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Symbol Digit Modalities Test
Symbol Digit Modalities Test
The Symbol Digit Modalities Test (SDMT) is currently a part of the Unified Huntington's Disease Rating Scale (UHDRS). A modified version of the SDMT is currently a part of the Brief Repeatable Battery of Neuropsychological Tests for Multiple Sclerosis (Rao, NMSS, 1991).
The Symbol Digit Modalities Test is copyrighted by Western Psychological Services:
Supplemental - Highly Recommended: Multiple Sclerosis (MS)
Highly recommended for studies involving neuropsychological testing for adult and pediatric MS patients; Huntington's Disease (HD), Highly recommended for cognitive assessments in HD studies.
The Symbol Digit Modalities Test is part of the MS Outcomes Assessments Consortium (MSOAC) Battery of tests, which is recommended as Supplemental – Highly Recommended for MS:
Timed 25 Foot Walk
9-Hole Peg Test
Sloan Low Contrast Letter Acuity
Symbol Digit Modalities Test
Each of the measures in the Consortium can be used individually as primary or secondary endpoints.
Supplemental: Sport-Related Concussion (SRC) and Traumatic Brain Injury (TBI)
|Short Description of Instrument||
Summary/Overview of Instrument: The SDMT measures the time to pair abstract symbols with specific numbers. The test requires elements of attention, visuoperceptual processing, working memory, and cognitive/psychomotor speed.
The SDMT is a measure of divided attention, visual scanning, and motor speed. This measure involves a coding key consisting of 9 abstract symbols, each paired with a number ranging from 1 to 9. The subject is required to scan the key and write down the number corresponding to each symbol as fast as possible. The number of correct substitution within 90 seconds is recorded. In the written version of the test the subject fills in the numbers that correspond to the symbols. In the oral version the examiner records the numbers spoken by the subject.
Construct measured: Processing speed, attention.
Generic vs. disease specific: Generic.
Intended use of instrument/purpose of tool: (cross-sectional, longitudinal, diagnostic, etc.): This test has been shown to predict group membership defined by processing speed deficits, such as brain-injured versus control samples, and has been used as a sensitive outcome in studies identifying predictors of longitudinal decline in elders.
This measure can be used in ages 8 to 91. It can be used in broad spectrum of TBI severity and type of injuries as long as the subject is sufficiently functional to be testable.
Means of administration: (paper and pencil, computerized): Written (oral form also available).
Trained examiners. A written or oral version of the test may be administered. Test can be completed in under 5 minutes.
Location of administration: (clinic, home, telephone, etc.): Clinical Setting.
Intended respondent: (patient, caregiver, etc.): Patient.
# of items: N/A.
# of subscales and names of sub-scales: N/A.
Type of scale used to describe individual items and total/subscale scores (nominal, ordinal, or [essentially] continuous): Continuous.
If ordinal or continuous, explain if ceiling or floor effects are to be expected if the measure is used in specific HD Subgroups. No ceiling or floor effects. Individuals with advanced HD may struggle to write legibly due to motor disability.
Scoring: (include reference to detailed scoring instructions, including calculation of a total score and subscale scores, and any limitations of scale or scoring posed by item nonresponse): The score is the number of correctly coded items from 0–110 in 90 seconds.
Standardization of scores to a reference population: (z scores, T scores, etc): Manual norms have been criticized because they are based on a sample of convenience and were collected in the 1970's. Other published norms are available based on age, education, and sex.
If scores have been standardized to a reference population, indicate frame of reference for scoring: (general population, HD subjects, other disease groups, etc). General population.
For multiple sclerosis most clinical research has employed the oral response form exclusively.
Strengths: Sensitive to changes in pre-manifest HD in cross-sectional and longitudinal studies. Easy to administer and score. Multiple forms available.
Weaknesses: More severe motor impairment may influence results. An oral form of the test is also available, although much less is known about this version in HD.
Special Requirements for administration: Stopwatch/clock.
Administration Time: Less than 5 minutes.
Translations available (e.g., Spanish, French, Other languages): Involves only geometric figures and numbers, and therefore can be administered to people who do not speak English.
Test-retest or intra-interview (within rater) reliability (as applicable): The test demonstrates strong reliability and validity coefficients (6-year interval). Test-retest reliability in other studies ranges between 29 days to 2 years (r=.70 to .91) (e.g., Smith et al., 1991).
Test-retest reliability in MS over two-weeks ranges from 0.85 to 0.98 (Benedict et al 2009; 2012)
Inter-interview (between-rater) reliability (as applicable): N/A.
Internal consistency: N/A.
Statistical methods used to assess reliability: Reliability coefficient.
Reliability data from the CAB study will be available by end of 2012 for 100 control, 100 pre-manifest, and 50 early HD subjects.
Validity: Content validity: SDMT correlates with oral versions (r=.78) (Smith et al., 1991). Construct validity: SDMT correlates well with the Wechsler Digit Symbol subtest (r=.62 to .91) (Hinton-Bayre et al., 1999).
Known Relationships to Other Variables: (e.g., gender, education, age, etc): Performance improves with IQ (Nielsen, 1989) and declines with age (Selnes, et al., 1991).
Smith A. (1982). Symbol digit modalities test: Manual. Los Angeles: Western Psychological Services.
Rao SM. (1991). A Manual for the Brief, Repeatable Battery of Neuropsychological Tests in Multiple Sclerosis. New York: National Multiple Sclerosis Society.
Rao SM. (1991). Neuropsychological Screening Battery for Multiple Sclerosis. New York: National Multiple Sclerosis Society.
Benedict RH, Smerbeck A, Parikh R, Rodgers J, Cadavid D, Erlanger D. Reliability and equivalence of alternate forms for the Symbol Digit Modalities Test: implications for multiple sclerosis clinical trials. Mult Scler. 2012;18(9):1320-1325.
De Monte VE, Geffen GM, May CR, McFarland K. Improved sensitivity of the rapid screen of mild traumatic brain injury. J Clin Exp Neuropsychol. 2010;32(1):28-37.
Hinton-Bayre AD, Geffen GM, Geffen LB, McFarland KA, Friis P. Concussion in contact sports: reliable change indices of impairment and recovery. J Clin Exp Neuropsychol. 1999;21(1):70-86.
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Stout JC, Paulsen JS, Queller S, Solomon AC, Whitlock KB, Campbell JC, Carlozzi N, Duff K, Beglinger LJ, Langbehn DR, Johnson SA, Biglan KM, Aylward EH. Neurocognitive signs in prodromal Huntington disease. Neuropsychology. 2011;25(1):1-14.
Wechsler D. (1997). Wechsler adult intelligence scale-III. New York: Psychological Corporation.
Document last updated April 2020