Introduction
Precise grading of the severity of global cognitive decline in mild dementia is important in view of developing new therapies for the earliest stage. However, evidence shows several instruments designed for this purpose to have inadequate precision or diminished capacity to distinguish between mild dementia and normal ageing (Vellas, Andrieu, Sampaio, Coley, & Wilcock, Reference Vellas, Andrieu, Sampaio, Coley and Wilcock2008), and consequently, also suboptimal accuracy for correct classification of mild dementia versus normal ageing.
The cognitive part of the Alzheimer's Disease Assessment Scale (ADAS-cog) (Rosen, Mohs, & Davis, Reference Rosen, Mohs and Davis1984) designed to assess dementia severity and predominantly used in clinical trials, the Cambridge Cognitive Examination (CAMCOG) (Roth et al., Reference Roth, Tym, Mountjoy, Huppert, Hendrie, Verma and Goddard1986) designed to screen for dementia and to assess dementia severity, and the Mini Mental State Examination (MMSE) (Folstein, Folstein, & McHugh, Reference Folstein, Folstein and McHugh1975) were all shown to have imprecise grading of dementia severity in mild dementia (De Jager, Milwain, & Budge, Reference De Jager, Milwain and Budge2002; Harrison et al., Reference Harrison, Minassian, Jenkins, Black, Koller and Grundman2007; Tombaugh & McIntyre, Reference Tombaugh and McIntyre1992). This reduces the capacity to detect improvement of cognitive function in mildly demented patients following therapy, it limits the screening for and monitoring of preclinical and incipient dementia, and it weakens the relationship between early global cognitive decline and other clinical outcome variables. At the same time, neuropsychological tests designed to assess memory and executive functioning were shown to better measure mild degrees of cognitive decline than the ADAS-cog (Harrison et al., Reference Harrison, Minassian, Jenkins, Black, Koller and Grundman2007) and to better discriminate between incipient dementia and normal ageing than the CAMCOG (De Jager et al., Reference De Jager, Milwain and Budge2002). Regarding the MMSE, Tombaugh and McIntyre (Reference Tombaugh and McIntyre1992) suggested adding neuropsychological tests to enhance its precision and accuracy in mild dementia.
Using the Rasch measurement theory (Fischer & Molenaar, Reference Fischer and Molenaar1995; Verhelst & Glas, Reference Verhelst and Glas1995), we examine whether adding neuropsychological tests to the ADAS-cog, CAMCOG, and MMSE improves the precision of grading mild degrees of global cognitive decline and the detection of mild dementia. We examine this in particular for neuropsychological tests that measure episodic or semantic memory and executive function. The Rasch measurement theory is well known in the field of educational measurement and is increasingly in use to improve the validity of patient reported outcomes (Hobart, Cano, Zajicek, & Thompson, Reference Hobart, Cano, Zajicek and Thompson2007; Jenkinson, Fitzpatrick, Garratt, Peto, & Stewart-Brown, Reference Jenkinson, Fitzpatrick, Garratt, Peto and Stewart-Brown2001).
We extend previous findings about the additional value of neuropsychological tests in three important ways. First, we compare the neuropsychological tests with the ADAS-cog, the CAMCOG, and the MMSE simultaneously rather than separately. Second, the Rasch measurement theory assumes a single underlying dimension of global cognitive ability that is common to the CAMCOG, ADAS-cog, MMSE, and neuropsychological tests. If corroborated, it permits quantitative comparisons of individuals’ total scores on either of these instruments. Third, we examine how all items and tests relate to global cognitive ability in terms of difficulty. We examine if neuropsychological tests are more difficult than the other instruments and, therefore, if they are more suitable to grade mild degrees of global cognitive decline.
