High Cerebrospinal Fluid Tau and Low Amyloid β42 Levels in the Clinical Diagnosis of Alzheimer Disease and Relation to Apolipoprotein E Genotype FREE

A CLINICAL diagnosis of Alzheimer disease (AD) typically requires a history from the patient and ideally an informant and results of mental status testing, physical and neurological examinations, a panel of laboratory tests, and a neuroimaging study to rule out other causes of dementia. The workup may include additional laboratory tests in atypical cases and more detailed psychometric testing and follow-up to increase the confidence of the diagnosis, especially when a patient presents with mild dementia. Current guidelines recognize an element of uncertainty in the clinical diagnosis with the terms probable or possible¹; definite AD is reserved for patients for whom results of a brain biopsy or autopsy show the requisite neuropathological features of AD. In recent series of patients with clinically diagnosed probable AD who underwent evaluation at research centers and were followed up to autopsy, the rates of agreement between the clinical and pathological diagnosis averaged about 85% to 90%.^2- 7 These studies generally involve an extensive clinical workup and longitudinal monitoring of the progressive cognitive decline in subjects to strengthen the diagnosis of AD. Such studies define the maximum in clinical accuracy but require resources that exceed those of routine clinical practice. Diagnosis at early stages of dementia is likely to be less accurate than after prolonged follow-up. The advent of cholinergic-based treatments for AD emphasizes the need for early and accurate diagnosis to allow initiation of therapy when it will be of most benefit to the patient.

There are several reasons to consider biological markers for AD, including strengthening the certainty of the clinical diagnosis, distinguishing AD from cognitive symptoms attributable to aging and from other dementias, allowing early and even presymptomatic diagnosis, and providing an index of disease activity. The efficacy of future therapies may in fact be best monitored by using surrogate biological markers, as has been shown strikingly in recent years for disorders such as acquired immunodeficiency syndrome. Such an approach may complement the evaluation of cognitive changes in AD and could streamline the evaluation of future drugs.

Biochemical markers for AD have been sought for many years.⁸ Recent progress has focused on analytes in cerebrospinal fluid (CSF) that are based on the pathologically altered proteins found in the brains of patients with AD. Specifically, studies have measured CSF levels of the microtubule-associated protein tau, the primary constituent of neurofibrillary tangles, and of a form of amyloid β protein Αβ ending at amino acid 42 (Αβ42), the major component of the parenchymal amyloid deposits. Several groups have now confirmed that tau levels are increased^9- 19 and Αβ42 levels are decreased^11,20- 21 in the CSF of patients with AD, compared with nondemented elderly control subjects. Clinical or demographic factors have not been shown to have a strong effect on variation in levels of these CSF markers, although contradictory effects of dementia severity on CSF tau levels have been reported.^10- 13,16

Apolipoprotein E (Apo E) genotype has a major effect on the risk for development of AD and the age of onset, with the Apo E ϵ4 allele increasing the risk and the Apo E ϵ2 allele protecting against it, relative to the Apo E ϵ3 allele.^22- 24 Protein interactions between Apo E and tau or Αβ have been proposed as mechanisms that could explain this genetic effect.^24- 25 Previous reports have not found an effect of Apo E genotype on CSF tau and Αβ42 levels, but the number of subjects in each genotype category was fairly small.^11- 12 We herein extend observations on the use of CSF tau and Αβ42 levels to distinguish patients with AD (AD group) from normal elderly controls (NC group) and patients with other neurological and dementing disorders (ND group). In addition, we explore the relationships between CSF tau and Αβ42 levels and clinical variables such as age, sex, dementia severity, and Apo E genotype.

Subjects were enrolled through AD research clinics and neurology services at 6 academic medical centers. Informed consent was obtained from all participants or their guardians as appropriate. All procedures were performed under institutional guidelines for studies involving human subjects. The evaluation included history from the subject (and an informant if cognitive problems or dementia were apparent), neurological examination, and cognitive screening using the Mini-Mental State Examination (MMSE).²⁶ The Hachinski Ischemic Index²⁷ was used to screen all patients with dementia. In subjects suspected of having AD or other dementia, a neuroimaging study and dementia screening blood tests were required. A standardized case report form that documented this information and the diagnosis was recorded for all subjects. In addition, clinical vignettes were provided for all patients thought to have dementia due to reasons other than AD or with atypical dementia syndromes. Diagnoses were made by the neurologist investigator(s) at each site (D.G., L.C., B.M., C.M.C., J.K., D.K., and R.G.), and vignettes were reviewed by a coordinating neurologist (D.G.) to verify subject eligibility and to standardize diagnostic classifications.

