Patient-level minimal clinically important difference based on clinical judgment and minimally detectable measurement difference: a rationale for the SF-36 physical function scale in the SPORT intervertebral disc herniation cohort.

STUDY DESIGN A proof of concept case study. OBJECTIVE To introduce and evaluate a method for identifying what constitutes a minimal clinically important difference (MCID) in the SF-36 Physical Function scale at the patient level. SUMMARY OF BACKGROUND DATA MCID has become increasingly important to researchers interested in evaluating patient care. Over the last 30 years, an array of approaches for assessing MCID has evolved with little consensus on which approach applies in any given situation. METHODS Three approaches for estimating standard errors of measurement (se) and a 30% change approach for establishing MCID were evaluated for the physical function (PF) scale with SPORT patients in the intervertebral disc herniations cohort. MCIDs for each se approach were then developed based on (1) these standard errors and (2) clinically relevant factors including: (a) baseline PF score and (b) acceptable risk for type I error. RESULTS.: Intervertebral disc herniations patients (N = 996) identified from the SPORT database met inclusion criteria. The se for the classic test theory (CTT)-based test level approach was 9.66. CTT-score-level and IRT-pattern-level standard errors varied depending on the score, and ranged between (2.73-7.17) and (5.96-16.2), respectively. As predicted, CTT-score-level se values were much smaller than IRT-pattern-level se values at the extreme scores and IRT-pattern-level se values were slightly smaller than CTT score-level se values in the middle of the distribution. Across follow-up intervals, the CTT-score-based approach consistently demonstrated greater sensitivity for identifying patients who were improved or worsened. Comparisons of CTT-based-score-level se and 30% improvement rule MCID estimates were as hypothesized: MCID values for 30% gains demonstrated substantially lower sensitivity to change for baseline PF scores in the 0 to 50 range but were similar to CTT-score-level-based MCIDs when baseline scores were above 50. CONCLUSION The CTT-based-score-level approach for establishing MCID based on the clinical relevance of the baseline PF score and the tolerance for erroneously accepting an observed change as reliable provided the more sensitive and theoretical compelling approach for estimating MCID at the patient level, which in turn will provide fundamentally important to the clinician regarding treatment efficacy at the patient level.