Method
Participants
We studied data from 2061 participants from 4 data sets: (I) the AMSTEL study, a population based study and two memory clinics (n = 852) (Frankfort et al., Reference Frankfort, Appels, De Boer, Tulner, Van Campen, Koks and Beijnen2006; Jonker, Schmand, Lindeboom, Havekes, & Launer Reference Jonker, Schmand, Lindeboom, Havekes and Launer1998; Walstra, Teunisse, Van Gool, & Van Crevel, Reference Walstra, Teunisse, Van Gool and Van Crevel1997), (II) three multicenter randomized controlled trials (n = 714) (Raskind, Peskind, Wessel, & Yuan, Reference Raskind, Peskind, Wessel and Yuan2000; Tariot et al., Reference Tariot, Solomon, Morris, Kershaw, Lilienfeld and Ding2000; Wilcock, Lilienfeld, & Gaens, Reference Wilcock, Lilienfeld and Gaens2000), (III) data from the diagnostic geriatric day clinic of the Slotervaart Hospital in Amsterdam (n = 182) (Meulen et al., Reference Meulen, Schmand, Van Campen, De Koning, Ponds, Scheltens and Verhey2004) and (IV) data from the Alzheimer Center of the VU University medical center in Amsterdam (n = 313) (see Van der Vlies et al., Reference Van der Vlies, Pijnenburg, Koene, Klein, Kok, Scheltens and van der Flier2007 for a description of part of the data and diagnostic workup). A total of 198 individuals (data set I, n = 23; data set II, n = 1; data set III, n = 47; data set 4, n = 127) responded to less than 20% of the items or tests and were excluded from further analysis, leaving data from 1863 individuals available. Dementia was diagnosed according to consensus guidelines (American Psychiatric Association, 1987; McKhann et al., Reference McKhann, Drachman, Folstein, Katzman, Price and Stadlan1984; McKeith et al., Reference McKeith, Galasko, Kosaka, Perry, Dickson, Hansen and Perry1996; Neary et al., Reference Neary, Snowden, Gustafson, Passant, Stuss, Black and Benson1998; Román et al., Reference Román, Tatemichi, Erkinjuntti, Cummings, Masdeu, Garcia and Hofman1993). Specific details on the diagnostic workup can be found in the original publications. All data were collected according to the principles of the Helsinki Declaration. Whenever data were collected outside the context of standard patient care, ethical approval was obtained from local medical ethical committees or institutional review boards.
Instruments
Data set I contained the CAMCOG, data set II the 13-item ADAS-cog, that is, the original 11-item ADAS-cog extended with a concentration task and a verbal delayed recall task (Mohs et al., Reference Mohs, Knopman, Petersen, Ferris, Ernesto, Grundman and Thal1997) and data sets III–IV neuropsychological tests. Data set III included the Trailmaking Test part A and B, (Reitan, Reference Reitan1955), the Visual Association Test (Lindeboom, Schmand, Tulner, Walstra, & Jonker, Reference Lindeboom, Schmand, Tulner, Walstra and Jonker2002), and the Stroop Color Word Test (Stroop, Reference Stroop1935). In addition to these tests, data set IV contained Animal Fluency (30s, 60s, 120s), Insect Fluency 60s (Lezak, Reference Lezak1995), the Mazes Test (Wechsler, Reference Wechsler1991), Digit Span Forward & Backward (Wechsler, Reference Wechsler1997), and the Digit Symbol Test (Wechsler, Reference Wechsler1997). The MMSE was available in every data set (see Table 1). Neuropsychological tests were administered by neuropsychologists or trained assessors under supervision of a neuropsychologist.
Table 1 Overview of MMSE, ADAS-cog, CAMCOG items and Neuropsychological tests
Notes. Domains: AT, Attention; CP, Constructional Praxis; EF, Executive Functioning; EM, Episodic Memory; IP, Ideational Praxis; LA, Language; OM, Orientation Memory; SM, Semantic Memory; WM, Working Memory. Instruments: MMSE, Mini Mental State Examination; ADAS-cog, Alzheimer Disease Assessment Scale; CAMCOG, Cambridge Cognitive Examination; NPE, Neuropsychological Examination. Item numbers in descriptions refer to original CAMCOG numbering, * As determined with the One Parameter Logistic Model (OPLM). c = correct answers: > cutoff point = 1, < cutoff point = 0, e = errors rating scales & ts = time in seconds: < cutoff point = 1, > cutoff point = 0. Data sets: 1, Amstel Study and Memory Clinics; 2, RCT data Janssen research foundation; 3, Neuropsychological test data diagnostic geriatric day clinic Slotervaarthospital; 4, Neuropsychological test data Alzheimer Center VU University Medical Center.
