Preoperative Disability Influences Effectiveness of MCID and PASS in Predicting Patient Improvement Following Cervical Spine Surgery

Introduction: Although minimal clinically important difference (MCID) and patient acceptable symptom state (PASS) are utilized to interpret Neck Disability Index (NDI), it is unclear if their utilization is dependent on the degree of preoperative disability.

Methods: Patients who underwent primary anterior cervical discectomy and fusion (ACDF), cervical disc replacement (CDR), or laminectomy were included. NDI and global rating change (GRC) data at 6 months/1 year/2 years were analyzed. GRC was collapsed to a dichotomous outcome variable – a) improved, b) not improved. Thresholds utilized were 10.5 for MCID (preoperative minus postoperative NDI) and 21 for PASS (postoperative NDI), respectively. Sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of MCID and PASS in terms of predicting improvement on GRC were calculated for the overall cohort and separately for patients with minimal (NDI < 30), moderate (NDI 30 – 49), and severe (NDI >50) preoperative disability. Two groups with patients who achieved PASS but not MCID and patients who achieved MCID but not PASS were analyzed.

Results: 141 patients (206 responses) were included. PASS had significantly greater sensitivity for the overall cohort (85% vs. 73% with MCID, p=0.02) and patients with minimal disability (96% vs. 53% with MCID, p< 0.001). Contrastingly, MCID had greater sensitivity for patients with severe disability (78% vs. 57% with PASS, p=0.05). Sensitivity was not significantly different for PASS and MCID in patients with moderate preoperative disability (83% vs. 92%, p=0.1). 17% of patients achieved PASS but not MCID and 9% of patients achieved MCID but not PASS. Most of these patients still reported improvement with no significant difference between the two groups (89% vs. 72%, p=0.13).

Conclusion : PASS is a better metric for patients with minimal preoperative disability and MCID is a better metric for patients with severe preoperative disability. Both metrics are equally effective for patients with moderate preoperative disability. Adequate interpretation of NDI using PASS and MCID metrics warrants individualized application as their utility is dependent on the degree of preoperative disability.