Target condition being diagnosed
Alzheimer's disease and related forms of dementia are common among older adults with a prevalence of 8% in individuals over the age of 65, increasing to a prevalence of approximately 43% in adults aged 85 and older (Thies 2012). Given the increasing number of older adults in most developing countries, the prevalence of dementia is expected to increase considerably in the coming years. Currently, an estimated 5.4 million Americans are diagnosed with Alzheimer's disease and this number is expected to increase to 6.7 million in by 2025 (Hebert 2003). Alzheimer's disease and related forms of dementia are incurable and result in considerable direct and indirect costs, both in terms of formal health care and lost productivity from both the affected individual and their caregivers (Thies 2012). Despite being incurable, there are several benefits to diagnosing Alzheimer's disease and related dementias early in the disease course. Most individuals with dementia, and their caregivers, would prefer to know a diagnosis of dementia, and earlier diagnosis of Alzheimer's disease allows these individuals to make decisions regarding future planning while they retain the capacity to do so (Prorok 2013). A diagnosis of dementia is also necessary to access certain services and support for individuals and their caregivers, and pharmacological treatments such as cholinesterase inhibitors (Birks 2006; Rolinski 2012) or memantine (McShane 2006; Wilkinson 2011) which may help slow the progression of the disease.
The diagnosis of Alzheimer's disease is clinical and based on a history of decline in cognitive functioning in memory and deficits in at least one other area of cognitive functioning (e.g. apraxia, agnosia, or executive dysfunction). There must be a decline from a previous level of functioning which results in significant social or occupational impairment (DSM 2000; McKhann 2011). A definitive diagnosis of Alzheimer's disease can only be achieved at autopsy, but a clinical diagnosis using standardized criteria is associated with a sensitivity of 81% and a specificity of 70% when compared to autopsy-proven cases (Knopman 2001).
Approximately 50% to 80% of all individuals with dementia are ultimately classified as having Alzheimer's disease (CSHA 1994; Blennow 2006; Brunnström 2009). While patients with dementias share common characteristics, subtle differences can help to provide a diagnosis in the absence of neuropathological examination. A smaller proportion of dementias are associated with dementia with Lewy bodies (Brunnström 2009) or Parkinson's disease dementia (Aarsland 2005). Dementia with a mixed etiology is present in 10% to 30% of cases (Crystal 2000; Brunnström 2009; Feldman 2003). Vascular dementias may occur more abruptly or present with a step-wise decline in cognitive functions over time and account for approximately 15% to 20% of dementias (Brunnström 2009; CSHA 1994; Feldman 2003; Lobo 2000). Patients experiencing frontotemporal dementia account for a smaller proportion of dementias (4% to 8%) and often present with problems in executive function and changes in behaviour, while memory is relatively preserved (Brunnström 2009; Greicius 2002).
The Mini-Cog is a brief cognitive test consisting of two components: a delayed three-word recall and the clock drawing test (Borson 2000). The Mini-Cog was initially developed in a community setting to provide a relatively brief cognitive screening test that was free of educational and cultural biases. Different scoring algorithms were tested to determine which combination had the optimal balance of sensitivity and specificity (McCarten 2011; Scanlan 2001). The Mini-Cog takes approximately three to five minutes to complete in routine practice (Borson 2000; Holsinger 2007; Scanlan 2001). The Mini-Cog has been reported to have little potential for bias in terms of education or language (Borson 2005; Borson 2000).
Dementia develops over a trajectory of several years. There is a presumed period when people are asymptomatic although disease pathology may be accumulating. Individuals or their relatives may first notice subtle impairments of short-term memory. Gradually, more cognitive deficits become apparent with difficulty completing complex tasks such as banking or medication management. At this point, the attribution of cognitive and memory symptoms to normal aging may cause delays in the diagnosis and treatment of Alzheimer's disease. This underscores the need for accurate, brief cognitive screening tests such as the Mini-Cog to help distinguish the cognitive changes associated with normal aging from the changes that might indicate dementia. Most older adults with memory complaints will first present to their general practitioner or other primary health care provider. Primary health care providers may then administer brief cognitive screening tests and, depending on the results of the cognitive screening, an individual may then have additional investigations or cognitive tests to determine if dementia is present. In some settings, a positive result on a brief cognitive screening test may result in a referral to a dementia specialist such as a neurologist, geriatrician, or geriatric psychiatrists. Some countries have also recommended that brief cognitive screening tests be administered to all older adults in order to help screen for undetected or asymptomatic cognitive impairment (Cordell 2013).