National Institute of Neurological and Communicative Disorders and Stroke–Alzheimer's Disease and Related Disorders Association criteria for probable AD were required. Patients thought to have very early dementia were included, provided that this diagnosis was supported by results of additional psychometric testing and that follow-up subsequent to the CSF collection showed progression to probable AD. Recruitment was aimed toward mild to moderate degrees of dementia. Only 7 (9%) of 82 patients with AD had MMSE scores below 10 (of a possible 30), whereas 24 (29%) had scores above 23.

Subjects were 50 years of age or older, without significant cognitive or neurological symptoms, and had normal results of a neurological examination. A score of at least 28 on the MMSE was required for eligibility. Sixty subjects were included in this group.

In addition to patients with diverse neurological conditions, patients with dementia disorders that typically enter into the differential diagnosis with AD were recruited. The neurological diagnoses were based on best clinical judgment. Dementia was diagnosed according to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition.²⁸ Wherever possible, research criteria were applied, eg, in cases of suspected frontotemporal dementia,²⁹ dementia with Lewy bodies,³⁰ and vascular dementia.³¹ Subjects with depression accompanied by symptoms or signs of impaired cognition (n=7) were included in this group of 74 patients, as the examining physicians thought that AD was unlikely to be the primary diagnosis. Depression was rated using the Hamilton Depression Scale, and a score of at least 10 was required.³²

Cerebrospinal fluid samples were collected using lumbar puncture and tested for protein and glucose levels and cell count. Samples with greater than 5 ×10⁵ red blood cells per liter or 5 ×10³ white blood cells per liter were excluded. Samples were stored at 4°C for up to 24 hours or frozen if longer storage times were needed, before being shipped to Athena Diagnostics, Inc, Worcester, Mass, for evaluation. Our study included 81 subjects from a previous study by Motter et al.¹¹ The original CSF samples from these subjects were stored at−80°C until reassay in our study. The concentrations of Αβ42 and tau in CSF do not change appreciably after storage for up to 2 years under these conditions (data not shown).

Levels of Αβ42 were quantified as described previously.¹¹ Briefly, the capture antibody 266, which recognizes Αβ residues 13 through 28, and the Αβ42–specific reporter antibody 277-2 were used as the sandwich enzyme-linked immunosorbent assay (ELISA) pair. Insignificant cross-reactivity with Αβ40 occurs using this assay. The Αβ42 peptide (Bachem, Torrance, Calif) used as the standard was dissolved in dimethyl sulfoxide, sonicated, filtered, and immediately diluted in specimen diluent to final working concentrations before freezing. Standards were stable for more than 1 year when prepared in this manner. Despite the fact that amino acid analysis was used in this study and the study by Motter et al¹¹ to establish the peptide stock concentrations, we observed a difference of approximately 2-fold in standard quantitation. Our absolute results were thus not directly comparable to those of Motter et al.¹¹ Although the Αβ42 values quantitated relatively higher than the original determinations, the paired values of the original and repeated assays were strongly correlated (Pearson r²=0.68; n=77).

Total levels of Αβ in CSF, which consist primarily of Αβ peptides ending at position 40, were not routinely analyzed in this study, as the study by Motter et al¹¹ showed that this measure, used alone or in a ratio with other measures, did not increase ability to discriminate the AD from the NC groups.¹¹

Tau levels in CSF were measured as previously described.^10- 11 The tau standard, prepared from human brain as previously described, is a different lot than used in the previous studies, yielding slightly different quantitation of samples, with higher levels than previously reported. Reassayed samples again showed a strong correlation with the original measures (Pearson r²=0.91).

Apolipoprotein E genotypes were determined using standard polymerase chain reaction methods with DNA derived from blood samples obtained from subjects.³³ Subjects consenting to blood donation included 76 in the AD, 53 in the NC, and 62 in the ND groups.

