Time that tells: critical clock-drawing errors for dementia screening

Clock-drawing tests are popular components of dementia screens but no single scoring system has been universally accepted. We sought to identify an optimal subset of clock errors for dementia screening and compare them with three other systems representative of the existing wide variations in approach (Shulman, Mendez, Wolf-Klein), as well as with the CDT system used in the Mini-Cog, which combines clock drawing with delayed recall.

The clock drawings of an ethnolinguistically and educationally diverse sample (N = 536) were analyzed for the association of 24 different errors with the presence and severity of dementia defined by independent research criteria. The final sample included 364 subjects with > or = 5 years of education, as preliminary examination suggested different error patterns in subjects with 0-4 years of education and inadequate numbers of normal controls for reliable analysis.

Eleven of 24 errors were significantly associated with dementia in subjects with > or = 5 years of education, and six were combined to identify dementia with 88% specificity and 71% sensitivity: inaccurate time setting, no hands, missing numbers, number substitutions or repetitions, or refusal to attempt clock drawing. Time setting was the most prevalent error at all dementia stages, refusal occurred only in moderate and severe dementia; and ethnicity and language of administration had no effect. All critical errors increased in frequency with dementia stage. This simplified scoring system had much better specificity than two other systems (88% vs 39% for Mendez's system - 63% for Shulman's) and much better sensitivity than Wolf-Klein's (71% vs 51%). Stepwise logistic regression found the simplified system to be more strongly predictive of dementia than the three other CDT systems of dementia. Substituting the new CDT algorithm for that used in the original CDT Mini-Cog improved the Mini-Cog's specificity from 89 to 93% with minimal change in sensitivity.

Only six errors need be assessed to capture most of the power of clock drawing to discriminate between people with dementia and normal subjects, and improves specificity over older systems in subjects with > or = 5 years of education. These errors require minimal conceptual classification and are easily detected and scored by non-specialists.