Demographically-adjusted normative data for Symbol Digit Modalities Test (SDMT) among Sri Lankan adults

Background: Symbol Digit Modalities Test (SDMT) is a simple pencil-and-paper substitution task that measures divided attention, visual scanning and processing speed. It is sensitive to cognitive impairment in a wide variety of neuropsychiatric conditions. Extant normative data for the SDMT, mainly based on Western populations, may not be applicable for Sri Lankans. Aims: We aimed to create sex-, age- and education-adjusted regression-based norms for the SDMT for Sri Lankan adults. Methods: Four-hundred and twenty-two community-living adults (220 women, 52.1%), aged 18 to 83 years, with 5 to 23 years of education, completed the study. We conducted multiple linear regression analyses with sex, age and years of education to predict the SDMT score. Results: The SDMT scores of the sample ranged from 5 to 72, with a mean (SD) score of 35.98 (12.87). The regression model [predicted SDMT score = 29.395 - (1.549 × sex) - (0.434 × age in years) + (2.207 × years of education)] explained 56.5% of the variance in SDMT scores (adjusted R2 = 0.565; F=183; p<0.001). Age explained 20.9% and years of education explained 18.8% of the SDMT score variance uniquely. Sex was not a significant predictor. Conclusion: We report regression-based norms for the SDMT for Sri Lankans aged 18–83 years, and supplement the regression equation with a Microsoft Excel-based calculator that produces predicted and standardized scores for individual test participants. These norms would assist clinicians in accurately interpreting SDMT test results, accounting for the variability introduced by sex, age and education.