Data were analyzed using commercially available software (S-plus, version 3.4; MathSoft, Inc, Seattle, Wash). For between-group comparisons of demographic, clinical, and CSF variables, Student t tests were used, with the Holm adjustment for multiple comparisons of post hoc tests. Discrete variables such as Apo E allele frequencies were compared using χ² tests. To determine the extent to which age, sex, dementia severity, or Apo E genotype influenced CSF Αβ42 or tau levels, we used multiple linear regression models to examine the AD, NC, and ND groups, followed by backward selection, to define which predictors were significant at a the level of P≤.05.

To examine the extent to which CSF tau and Αβ42 levels individually distinguished the AD from the NC groups, we first used receiver operating characteristic curve analysis to determine cutoffs for Αβ42 and tau levels that maximized overall classification. To develop predictive cutoffs that optimized the combined use of Αβ42 and tau levels, we used classification tree statistics in the software, specifically the RPART routines developed by Therneau and Atkinson.³⁴ The approach uses recursive partitioning, a nonparametric computation that considers all possible binary splits of the data in pursuit of optimal classification.³⁵ Classification trees typically achieve a similar degree of accuracy as logistic regression in classification analyses and, in addition, provide an intuitive classification process that can be readily applied to clinical diagnosis. The analysis considered Αβ42 and tau levels simultaneously to create cutoff values that maximized heterogeneity in the classification of the AD and NC groups, generating a decision tree. To justify branches and prevent overfitting of the data in the classification model, an algorithm in RPART was used to find the most parsimonious model that optimized predictive ability. The cost of adding each successive branch to the model was weighed against its value in improving classification in a validation process that analyzed 10 successive subsets of 90% of the data set, each of which omitted a different 10% of the subjects.

The analysis included data from a total of 82 subjects in the AD group, 60 in the NC group, and 74 in the ND group. Of these, 31 subjects in the AD group, 19 in the NC group, and 31 in the ND group had been included in the previous study by Motter et al¹¹ and had sufficient stored CSF for reassay; updated clinical information was taken into account to verify clinical diagnoses for these subjects. Table 1 shows that the AD and NC groups did not differ significantly regarding age and sex. All 3 groups covered a wide age range (42-85 years [median age, 73 years] for the AD group; 52-82 years [median age, 70 years] for the NC group; and 48-89 years [(median age, 65 years] for the ND group). The ND group was slightly younger than the NC and AD groups. The AD group had a mean±SD MMSE score of 19.3±6.8 and included 24 subjects with very mild dementia, defined as an MMSE score of at least 24. Follow-up information and results of detailed psychometric testing on these mildly demented subjects indicated progressive cognitive and functional decline consistent with AD.

The Apo E ϵ4 allele frequencies of 41% in the AD group and 17% in the NC group were similar to those of published clinical series^36- 37 and showed the expected increase in Apo E ϵ4 allele frequency associated with AD. However, the Apo E allele distribution in the ND group differed significantly from that of the NC group (χ², P=.04) and included 7 patients homozygous for the Apo E ϵ4 allele. This raised the possibility that unrecognized cases of AD existed among the ND group. The ND group included patients with the following clinical diagnoses: frontotemporal dementia (n=19), Parkinson disease (PD) (with or without dementia) (n=10, of whom 5 were clinically demented), vascular dementia (n=7), depression (n=7), progressive aphasia (n=6), Lewy body dementia (n=3), amyotrophic lateral sclerosis (n=3), progressive supranuclear palsy (n=4), cortical-basal ganglia degeneration (n=2), cerebellar ataxia (n=3), mild cognitive impairment (n=3), nonprogressive amnesia after head trauma (n=1), normal-pressure hydrocephalus (n=1), dementia with motor neuron disease (n=1), multiple sclerosis with dementia (n=1), cognitive symptoms after treatment of Lyme disease (n=1), ganglioglioma with partial complex seizures (n=1), and myotonic dystrophy (n=1).

