Variability of minimum clinically important difference threshold values in lumbar microdiscectomy literature: a systematic review

BACKGROUND CONTEXT: Patient reported outcome measures (PROMs) have been increasingly utilized to evaluate outcomes of spinal surgery procedures such as lumbar microdiscectomy. To interpret PROMs in a clinically meaningful way, threshold values such as the minimum clinically important difference (MCID), substantial clinical benefit (SCB), patient acceptable symptom state (PASS), and others are used to describe PROM score changes that have clinical relevance. However, there are no standard threshold values for these metrics, which leads to potential variability in results and their clinical interpretation. PURPOSE: The purpose of this study was to evaluate the variability of common threshold values reported in lumbar microdiscectomy literature. STUDY DESIGN/SETTING: Systematic review. METHODS: Pubmed and Embase were searched for studies from January 1, 2000 - May 1, 2024 that reported threshold values for PROMs following lumbar microdiscectomy. Patient demographics, study characteristics, threshold values, and threshold calculation methods were extracted for all PROM outcomes. RESULTS: A total of 45 relevant studies were identified, collectively studying 85,954 patients undergoing lumbar microdiscectomy. The average age of included patients was 46.7±5.9 years with an average BMI of 26.9±1.9. There were 121 MCID threshold values reported, 8 PASS thresholds, 3 SCB, 3 MIC, and 2 MCRC. A total of 20 unique PROMs were reported, which most commonly included ODI, VAS-leg, VAS-back, and SF-12 PCS. Forty-five of the included studies referenced previous threshold values, while 3 studies (all MCID) calculated novel threshold recommendations. Among these 3 studies, there were 10 novel MCID thresholds reported for 4 different PROMs. The range of MCID thresholds for the most common PROMs were 20 (2.0–22.0) for ODI, 4.7 (1.1–5.8) for VAS-leg, 1.8 (1.2–3.0) for VAS-back, and 2.8 (2.9–5.7) for SF-12 PCS. PASS, SCB, MIC, and MCRC thresholds did not demonstrate variability in the literature. CONCLUSION: The MCID threshold values used in the lumbar microdiscectomy literature display considerable variability, especially for ODI PROMs. This variability is likely due to variations in calculation methods, the generalization of these values from other spine pathologies to microdiscectomy, and certain inherent limitations of MCID thresholds. This study highlights the importance of standardizing PROM threshold values for more accurate assessments of lumbar microdiscectomy outcomes.