The clock-drawing test (CDT) has become one of the standard cognitive screening tools used around the world. Like all cognitive screening tests, it is designed to detect early brain changes to determine if an individual may be suffering from a form of dementia. The test uses a pre-drawn circle and consists of asking the patient to put in the numbers of the clock and then to set the time to ‘10 past 11’. How did this particular test achieve such popularity among clinicians and why is it so useful?
Originally, the clock-drawing test was cited in a leading neurology textbook as a means of specifically assessing parietal lobe function in the brain, because that is the location of visuospatial ability. However, in the 1980s, our group at Sunnybrook (among others) began using the clock-drawing test and assessing its value as a broader cognitive screening tool comparable to the widely used Mini Mental State Examination (MMSE). The MMSE is a 30-item test that assesses orientation to time and place, attention, short term memory, verbal skills, and visuospatial ability. It takes about 10 minutes to administer.
The CDT’s acceptability, shorter time (2 minutes), and ease of administration have made it very popular. Drawing a clock requires multiple cognitive functions (not just visuospatial ability). It requires: visual memory of a clock, planning ability and concentration in order to place the numbers evenly around the clock circle, abstract ability to set the time using the symbol of hands which point to the number ‘2’ to represent 10 minutes past the hour. The latter task is very sensitive to cognitive impairment as affected individuals often ‘pull’ the minute hand to point to the number ‘10’ rather than ‘2’.
It turns out that clock-drawing correlates very well with the MMSE. It takes less time to administer and is generally very well accepted. It is as sensitive to cognitive impairment as the MMSE and picks up changes in cognition over time. Moreover, it tests frontal-executive, higher-level brain functioning (planning and abstract ability) in a way that the MMSE does not. Also, the visual impact of abnormal clocks is often an eye opener for families who may not have been aware of the full extent of cognitive dysfunction. The clock test is frequently used in combination with the MMSE as a screening battery.
Clock-drawing is, of course, subject to the same limitations as all screening tests, including the MMSE. It is affected by level of education and should not be used alone for diagnostic purposes or for assessing the severity of cognitive impairment. However, like all good screening tests, it does tap into multiple cognitive functions and can provide a ‘signal’ that calls for further inquiry or more investigations.
Importantly, it also establishes a baseline for future monitoring to determine if cognitive impairment is worsening and thus increasing the likelihood of an underlying dementia. This type of monitoring using the CDT is best done by a health care professional.
Finally, the often asked question: “What will happen to the generation of kids who have grown up with digital clocks?” Well, for them, this test may indeed become obsolete. However, we still have many more years of the current cohort of middle-aged and elderly people for whom this test will still be useful.