Validation of Minimal Clinically Important Difference (MCID) for University of Pennsylvania Smell Identification Test (UPSIT).

Background: The University of Pennsylvania Smell Identification Test is widely used to measure change in olfactory function, but a minimal clinically important difference (MCID) has not been well-established. A study published in 1997 regarding patients with head trauma reported an MCID of 4 but did not detail the methods used in the calculation.

Objective: To validate the MCID for UPSIT in patients with postviral, sinusitis, and procedure-associated olfactory loss.

Methods: This was a secondary analysis of prospectively collected data from 5 clinical research studies related to olfactory function. Three studies included subjects with COVID-19-related olfactory dysfunction, one with chronic sinusitis subjects, and one with subjects undergoing transsphenoidal surgery. All subjects had completed a baseline and follow-up UPSIT, baseline and follow-up Clinical Global Impression-Severity (CGI-Severity), and a follow-up CGI-Improvement. Both distribution- and anchor-based methods were used to determine the MCID of UPSIT. Distribution-based method calculated MCID using half standard deviation of baseline UPSIT and delta UPSIT scores. Clinical-anchor method determined MCID by comparing delta UPSIT scores between consecutive CGI-I clinical categories ranging from very much better to very much worse.

Results: The study population comprised 295 subjects. Subjects had a mean (SD) baseline UPSIT score of 27 (7.5), and follow-up score of 28 (7.9), and a mean UPSIT change of 0.6 (5.8). Half the baseline UPSIT SD was 3.75 and half the delta UPSIT SD was 2.9. With the anchor-based approach, an MCID of 4 was defined as clinically meaningful by exploring the relationship between delta UPSIT and CGI-Improvement. Using a more conservative approach based on the MCID values identified from both methods, we determined that a change of 4 or greater is the appropriate MCID for UPSIT.

Conclusion: Investigators in the future should use 4 as MCID for UPSIT and report the percentage of study subjects who achieve a clinically meaningful difference.

Level of evidence: III.