Estimation of Item Difficulties and Individual Abilities
Two approaches from the Rasch measurement theory were used in conjunction with each other to pool the data sets and to estimate an underlying dimension of global cognitive ability common to all items or tests. A problem for pooling of data sets which have only part of their items in common are the resulting blocks of missing data where the data sets have unique rather than common items. Fortunately, as long as data sets have items in common, they can still be pooled and the whole item set can be equated and analyzed with the common items. The method is hence called “common item equating” (Kelderman, Reference Kelderman1988). See Holman, Lindeboom, Glas, Vermeulen, & De Haan (Reference Holman, Lindeboom, Glas, Vermeulen and De Haan2003) for application and discussion. In our study, the MMSE items (all four data sets), animal fluency 60s (Data sets I and IV), the Stroop Color Word test, Trailmaking Test and the Visual Association Test version A (Data set III and IV) were the common items or tests.
Subsequently, the One Parameter Logistic Model (OPLM) (Verhelst & Glas, Reference Verhelst and Glas1995), a Rasch measurement model, was used to estimate a single difficulty level for each item on this dimension of global cognitive ability. In addition, the OPLM estimates an ability level for each individual on this same dimension. The item difficulty and individual ability estimates are maximum likelihood estimates of the log transformed odds (log p/(1-p)) of responding correctly. In the OPLM, the probability to respond correctly is modeled as a logistic function or s-shaped curve of the difference between the estimated item difficulty and an individual's estimated ability. By definition, the item difficulty corresponds to an ability level at which the probability of responding correctly is 50%. Someone with less ability than the item difficulty has a lower probability of a correct response; someone with more ability than the item difficulty has a higher probability of a correct response. With an increase in ability, the probability to respond correctly to any item increases, also for more difficult items. With an increase in item difficulty, the probability to respond correctly decreases for anyone even for more able individuals. With respect to these logistic s-shaped curves, a potential problem of the basic Rasch model is that the curves are assumed to have equal slopes for all of the items. However, this assumption is not realistic as some items may have better capacity to discriminate between the estimated ability levels, that is, have steeper slopes than items with less capacity, that is, those with lower slopes. The OPLM resolves this problem by extending the basic Rasch model with a slope, one for each item, to express its capacity to discriminate between estimated ability levels.
Analyses
Preparatory analyses as described elsewhere were done to facilitate the analysis (Wouters, Van Gool, Schmand, Zwinderman, & Lindeboom, Reference Wouters, Van Gool, Schmand, Zwinderman and Lindeboom2009). One item (Q64, see Table 1) was excluded from further analyses, because 95% of the responses were correct, precluding reliable difficulty estimates. Items scoring the number of seconds or errors were recoded in the scoring direction of the CAMCOG and MMSE which score the number of correct responses. Deciles were calculated for the continuous items (e.g., those measuring time in seconds) to obtain ten response categories. Categories of items were reduced by collapsing adjacent ones with the rationale as described elsewhere (Wouters, Van Gool, Schmand, & Lindeboom, Reference Wouters, Van Gool, Schmand and Lindeboom2008). For matter of simplicity, all items that were still polytomous after these revisions were then dichotomized to get a single cutoff point (incorrect vs. correct) for each item (see Table 1) for which we estimated a single difficulty. The validity of the item difficulties as measures of the underlying dimension of global cognitive ability was examined.
Under the assumption that a single dimension of global cognitive ability explains the data, it is expected (a) that with an increase in ability or total score, a patient's probability to respond correctly to a cognitive task should also increase, and (b) for the common items, the chance to respond correctly for people with equal ability levels should be similar in all data sets. The second expectation reflects the absence of a phenomenon called differential item functioning which suggests that the common items are not equally difficult across data sets. Both expectations, if met, would corroborate the common dimension of global cognitive ability. Using χ 2-tests implemented in the OPLM software package (Verhelst & Glas, Reference Verhelst and Glas1995), the fit of the individual items and overall fit to these expectations was examined. It should be noted that good fit supports the null hypothesis that the observed values are according to the expected values by the OPLM. Thus, p values > .01 are desirable, because they indicate acceptable fit. This is unlike the usual “aim” to detect significant p values. Good fit was defined as p > .01 rather than p > .05 because of the large number of items.