Cerebrospinal fluid levels of Αβ42 were significantly lower in the AD group compared with the other groups (Table 1). Mean levels of Αβ42 were lowest in the AD group, intermediate in the ND group, and highest in the NC group. We analyzed the effects of age, sex, severity of dementia, and Apo E ϵ4 allele on Αβ42 levels using multiple linear regression. In the AD group, age and sex were not significant covariates, while MMSE scores showed a slight positive association (higher MMSE scores correlated with higher Αβ42 levels; r²=0.12; P =.007). The strongest covariate was Apo E ϵ4, which showed a highly significant negative correlation with Αβ42 level dependent on the number of Apo E ϵ4 alleles. All of these covariates accounted for only a small amount of the variance of Αβ42 level, since overall, r²=0.33 (P<.001). The extent of the correlation due to Apo E ϵ4 copy number alone was 0.26. As shown in Figure 1, CSF Αβ42 levels in the AD group were highest in subjects with no Apo E ϵ4 alleles (1.067×10⁻⁶±4.49×10⁻⁷ g/L [1067±449 pg/mL]), intermediate in patients with a single Apo E ϵ4 copy (7.72×10⁻⁷±2.65×10⁻⁷ g/L [772±265 pg/mL]), and lowest in patients homozygous for the Apo E ϵ4 allele (5.66×10⁻⁷ g/L±2.01×10⁻⁷ g/L [566±201 pg/mL]). Levels of CSF Αβ42 in the subgroup of the AD group with no Apo E ϵ4 alleles were significantly lower than in the NC group (P<.001). For Αβ42 levels among the NC group, multiple linear regression showed a minimal decline with age (r²=0.11; P=.04), whereas sex and Apo E ϵ4 allele count (0 or 1) had no significant effects; the model accounted for a very small amount of variance (overall, r²=0.16; P=.04). Among the ND group, MMSE score was positively associated (P=.02) and Apo E ϵ4 allele count was negatively associated (P=.02) with Αβ42 levels, whereas age and sex were not significant for the overall model (r²=0.24; P<.001).

Levels of CSF tau differed significantly among all 3 groups (overall analysis of variance, Table 1) and significantly between AD vs NC groups and AD vs ND groups based on results of post hoc tests. Mean levels of tau showed the opposite gradient to Αβ42, ie, highest in the AD group, intermediate in the ND group, and lowest in the NC group. In the AD group, multiple linear regression models examined the covariates of age, sex, Apo E genotype, and MMSE score in relation to tau levels (overall, r²=0.26; P<.001). Significant predictors were sex (male more than female; P=.03), 1 Apo E ϵ4 allele (P<.001), and 2 Apo E ϵ4 alleles (P=.04). The Apo E effects were in opposite directions, however, with increased tau levels in patients heterozygous for the Apo E ϵ4 allele compared with those without the Apo E ϵ4 allele, and decreased tau levels in patients homozygous for the Apo E ϵ4 allele compared with those without the Apo E ϵ4 allele. The MMSE scores and age were not significant predictors of tau levels. Each predictor contributed to a small extent (eg, partial r²=0.12, 1 Apo E ϵ4 allele). Similar models examining CSF tau levels among the NC group showed no significant effects of age, sex, or Apo E (overall, r²=0.12). Among the ND group, the overall correlation coefficient was only 0.09, and no predictors attained statistical significance.

We examined CSF Αβ42 and tau levels individually for their ability to discriminate between the AD and NC groups. For Αβ42 levels, the optimal cutoff for differential classification was 1.0315×10⁻⁶ g/L (1031.5 pg/mL), which was used to classify correctly 115 (81.0%) of 142 subjects in the AD and NC groups, overall, with sensitivity of 78% and specificity of 83% for the diagnosis of AD. For tau levels alone, the best cutoff was 5.025×10⁻⁷ g/L (502.5 pg/mL), which was used to classify correctly 97 (68.3%) of 142 subjects in the AD and NC groups, overall, with sensitivity of 57% and specificity of 83% for the diagnosis of AD.

Better resolution of the groups was realized by applying the binary tree–structured classification system, in which tau and Αβ42 levels were considered simultaneously. This produced a tree (Figure 2) that correctly classified 121 (85.2%) of 142 subjects, with sensitivity of 90% and specificity of 80% for the diagnosis of AD. Because Apo E ϵ4 influenced Αβ42 levels, a separate classification tree analysis used Apo E ϵ4 allele count and CSF Αβ42 and tau levels as possible predictors. The Apo E ϵ4 allele count did not enter into the optimal tree, which remained identical to that in Figure 2.