We are not including alternative tests in this review because there are currently no standard tests available for the diagnosis of dementia.
The Cochrane Dementia and Cognitive Improvement Group (CDCIG) is conducting a series of diagnostic test accuracy reviews of biomarkers and scales (see list below). Although the CDCIG is conducting reviews on individual tests compared to a reference standard, we plan to compare our results in an overview.
18F-FDG-PET (positron emission tomography F-2-fluoro-2-deoxy-D-glucose)
11C-PIB-PET (positron emission tomography Pittsburgh compound-C)
sMRI (structural magnetic resonance imaging)
Neuropsychological tests (Mini Mental State Examination (MMSE); Montreal Cognitive Assessment (MoCA))
Informant interviews (Informant Questionnaire on Cognitive Decline in the Elderly (IQCODE); AD8)
FP-CIT SPECT (fluoropropyl-carbomethoxy-iodophenyl-tropane single-photon emission tomography)
Cognitive diagnostic tests are required to assess cognition and assist in diagnosing conditions such as mild cognitive impairment and dementia. Comprehensive neuropsychological evaluation conducted by psychologists or dementia specialists such as general psychiatrists, geriatric psychiatrists, geriatricians, or neurologists would be the most accurate clinical procedure for assessing cognition and diagnosing dementia in older adults. However, these specialized resources are scarce and expensive and as such are not practical for routine use in the evaluation of cognitive complaints (Pimlott 2009; Yaffe 2008). While there are some cognitive tests that can be performed by health care providers who are not dementia specialists, many of these tests are time-consuming and may not be practical to use routinely in primary care or community settings (Brodaty 2006; Harris 2009; Pimlott 2009). As such, brief but relatively accurate cognitive screening tests are required for health care providers in community and primary care settings to identify individuals who may require more in-depth evaluation of cognition.
Utilizing a standard diagnostic test also promotes effective communication between health care providers. The sensitivity and specificity of such tests vary depending upon the setting in which they are utilized (Holsinger 2007). Some studies have found that the majority of older adults with dementia in primary care are undiagnosed (Boustani 2005; Sternberg 2000). In addition, many primary care providers have difficulty in accurately diagnosing dementia, and mild dementia is particularly under-diagnosed (van den Dungen 2011). Early diagnosis and treatment of dementia has numerous clinical and economic benefits for the patient, their community, and the public healthcare system (Bennett 2003; Thies 2012). Accurate diagnosis of dementia is also important in order to initiate dementia therapeutics including both non-pharmacological and pharmacological treatments, such as cholinesterase inhibitors (Birks 2006; Rolinski 2012) or memantine (McShane 2006). A brief and simple cognitive screening test that could be used in routine community settings would allow healthcare professionals or lay people to screen older adults initially for the presence of dementia. Individuals that screen positive for cognitive decline on the Mini-Cog may then be further investigated for the presence of dementia using additional cognitive tests or other investigations. Given that the Mini-Cog is brief, widely available, easy to administer, and has been reported to have reasonable test accuracy properties (Brodaty 2006), it may be well suited for use as a cognitive screening test in community settings. Other cognitive screening tests that may also be suitable for use in the community or primary care settings include the Mini Mental State Examination (Holsinger 2007), the General Practitioner Assessment of Cognition (GPCOG), or the Memory Impairment Screen (Brodaty 2006). The current review will examine the diagnostic accuracy of the Mini-Cog in community settings. Separate diagnostic test accuracy reviews are being undertaken for primary and secondary care settings.