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Symbol Digit Modalities Test
Please visit this website for more information about the instrument: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.
NeuroRehab Supplemental - Highly Recommended
Recommendations for Use: Indicated for studies requiring a measure for processing speed.
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, and includes:
  • Timed 25 Foot Walk
  • 9-Hole Peg Test
  • Sloan Low Contrast Letter Acuity
  • Symbol Digit Modalities Test<//li>
Each of the measures in the Consortium Battery can be used individually as primary or secondary endpoints.
Supplemental:  Mitochondrial Disease (Mito), 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 sustained attention, processing speed, 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 participant 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 an 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 and/or oral. (Classic administration is written form followed by oral form.)
Trained examiners. A written or oral version of the test may be administered. When significant upper extremity motor impairment exists, examiners might consider substituting the oral version (or adding it to allow for comparison).
Test can be completed in under 5 minutes.
Location of administration: (clinic, home, telephone, etc.): Clinical Setting.
Intended respondentt: Patient.
# of items: N/A.
# of subscales and names of sub-scales: N/A.
Comments/Special Instructions
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.
NeuroRehab Specific: It is broadly used in the literature, has alternate forms and appropriate norms, new norms project completed.
Scoring and Psychometric Properties
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.
Reliability: 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).
Strengths: Brief, easy to administer. Oral form allows for valid assessment of processing speed with minimal impact of peripheral motor. Multiple forms available. Updated norms correct for age, sex, and education. A good way to measure processing speed in a more unstructured manner than other processing speed measures. Sensitive to changes in pre-manifest HD in cross-sectional and longitudinal studies. Widely used in multiple sclerosis and for individuals with motor impairment.
Weaknesses: More severe motor impairment may influence results, especially on the written version. An oral form of the test is also available, although much less is known about this version when used alone.The manual has old norms and new publications of norms should be used instead (i.e., Strober et al., 2020)
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.
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).
Key Reference:
Smith A. Symbol Digit Modalities Test: Manual. Los Angeles: Western Psychological Services; 1982.
Additional References:
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 Sep;18(9):1320-5.
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 Jan;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 Feb;21(1):70-86.
Knopman DS, Mosley TH, Catellier DJ, Coker LH; Atherosclerosis Risk in Communities Study Brain MRI Study. Fourteen-year longitudinal study of vascular risk factors, APOE genotype, and cognition: the ARIC MRI Study. Alzheimers Dement. 2009 May;5(3):207-14.
Mahant N, McCusker EA, Byth K, Graham S; Huntington Study Group. Huntington's disease: clinical correlates of disability and progression. Neurology. 2003 Oct 28;61(8):1085-92.
Rao SM. A Manual for the Brief, Repeatable Battery of Neuropsychological Tests in Multiple Sclerosis. New York: National Multiple Sclerosis Society; 1991.
Rao SM. Neuropsychological Screening Battery for Multiple Sclerosis. New York: National Multiple Sclerosis Society; 1991.
Selnes OA, Jacobson L, Machado AM, Becker JT, Wesch J, Miller EN, Visscher B, McArthur JC. Normative data for a brief neuropsychological screening battery. Multicenter AIDS Cohort Study. Percept Mot Skills. 1991;73(2):539-550.
Strober LB, Bruce JM, Arnett PA, Alschuler KN, Lebkuecher A, Benedetto MD, Cozart J, Thelen J, Guty E, Roman C. Corrigendum to: A new look at an old test: Normative data of the symbol digit modalities test-Oral version [Multiple Sclerosis and Related Disorders 43 (2020) 102154]. Mult Scler Relat Disord. 2020 Aug;43:102301
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. Wechsler adult intelligence scale-III. New York: Psychological Corporation. 1997.
Document last updated March 2024