To facilitate interpretation, the item difficulties and patient ability levels expressed in log transformed odds (the unit of measurement of the OPLM) were linearly transformed to resemble a T-score metric (M 50; SD 10). To infer which items had the best precision for which ability level, the item difficulties were plotted against the mean ability levels (+ and – 2 standard deviations) of normal ageing individuals, of patients with MCI or mild dementia (total score MMSE > 22) and of patients with more advanced dementia (total score MMSE < 22).
We examined the level of precision of the ability estimates along the dimension using the so called “information” of the OPLM ability estimates (Mungas, Reed, & Kramer, Reference Mungas, Reed and Kramer2003). Amount of information was calculated as one divided by the error variance of each ability estimate and plotted against each ability level. As a rule, the higher the level of information, the more reliable the ability estimate. The amount of information of a certain ability level is related to classical reliability. Information levels of 10, 20, 30, and 60 would correspond to classical reliability levels of respectively 0.50, 0.75, 0.83, and 0.92 in samples with an observed variance in ability of approximately 0.20. However, the advantage of information compared to classical reliability is that it can be calculated for each ability level, which results in informative trends showing not only how reliable a test is overall, but also how reliable it is for a range of ability levels. For classical reliability this is not possible, since classical reliability reflects the proportion of true variance of the total observed variance of ability. Also, as classical reliability is based on variance, it is variance dependent. Consequently, a test that was found to have adequate reliability in a particular sample can have quite disappointing reliability in samples with lower variance in ability. We compared the information of the ability estimates based on the neuropsychological tests and MMSE items (data sets III-IV) with that of the ability estimates based on the CAMCOG and MMSE (data set I), the ADAS-cog and MMSE (data set II) (all items were dichotomized, see above) and also with that of the original polytomous MMSE (all data sets), CAMCOG (data set I), and ADAS-cog (data set II).
Diagnostic accuracy of the ability estimates was examined in two series of ROC curve analyses. First, the diagnosis of dementia was taken as the reference standard. Second, the diagnosis of either MCI or mild dementia, defined according to MMSE cutoff points ranging from 20 to 25, was taken as the reference standard. Thus, the reference standard was actually a series of composite diagnoses. The p values were obtained by bootstrapping, that is, calculating Areas Under the Curve (AUCs) for 1000 samples drawn with replacement from the actual sample. Accuracy could not be examined for ability estimates based on the MMSE and the ADAS-cog items from data set II because that data set had only AD patients.
Results
Demographic and Clinical Characteristics
Of the 1863 individuals included in the analysis, 585 (31.4%) were normal ageing individuals or had subjective complaints only, 66 (3.5%) were diagnosed with MCI, 1012 (54.3%) with AD, 133 (7.1%) with another form of dementia, and 67 (3.6%) had an unknown or psychiatric diagnosis. Individuals from data set IV were higher educated, younger and less cognitively impaired (see Table 2).
Table 2 Demographic and clinical characteristics of the participants from the four data sets
Notes. * Education measured on a 7-point scale, MCI = mild cognitive impairment; AD = Alzheimer's disease; MMSE = Mini Mental State Examination.
Validity Analyses
The validity of all item difficulties was examined (except Q64, see Methods). Several of the common items had biased difficulties, that is, systematic differences in difficulty across the data sets and were removed from the data set where they had bias (data set I: Q3, Q6, Q7, data set II: Q3, Q4, Q5, Q6, Q8, Q9, and Q10, data set III: Q9, Q11, Q55, and Q59 and data set IV: Q1, Q11, Q26, and Q54). Hence they were abandoned as common items and not used to equate the four data sets. However, they were retained in the remaining data set(s) to keep as many items as possible in the overall data set and because their difficulty could be validly estimated. For six items, difficulty levels could not be validly estimated (Q2, Q19, Q29, Q30, Q45, and Q47); these items were excluded from the whole data set (see Table 1).
Item Difficulties versus Patient Abilities
The neuropsychological tests had higher difficulty levels (M = 59.62; SD = 3.68) than the MMSE (M = 48.06; SD = 8.83), the CAMCOG (M = 49.92; SD = 6.64), and the ADAS-cog items (M = 38.93; SD = 9.10). Along the dimension of global cognitive ability, the most difficult neuropsychological tests measured memory (e.g., Q59-60, Q62-63) and executive functioning (e.g., Q56-Q58). A graphical inspection of Figure 1, showed the difficulties of the neuropsychological tests to be in the ability range of normal ageing individuals and patients with MCI or mild dementia.
