The Clock Drawing Test, or CDT, is a quick screening method used by healthcare professionals to assess certain cognitive abilities. It provides a brief snapshot of brain functions related to memory, executive function, and visuospatial skills. The test is valued for its simplicity and the speed at which it can be administered, often taking only a couple of minutes.
Administering and Scoring the Test
The test is typically administered in one of two ways. A person may be given a blank piece of paper and asked to draw a clock from memory, a method known as the “free-drawn” command. Alternatively, they might be provided with a pre-drawn circle and instructed to fill in the numbers and hands. A common instruction is, “Please draw a clock, put in all the numbers, and set the hands to a specific time,” such as ten past eleven. This specific time is chosen because it requires the person to process that “10” minutes corresponds to the number “2” on the clock face.
Scoring the CDT involves various standardized systems which help clinicians evaluate the drawing objectively. These systems award points for correctly placing numbers, drawing the hands to the specified time, and creating a complete clock face. For example, a simple pass-fail system might be used where a completely correct clock receives full points and any error results in a score indicating an abnormal result.
Features of an Abnormal Drawing
An abnormal clock drawing can present with several distinct types of errors, each pointing to different aspects of cognitive function. These inaccuracies are categorized to help clinicians understand the potential deficit and provide more specific information than a simple score.
Visuospatial and planning deficits are among the most common errors. These include a misshapen or disproportionately large or small clock circle, numbers crowded into one half of the clock, or numbers written outside the circle entirely. For instance, a person might squeeze all the numbers from 1 to 12 into the right side of the clock, leaving the left side empty, an error known as hemispatial neglect. Difficulties with the spatial layout of numbers are often associated with issues in the parietal lobe of the brain.
Conceptual errors reflect a misunderstanding of the properties of a clock. This can manifest as missing numbers, representing the time digitally (e.g., writing “11:10”) instead of drawing hands, or placing the hands incorrectly. A specific conceptual error, known as a stimulus-bound response, occurs when a patient asked to set the time to “ten past eleven” draws the minute hand pointing to the number 10 instead of the 2. Such mistakes suggest difficulty with abstract thinking and executive functions, which are often linked to the frontal lobe.
Another revealing feature is perseveration, which involves the inappropriate repetition of a response. In the CDT, this might appear as drawing more than two hands or continuing to write numbers past 12, such as 13, 14, and 15. This type of error indicates a problem with terminating an action and is linked to impairment of executive functions managed by the prefrontal area of the brain.
Conditions Linked to Abnormal Results
Difficulties with the CDT are frequently observed in individuals with Alzheimer’s disease, where errors in spatial planning and conceptualization are common. Patients with vascular dementia, which results from impaired blood flow to the brain, may also show significant deficits in their drawings. Similarly, individuals with Parkinson’s disease can exhibit errors, often related to motor control and planning, such as producing an unusually small clock. The test can also detect cognitive changes following a stroke or a traumatic brain injury.
It is important to understand that performance on the CDT can be influenced by other factors, such as level of education and age. Therefore, an abnormal drawing is not definitive proof of a specific disease.
What Happens After an Abnormal Test
Receiving an abnormal result on the Clock Drawing Test is a prompt for further medical evaluation, not a final conclusion. The CDT is a screening instrument, meaning its purpose is to identify individuals who might have a cognitive issue that requires a more detailed examination. The result is one piece of information in a larger diagnostic puzzle.
Following an abnormal CDT, a healthcare provider will typically recommend a comprehensive assessment. This evaluation often begins with more detailed neuropsychological testing to measure memory, attention, and problem-solving skills more thoroughly. A physical examination, a review of medical history, and blood tests are also standard procedures to rule out other causes of cognitive symptoms.
To get a clearer picture of the brain’s structure and function, brain imaging studies like an MRI or CT scan may be ordered. These scans can help identify signs of stroke, tumors, fluid buildup, or patterns of brain atrophy associated with different types of dementia. The combination of these results helps clinicians arrive at an accurate diagnosis and develop an appropriate management plan.