”. On the y-axis, percentage scores between baseline and 12-month follow-up are presented, which were calculated as follows: ((baseline score − follow-up score)/baseline score) × 100). Positive scores indicate an improvement at follow-up compared to baseline, while negative percentages indicate a worse outcome at follow-up compared to baseline. MCID values were calculated and are presented on the plot by the dashed blue lines. For MQS, the MCID is located at 41.18% and for the MME at 28.20%. Abbreviations. MME: morphine milligram equivalents; MQS: Medication Quantification Scale III.