Fig. 1 Items in terms of their difficulty and persons in terms of their ability level arranged on the dimension of global cognitive ability (X-axis). Upper panel: Mean person ability levels (+ and − 2 standard deviations) for demented individuals (MMSE <22; n = 861; MMSE >22; n = 284) and non-demented individuals (MMSE <22; n = 64; >22, n = 594) in relation to the estimate of overall cognitive ability for the entire population (n = 1803) (on X-axis, mean: 50, SD 10, by definition). Lower panel: MMSE items (squares) and MMSE cutoff points, ADAS-cog items (diamonds), CAMCOG items (circles) and neuropsychological tests (stripes) arranged on the dimension of global cognitive ability. Item numbering corresponds with that of Table 1. Correlation ability estimates (0–100) and MMSE total score = 0.86.
Precision of the Ability Estimates
Plotting the ability estimates against their amount of information revealed the ability estimates based on the dichotomized MMSE items and either the dichotomized CAMCOG, ADAS-cog and neuropsychological tests to have consistently more information than the original MMSE alone (see Figure 2a–d). The abilities based on the dichotomized neuropsychological tests and MMSE items (Figure 2cd) also had more information in the ability range above average than the combinations of CAMCOG or ADAS-cog plus MMSE items (both dichotomous), and the polytomous ADAS-cog and CAMCOG (Figure 2ab).
Fig. 2 Information, that is, 1/(SE2ability), (Y-axis, see Methods) along the range of global cognitive ability (X-axis, analogous to Figure 1) of the dichotomized MMSE plus either the dichotomized CAMCOG (a), the ADAS-cog (b) or the two data sets with neuropsychological tests (c–d) and of the MMSE, CAMCOG, the extended and original ADAS-cog in their polytomous form.
Accuracy
For the diagnosis of dementia, including all patients at all levels of global cognitive ability, the OPLM ability estimates did not have better accuracy than the MMSE alone. Neither when they were based on the CAMCOG and MMSE items (both AUCs 0.93, bootstrapped p value >.05) nor when they were based on the neuropsychological tests plus the MMSE items (AUCs 0.91 vs. 0.88, bootstrapped p value >.05).
However, if the analysis was restricted to the accuracy of diagnosing mild dementia according to the MMSE cutoff points 20–25, the OPLM ability estimates detected dementia better when based on the combined dichotomized CAMCOG and MMSE items than when based on the polytomous MMSE for the cutoff point of 25 (bootstrapped p values <.05). AUCs of ability estimates based on CAMCOG and MMSE items ranged from 0.88 to 0.79, AUCs of the original MMSE ranged from 0.86 to 0.64. When the ability estimates were based on the dichotomized neuropsychological tests and the MMSE, they detected dementia consistently better than the polytomous MMSE for the cutoff points 21–25 (all bootstrapped p values <.05). AUCs of the neuropsychological tests and MMSE ranged from 0.86 to 0.80, while AUCs of the MMSE alone ranged from 0.81 to 0.66 (see Figure 3).
Fig. 3 Diagnostic accuracy of the ability estimates expressed as Areas under the Curve obtained with ROC analysis (Y-axis) and the original MMSE-score (X-axis) for the diagnosis of MCI and mild dementia according to the MMSE cutoff points of 20–25. Upper panel: circles represent ability estimates based on the MMSE plus the CAMCOG, triangles represent MMSE. Lower panel: circles represent ability estimates based on the MMSE plus the neuropsychological tests, triangles represent MMSE. Asterisk (*) above MMSE cutoff points indicate a significant difference between AUCs.
