Audit of the Addenbrooke's Cognitive Examination (ACE) in clinical practice: 2. longitudinal change

The Addenbrooke’s Cognitive Examination (ACE) is a brief ‘bedside’ cognitive assessment scale reported to be useful for the early diagnosis of dementia and the differentiation of Alzheimer’s disease (AD) from frontotemporal dementia (Mathuranath et al., 2000). Sensitivities of 93% and 82%, and specificities of 71% and 96%, were calculated for the clinical diagnosis of dementia for ACE cutoff scores of 88/100 and 83/100 respectively (Mathuranath et al., 2000), figures confirmed in a larger, independent, cohort (Larner, 2005). In practice, however, clinical diagnosis is not always clearcut; the demonstration of cognitive decline, rather than simply cognitive impairment, may be required to resolve uncertainty and substantiate a diagnosis of dementia, particularly in patients initially diagnosed with mild cognitive impairment (MCI) (Winblad et al., 2004). In the challenging group of patients with prominent mood-related symptoms, ACE scores below 88 have been reported to predict eventual diagnosis of degenerative dementia (Dudas et al., 2005). A good screening test for dementia is not, however, necessarily a good test for tracking cognitive change. No prior data on this specific point having been identified, an audit of longitudinal use of the ACE (original version) was undertaken. Over a three and a half year period (February 2002– July 2005 inclusive), all consecutive new referrals to the Cognitive Function Clinic were administered the ACE, as well as standard patient (and, where possible, informant) semi-structured interview, structural brain imaging (CT MRI), and, when necessary, formal neuropsychological assessment. Repeat ACE was undertaken at follow-up in patients whose diagnosis was uncertain at first assessment, or in whom the initial diagnosis was MCI. Widely accepted clinical diagnostic criteria for dementia, subtypes of dementia, and MCI were used. Of 285 patients assessed with the ACE over the study period, 23 (M:F1⁄4 13:10) had more than one assessment. Number of ACE assessments ranged from 2–5 (mode 2, median 2); time between first and last assessment ranged from 7–36 months (mode 8, median 13). Initial diagnoses were MCI (15), probable AD (5), probable vascular dementia (1), circumscribed amnesia secondary to profound hypoglycaemia in an insulin-dependent diabetic (1), and REM sleep behaviour disorder (REMBD) as a possible harbinger of dementia with Lewy bodies (1). Hence, at first assessment, six patients were suspected to have dementia, 17 were not demented. Based on patient and caregiver report and clinical judgement, 16 patients declined over the follow-up period, six remained static, and one improved. Final diagnoses were: AD (15); MCI (4); vascular dementia (1); circumscribed amnesia (1); idiopathic REMBD (1); and depressive pseudodementia (1). Hence, at the final assessment, 16 patients were clinically confirmed to have dementia, seven were not demented. On the ACE, 17 patients had declined, four remained static ( 2 point change in ACE score), and two improved. The two clinical/ACE mismatches were both in patients with MCI which did not progress clinically despite worsening or improvement in the ACE score. Applying the principles of evidence-based diagnosis (Qizilbash, 2002), various parameters were calculated from clinical diagnoses and ACE scores at cutoffs of 88 and 83 (Table 1). Comparing first and last assessments there was improvement in test accuracy, sensitivity remained at ceiling, false positive rate was high and specificity low consistently, false negative rate remained at floor, positive predictive value rose, negative predictive value remained at ceiling, false alarm rate fell and false reassurance rate remained at floor. Positive likelihood ratios were unimportant (<2) but negative likelihood ratios were very large. Using a lower cutoff value of 75/100, perhaps justified in a group of patients known to be cognitively impaired at outset and hence requiring reassessment, specificity and INTERNATIONAL JOURNAL OF GERIATRIC PSYCHIARTY