Short Cognitive Tests in the Diagnosis of Dementia
When a professional asks a patient to perform various physical tasks, it is self-evident that they are examining the patient, not performing a diagnostic test. The same is true for cognitive tasks: short cognitive tests are part of the examination, albeit one aided by standardisation. Once this is understood then many of the problems associated with short cognitive tests and their interpretation are highlighted.
The limitations of short cognitive tests are the same as for any form of physical examination. Unless they are combined with a history they can be surprisingly unhelpful in making a diagnosis. This can be illustrated by comparing the diagnosis of a stroke to that of dementia.
When a patient presents with unilateral weakness, an examination alone gives much less information that a full assessment. The briefest history clarifies the diagnosis: the statement "Mrs Jones was fine last night but awoke this morning aphasic and weak down the right side" strongly suggests a stroke. In contrast a physical sign in isolation: such as an extensor right plantar response is of limited use and certainly should not be used alone to diagnose stroke.
Similarly a cognitive score alone should not lead to a diagnosis. A very brief history such as "Mr Jones has begun to ask the same question several times an hour" in addition to poor episodic memory on a short test strongly suggests AD, a score of 22/30 on the MMSE should not be used alone to diagnose AD.
Most short cognitive test results are summarised by a number with a top score between 4 and 100, the scores are not points on an absolute scale and scoring schemes for short tests are subjective and subject to manipulation for external aims, such as ensuring that the total score adds up to a round number. The reduction of human cognitive function to a single figure has its uses: a numerical value can help assimilate information about a patient rapidly and experienced clinicians have an idea of what a MMSE of 15 or 25 suggests about a patient with AD. However, reductionism can lead to the belief that a score of 24 on the MMSE is normal while 23 signifies dementia (particularly in validation studies). Cut-offs are a rough guide and should not be determined in one clinical scenario and applied in another.
The pattern of the scoring is often more important than the overall score. A native French speaker scoring 87/100 on the ACE-III administered in English, with reduced verbal fluencies and slight difficulty in naming line drawings probably has normal cognition. An English professor scoring 87/100 with all points lost on recall and recognition of material learnt within the test probably has early AD.
External circumstances can influence the score. In the MMSE 5 points (17% of the total) are awarded for orientation in place so a patient tested in hospital may score fewer points than at home.
Education and knowledge of the test language will influence the scores. Short cognitive tests cannot be entirely independent of educational influence whatever the authors claim (a person who has not been taught to draw a clock will not be able to do one in a test). Devising easy tests in the native language in countries with universal secondary education should minimise the effect of education and language as shown in the original TYM study. In lower income countries the level of education is likely to have a larger influence on the score of cognitive tests, this was shown in the Greek TYM validation study conducted largely in rural areas with limited education. Minority ethnic groups could be disadvantaged in short tests unless they are fluent in the local language and familiar with local culture.
Anxiety, attention, motivation and physical handicaps will influence results.
A single test will not suffice, a test far too easy for a graduate with mild problems in clinic, may be far too difficult for an elderly inpatient with learning difficulties.
Limitations of Short Cognitive Tests
Short Cognitive Tests are Part of the Examination of the Patient, They are not Diagnostic Tests
When a professional asks a patient to perform various physical tasks, it is self-evident that they are examining the patient, not performing a diagnostic test. The same is true for cognitive tasks: short cognitive tests are part of the examination, albeit one aided by standardisation. Once this is understood then many of the problems associated with short cognitive tests and their interpretation are highlighted.
The limitations of short cognitive tests are the same as for any form of physical examination. Unless they are combined with a history they can be surprisingly unhelpful in making a diagnosis. This can be illustrated by comparing the diagnosis of a stroke to that of dementia.
When a patient presents with unilateral weakness, an examination alone gives much less information that a full assessment. The briefest history clarifies the diagnosis: the statement "Mrs Jones was fine last night but awoke this morning aphasic and weak down the right side" strongly suggests a stroke. In contrast a physical sign in isolation: such as an extensor right plantar response is of limited use and certainly should not be used alone to diagnose stroke.
Similarly a cognitive score alone should not lead to a diagnosis. A very brief history such as "Mr Jones has begun to ask the same question several times an hour" in addition to poor episodic memory on a short test strongly suggests AD, a score of 22/30 on the MMSE should not be used alone to diagnose AD.
Reducing Human Cognition to a Single Number
Most short cognitive test results are summarised by a number with a top score between 4 and 100, the scores are not points on an absolute scale and scoring schemes for short tests are subjective and subject to manipulation for external aims, such as ensuring that the total score adds up to a round number. The reduction of human cognitive function to a single figure has its uses: a numerical value can help assimilate information about a patient rapidly and experienced clinicians have an idea of what a MMSE of 15 or 25 suggests about a patient with AD. However, reductionism can lead to the belief that a score of 24 on the MMSE is normal while 23 signifies dementia (particularly in validation studies). Cut-offs are a rough guide and should not be determined in one clinical scenario and applied in another.
Interpreting Results
The pattern of the scoring is often more important than the overall score. A native French speaker scoring 87/100 on the ACE-III administered in English, with reduced verbal fluencies and slight difficulty in naming line drawings probably has normal cognition. An English professor scoring 87/100 with all points lost on recall and recognition of material learnt within the test probably has early AD.
External circumstances can influence the score. In the MMSE 5 points (17% of the total) are awarded for orientation in place so a patient tested in hospital may score fewer points than at home.
Education and knowledge of the test language will influence the scores. Short cognitive tests cannot be entirely independent of educational influence whatever the authors claim (a person who has not been taught to draw a clock will not be able to do one in a test). Devising easy tests in the native language in countries with universal secondary education should minimise the effect of education and language as shown in the original TYM study. In lower income countries the level of education is likely to have a larger influence on the score of cognitive tests, this was shown in the Greek TYM validation study conducted largely in rural areas with limited education. Minority ethnic groups could be disadvantaged in short tests unless they are fluent in the local language and familiar with local culture.
Anxiety, attention, motivation and physical handicaps will influence results.
A single test will not suffice, a test far too easy for a graduate with mild problems in clinic, may be far too difficult for an elderly inpatient with learning difficulties.
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