For a biomarker test to be a useful adjunct in the clinical workup of AD, it should have high sensitivity. In addition, given the consequences of a positive diagnosis, very high specificity is desirable. To achieve a useful balance of sensitivity and very high specificity, we selected terminal nodes of the classification tree analysis as being diagnostically useful only if they contained relatively pure AD or NC groups (specificity of 85% or higher). Terminal nodes of the tree with specificities below 85% were considered to be undefined. This created 3 classification zones as shown in Figure 3. Applying this system to the cohorts yielded an AD zone with 76.8% sensitivity and 93.3% specificity and a non-AD zone with 70% sensitivity and 91% specificity. Overall, using the 3-zone classification system, 63 subjects in the AD group and 42 subjects in the NC group (73.9%) were correctly classified, and 11 (7.7%) were incorrectly classified. Application of the zonal classification system to the 24 patients with very mild AD (MMSE scores ≥24 of 30) resulted in 15 classified in the AD zone, 3 in the non-AD zone, and 6 in the undefined zone.

One interesting footnote to this group concerned a subject whose test results were positive for AD using CSF markers, but who had cognitively normal test results at the time of lumbar puncture. In the 30 months after the CSF test was performed, symptoms consistent with a diagnosis of mild AD, including decline in cognitive test scores, developed in this subject.

The ND group was also classified by applying the high-specificity 3-zone scheme used to differentiate the AD from the NC groups (Figure 3). Twenty-six subjects were in the AD zone; 29, in the non-AD zone; and 19, in the undefined zone (Table 2). This suggests that the AD CSF profile is not specific for AD (false positive) and may occur in a variety of conditions that damage the brain, or that AD neuropathological features may be present in many subjects with dementia and that their contribution to the dementia was underrecognized by clinicians required to make a single diagnosis (in these cases, the CSF tests yield true positive results, at variance with the clinical diagnoses). To examine these possibilities, we considered the specific clinical diagnoses and Apo E genotypes among the ND group. First, classification as AD or non-AD zone varied according to whether patients were demented. Very few patients without dementia (MMSE score >27 and no significant memory or cognitive complaints) were classified as having AD based on CSF test results (3 of 18), compared with patients with conditions associated with dementia or memory problems (23 of 56). The breakdown into specific diagnoses (Table 3) shows that within each diagnostic group, fewer of the ND group were classified as having AD using CSF levels of Αβ42 and tau. Among these disorders were dementias that are clinically difficult to distinguish from AD, ie, frontotemporal dementia, vascular dementia, progressive aphasia, and PD with dementia. Two of 3 patients with mild cognitive impairment, a possible prodrome of AD, had an AD profile. Further evidence that subjects in the ND group with an AD profile actually may have AD was provided by the Apo E ϵ4 allele frequencies. Overall, the ND group had a slightly elevated Apo E ϵ4 allele frequency (27%, including 7 patients homozygous for the Apo E ϵ4 allele) compared with the NC group. In the ND subgroup classified in the AD zone, the Apo E ϵ4 allele frequency was 45%, similar to the 41% of the AD group. Of the 7 patients homozygous for the Apo E ϵ4 allele among the ND group, 5 were classified in the AD zone. Conversely, in the ND subgroup in the non-AD zone, the Apo E ϵ4 allele frequency was 22%, similar to the NC group value of 17%. Whereas presence of the Apo E ϵ4 allele alone is not synonymous with AD, its increased frequency in this subset of the ND group strongly increased the likelihood of an admixture of AD.

Only neuropathological examination will reveal whether the CSF tests or the clinician is correct in these cases where the 2 sets of results disagree. In the course of our study, 2 subjects in the ND group classified in the AD zone using CSF test results have undergone autopsy. One had rapidly progressive dementia accompanied by amyotrophy with onset at 81 years of age, clinically diagnosed as atypical amyotrophic lateral sclerosis–dementia complex, possibly frontal lobe dementia; autopsy 6 months later revealed plaques and tangles in sufficient quantities to meet a neuropathological diagnosis of AD. The second subject had seizures and cognitive decline. A right temporal lobe ganglioglioma was found on biopsy, and examination of the temporal lobe tissue removed during surgery to resect the tumor did not indicate the presence of AD. Another 4 patients in the ND group who were not classified in the AD zone using CSF markers have since undergone autopsy, and all were confirmed not to have AD. Seven autopsy-confirmed AD cases were AD positive based on results of CSF testing.