Discussion
We found different rates of measurement precision for the MMSE, the CAMCOG, and the ADAS-cog and specific neuropsychological tests (see Figure 2). Adding the CAMCOG to the MMSE improved the precision in the below average to average range of estimated global cognitive ability (Figure 2a; T-score range of approximately 35 to 60), whereas the precision in the below average range of estimated global cognitive ability was improved when the ADAS-cog was added to the MMSE (Figure 2b; T-score range of approximately 30 to 40). When we added the neuropsychological tests to the MMSE, the precision at levels of estimated global cognitive ability above average was improved (Figure 2c and 2d; T-score ranges of approximately 50 to 60). Moreover, the accuracy of detecting mild dementia improved consistently after adding the neuropsychological tests to the MMSE (Figure 3 lower panel) and also after adding the CAMCOG (Figure 3 upper panel). More neuropsychological tests than CAMCOG, MMSE, and ADAS-cog items had estimated difficulty levels in the range of global cognitive ability seen in normal ageing to mild cognitive impairment and in mild dementia (Figure 1, T-score range of 50–65).
These findings are important given that the usefulness of cognitive tests designed to grade the severity of dementia was questioned for early dementia (Vellas et al., Reference Vellas, Andrieu, Sampaio, Coley and Wilcock2008). The accompanying appeal was to improve their measurement precision to increase their usefulness for diagnosis and evaluation of therapy effects in patients with early dementia.
Our results are consistent with other findings (Harrison et al., Reference Harrison, Minassian, Jenkins, Black, Koller and Grundman2007; De Jager et al., Reference De Jager, Milwain and Budge2002; Tombaugh & McIntyre, Reference Tombaugh and McIntyre1992), yet they also extend current knowledge in two ways. First, we examined these three widely used instruments together rather than separately, as was done previously, using a common dimension of global cognitive ability. The advantage of an overall dimension of global cognitive ability is obvious: it can be measured simply with a total score. This is practical for detecting dementia, monitoring patients, and evaluating effects of medication. Our results suggest to use the most difficult CAMCOG items and neuropsychological tests for the screening and grading of early and mild dementia, and to administer the easier CAMCOG and MMSE items and most ADAS-cog tests in patients with moderate to advanced dementia. Second, our results do not suggest replacement of the MMSE, the ADAS-cog, or the CAMCOG with more complex neuropsychological tests. After all, adding the neuropsychological tests did not improve accuracy and precision in already progressed dementia. Based on our results, a practical approach to obtain a more precise estimate of early decline of overall cognitive ability than obtained with, for example, the MMSE alone, could be to first administer the MMSE as the set of common items to every patient. Subsequently, only the highest scoring patients with, say, an MMSE score of 25–30, could be administered neuropsychological tests of executive functioning, language, and episodic and working memory, that would likely give additional information. On the other hand, lower scoring patients with, say, an MMSE score below 20, could be administered the easier items from the ADAS-cog or CAMCOG. Using the common item equating approach as described here, it would still be possible to compare patients’ estimates of global cognitive ability despite the fact that for some patients the MMSE was complemented with neuropsychological tests, whereas for others it was complemented with the ADAS-cog or CAMCOG.
Our findings are limited in some respects. First, in the memory clinics diagnosis was made using the cognitive test results. Thus, the clinical diagnosis of dementia as the reference standard and the cognitive tests as the index tests were not totally independent. However, dementia assessment encompasses much more than cognitive testing alone. It also includes patient history taking, informant interview, assessment of activities of daily living, neurological and psychiatric examination, laboratory testing, and neuroimaging. Second, other tests than the ones studied here may have better psychometric properties and accuracy in detecting early dementia. Third, item difficulties may have been influenced in part by the dichotomous recoding based on the OPLM analyses. This affected the precision of the ADAS-cog compared to the polytomous ADAS-cog (see Figure 2). However, even in its polytomous form the ADAS-cog had less precision in mild dementia than the neuropsychological tests. Finally, the relatively small number of patients with other forms of dementia than AD is a potential limitation for the external validity of the results.
Taken together, our findings demonstrate that grading precision of global cognitive ability and diagnostic accuracy for early and mild dementia can be improved by adding neuropsychological tests. At the same time, administration time can be kept short using subtests with difficulty levels that are tailored to the patient's global cognitive ability.
Acknowledgments
Part of the findings were presented as a poster at the 38th annual meeting of the International Neuropsychological Society, February 3–6, 2010, Acapulco de Juárez, México. This study was funded by a grant from the Internationale Stichting Alzheimer Onderzoek (ISAO). We thank Prof. Jonker and associates and the Janssen research foundation for access to the data. We also thank the Dutch Council for Educational Research and Prof. Verhelst for making the OPLM software available to us. The authors report no conflicts of interest.
