Alzheimer's Assessment Scale May Not Be Accurate Enough

Abstract

Portions of one of the most commonly used tests to measure cognitive performance in Alzheimer's disease trials may be too easy and may not accurately assess the range of patients' cognitive abilities or detect their change over time, according to two complementary studies. Analyses of Alzheimer's Disease Assessment Scale – Cognitive Behavior Section (ADAS-Cog) scores measured in 193 patients with mild disease who participated for 2 years in the Alzheimer's Disease Neuroimaging Initiative detected limitations of the scale that could be improved, reported Dr. Jeremy Hobart of the Clinical Neurology Research Group at Plymouth (England) University Peninsula Schools of Medicine and Dentistry and his colleagues. The investigators used observational data from the initiative to show that out of 675 measurements made at time points of 0, 6, 12, and 24 months, 8 of the scale's 11 components (all except word recall, word recognition, and orientation) had a skewed distribution of scores (Alzheimers Dement. 2013;9[1 Suppl]:S4-9). The mean age of patients was 74 years and participants had a mean Mini-Mental State Examination score of 23. More than three-fourths of the participants with mild Alzheimer's disease in the study scored either 0 or 1 on the majority of ADAS-Cog components. Dr. Hobart and his associates remarked that this would mean that few or no cognitive problems were detected. “However, as there is almost certainly greater variance in patient ability, this finding points to a limitation in the ADAS-Cog score function – namely that the ADAS-Cog, in its current form, is not subtle enough to record and monitor variance in the mildest stages of AD-type dementia.” In a second study that analyzed how accurately ADAS-Cog measured cognitive performance in the same sample of patients, the investigators tried to avoid the limitations imposed by classical means of assessing reliability and validity of scales. They did this by using a method, called Rasch Measurement Theory, that can expose anomalies in a scale that can be improved and then retested. The range of cognitive performance measured by the 11 ADAS-Cog components suboptimally targeted the range of cognitive performance observed in patients in the sample. In six of the components, the integer-based scoring method used to assign cognitive performance did not reflect a continuum of performance as it was intended to, but instead indicated that some component scores were much more likely to occur than others, meaning that a higher score on one of these components did not confirm more cognitive impairment. These gaps in some of the components' abilities to measure cognitive performance means that their precision is limited and that the raw scores of the ADAS-Cog do not have a linear relationship, the investigators said (Alzheimers Dement. 2013;9[1 Suppl]:S10-20). Both studies were supported in part by grants from an anonymous foundation, the U.K. National Institute for Health Research, and the U.S. National Institutes of Health. LTC Perspective“The instruments that we use to evaluate cognition in both research and clinical care need refinement,” said Dr. David Smith, CMD, president of Geriatric Consultants of Central Texas. “The work described is on target for that task.“Hobart and colleagues have correctly identified the most important shortfall of current instruments, a somewhat deficient utility in stratifying mild dementia (let alone minimum cognitive impairment of the amnestic and nonamnestic types).“In clinical care and especially in targeted screening, one of the barriers consistently identified by primary care physicians is the time required to administer the instruments. Components of testing that lack utility need to be replaced by components that are useful, in the interest of efficiency as well as accuracy. This will incentivize primary care physicians to use them.” “The instruments that we use to evaluate cognition in both research and clinical care need refinement,” said Dr. David Smith, CMD, president of Geriatric Consultants of Central Texas. “The work described is on target for that task. “Hobart and colleagues have correctly identified the most important shortfall of current instruments, a somewhat deficient utility in stratifying mild dementia (let alone minimum cognitive impairment of the amnestic and nonamnestic types). “In clinical care and especially in targeted screening, one of the barriers consistently identified by primary care physicians is the time required to administer the instruments. Components of testing that lack utility need to be replaced by components that are useful, in the interest of efficiency as well as accuracy. This will incentivize primary care physicians to use them.”