The 9 patients in the ND group with Αβ42 levels below 7.61×10⁻⁷ g/L (761 pg/mL), a classification tree zone occupied almost exclusively by patients with AD in the AD vs NC group comparison, consisted of 4 patients with frontotemporal dementia and 1 patient each for diagnoses of primary progressive aphasia (Apo E ϵ4/ϵ4), depression with cognitive changes (MMSE score, 21; Apo E ϵ4/ϵ4), vascular dementia, PD with depression and memory complaints (Apo E ϵ4/ϵ4), and Lyme disease followed by cognitive complaints (MMSE score, 20; Apo E ϵ3/ϵ4). All of these patients presented with dementia or cognitive impairment, while none of the cognitively normal subjects in the ND group had an Αβ42 level of less than 7.61×10⁻⁷ g/L (761 pg/mL). Alzheimer disease likely contributed to the clinical picture in some of these cognitively impaired patients.

Our study is consistent with and extends the results of a previous report showing that Αβ42 levels are decreased and tau levels are increased in CSF in AD.¹¹ The larger number of subjects in our report, recruitment of very mildly demented individuals, and the larger number of subjects with Apo E genotype provided adequate statistical power to examine the effects of a number of covariates. Across an age range of 52 to 82 years, cognitively normal individuals showed only a small trend toward increased CSF tau levels and a minimal effect on Αβ42 levels. Dementia severity did not influence tau levels, as we and others^11- 12 have previously reported, and in fact tau levels have been shown to be elevated in very mild AD.^38- 39 Interestingly, the extent of cognitive impairment slightly affected Αβ42 levels. The decrease in this marker with increasing dementia is consistent with a reduction in viable neurons producing Αβ42 or a larger fraction of Αβ42 being sequestered in plaque or otherwise unable to enter the CSF. The fact that total Αβ levels do not decrease in the CSF of patients with AD¹¹ favors the latter argument.

Apolipoprotein E genotype was the major significant covariate related to Αβ42 levels in the AD group in an Apo E ϵ4 allele dose-dependent fashion. Patients homozygous for the Apo E ϵ4 allele had the lowest mean level of Αβ42, while those lacking an Apo E ϵ4 allele had the highest level. Regardless of whether subjects had 0, 1, or 2 Apo E ϵ4 alleles, the AD group showed lower Αβ42 mean levels than the NC group. One possible explanation for the genotype effect is that the frequency of misdiagnosis may be higher in subjects lacking an Apo E ϵ4 allele,³⁷ artifactually raising the group mean. However, tau levels did not show a similar Apo E ϵ4 dose dependence, and we would have expected both measures to be similarly distorted by misdiagnoses. Failure to find this genotypic effect on Αβ42 levels in an earlier study¹¹ was likely due to the insufficient number of patients with AD lacking an Apo E ϵ4 allele, as that study had an unusually high Apo E ϵ4 frequency (58%).

It was suggested originally that the decrease in CSF Αβ42 levels was related to the specific deposition of this protein, as opposed to the bulk Αβ levels, in amyloid plaque.¹¹ Our data strengthen this idea by linking Αβ42 levels inversely to Apo E ϵ4 gene dosage. The plaque burden in AD has been well documented to increase with Apo E ϵ4 gene dosage,^40- 41 consistent with the lowering of CSF Αβ42 levels being most pronounced in patients homozygous for the Apo E ϵ4 allele. A link between CSF Αβ42 levels and plaque burden would also explain the diagnostic specificity for AD associated with the Apo E genotype and CSF Αβ42 levels. Ongoing studies will use neuropathological correlates to further test this idea.

The undefined zone in Figure 3 included patients who showed high CSF tau and Αβ42 levels. Numerous studies have shown elevation of CSF tau levels can occur not only in AD but also in other neurological and neurodegenerative disorders. The tau released into CSF has been shown, in patients with AD or other disorders, to be a fragment lacking much of the portion of tau thought to be critical in paired helical filament formation.⁴² Only rarely have elevated CSF tau levels been observed in normal controls, suggesting that the finding of high CSF tau levels indicates an underlying neuropathological process.

The ND group included the most diagnostically challenging patients, and CSF Αβ42 and tau levels could not be fully interpreted in the absence of autopsy examination. Thus the patients with non-AD dementias enrolled in our study may not be typical of those seen in community practice settings. Although AD is by far the most common cause of dementia, its distinction from other conditions associated with dementia can be clinically difficult. One factor that may lead to misdiagnoses is the lack of robust clinical criteria for identifying patients with specific non-AD dementing conditions. Another confounding factor is that AD may coexist with other causes of dementia, making the clinical diagnosis of non-AD dementias more uncertain than that of probable AD. For example, studies of dementia conditions clinically diagnosed as frontotemporal dementia,^29,43 progressive aphasia,^44- 45 progressive supranuclear palsy,⁴⁶ and cortical-basal ganglia degeneration⁴³ have all revealed a few patients who have AD lesions at autopsy, alone or in conjunction with another degenerative condition. In vascular dementia and dementia associated with PD, coexistence of a significant burden of AD lesions is a common finding.^47- 48

In view of these reservations, interpretation of the CSF data in the ND group can only be tentative. Nonetheless, we identified several interesting observations and trends. First, there was a high Apo E ϵ4 allele frequency among the ND group, especially those with an AD-like profile of CSF Αβ42 and tau levels (Table 3). In clinical series, association between the Apo E ϵ4 allele and disorders such as PD⁴⁹ and Pick disease^50- 51 have not been found, and in autopsy series, the presence of the Apo E ϵ4 allele in non-AD dementias can be related to coexisting AD abnormalities.⁵² This is consistent with the possibility that patients with non-AD dementia and the Apo E ϵ4 allele are more likely to harbor AD abnormalities, which fits with the striking increase in Apo E ϵ4 allele frequency (45%) in such patients with an AD-CSF profile. Second, no particular subgroup of the ND group was classified consistently as having AD. Third, only 9 patients in the ND group were classified as having AD because of high CSF tau values (Αβ42, 7.61×10⁻⁷-1.031×10⁻⁶ g/L [761-1031 pg/mL]; tau, >5.03×10⁻⁷ g/L [>503 pg/mL]), whereas 16 were classified as having AD because of Αβ42 levels below 7.61×10⁻⁷ g/L (761 pg/mL). Therefore, low CSF Αβ42 levels were more important than tau levels in determining the classification of the ND group. As there is no evidence that Αβ deposition occurs nonspecifically in other neurological conditions besides AD, the CSF results hinted at undiagnosed AD rather than false positive results in the ND group, assuming the lowering of CSF Αβ42 levels is related to plaque formation.

Our study has confirmed and extended earlier findings that low Αβ42 and high tau levels characterize the CSF of most patients with AD and supports several potential clinical applications of these measures. In patients suspected of having AD, but in whom the clinical diagnosis is uncertain, CSF Αβ42 and tau levels can be used to strengthen the diagnosis. When more than 1 potential cause of dementia is suspected, these CSF markers may help to establish that AD is a contributory factor. With the advent of symptomatic treatment for cognitive symptoms of AD, and as drugs are developed to slow the progression of AD, very early diagnosis is important to define the earliest window of opportunity to apply such treatments. At this early stage of AD, it is more difficult to document a history of progressive cognitive and functional decline or to demonstrate deficits on cognitive testing. This often raises the clinical question of whether symptoms represent age-associated changes or AD, a decision that the biomarker tests may help to resolve. A major future focus of further CSF studies and studies of other biological markers will be subjects with mild cognitive impairment or with risk factors for dementia combined with minimal cognitive symptoms. In this regard, it is encouraging that most patients with AD and very mild dementia (MMSE score ≥24) were classified as having an AD CSF profile, indicating that measuring CSF Αβ42 and tau levels may be helpful in diagnosing very early or mild AD.

Accepted for publication December 2, 1997.

Supported in part by Athena Neurosciences, Inc, South San Francisco, Calif, and Athena Diagnostics, Inc.

Reprints: Peter Seubert, PhD, Athena Neurosciences, Inc, 800 Gateway Blvd, South San Francisco, CA 94080.