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RAPID COMMUNICATION |
1 Departments of Neurology and 2 Geriatric Medicine, 3 Laboratory of Pediatrics and Neurology, Radboud University Nijmegen Medical Centre, The Netherlands.
Address correspondence to Marcel M. Verbeek, PhD, MSc, Radboud University Nijmegen Medical Centre, Laboratory of Pediatrics and Neurology (830 LKN), P.O. Box 9101, 6500 HB Nijmegen, The Netherlands. E-mail: m.verbeek{at}cukz.umcn.nl
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Background. The differentiation of Alzheimer's disease (AD) from vascular dementia (VaD) is hampered by clinical diagnostic criteria with disappointing sensitivity and specificity. The objective of this study was to investigate whether cerebrospinal fluid (CSF) levels of total tau protein (t-tau), amyloid ß42 protein (Aß42), and tau phosphorylated at threonine 181 (p-tau181) are useful biomarkers to distinguish AD patients from VaD patients.
Methods. We measured CSF levels of p-tau181, Aß42, and t-tau in 86 patients with a clinical diagnosis of AD or VaD and in 30 control participants.
Results. Optimal differentiation between AD and VaD was achieved by using the ratio of the CSF levels of Aß42 and p-tau181 (Q Aß42/p-tau) with sensitivity, specificity, positive and negative predictive values all 
85%.
Conclusions. Our results support further efforts to prospectively validate the use of Q Aß42/p-tau as a biomarker to discriminate between AD and VaD.
The clinical diagnostic criteria currently used for AD and VaD (1,2) have disappointing sensitivity and specificity (3,4), often leading to the unequivocal diagnosis "mixed dementia," indicating clinical features of AD, but with multiple vascular lesions at brain imaging and/or cardiovascular risk factors. Although AD and VaD are clearly different diseases (e.g., as exemplified by genetics), both seem to share vascular risk factors such as atherosclerosis and smoking (5). Finally, AD may present with vascular comorbidity, which complicates the diagnostic work-up of AD patients. So, how to disentangle AD from VaD?
Cerebrospinal fluid (CSF) analysis of amyloid ß42 protein (Aß42) and total tau protein (t-tau), have been advocated as diagnostic biomarkers. T-tau levels are elevated and Aß42 levels decreased in CSF of AD patients compared to normal control (NC) participants (6,7). The combination of CSF t-tau and Aß42 yields a highly accurate differentiation between AD and NC (sensitivity 50%–94%, specificity 83%–100% (6). However, CSF-based differentiation of AD from VaD remains a challenge; specificity was only 48% versus VaD (8). Therefore, additional biomarkers are clearly needed. Quantification of hyperphosphorylated tau (p-tau) in CSF may be such a biomarker. CSF p-tau181 concentrations improve the discrimination of AD from dementia with Lewy bodies (DLB) (9), but its validity in discriminating AD from VaD has not extensively been studied. In this retrospective case–control study we analyzed CSF levels of t-tau, Aß42, and p-tau181 in NC participants and patients with clinical AD and VaD, to achieve an optimal differentiation between AD and VaD.
METHODS
Patients
Patients with mild to moderate AD (n = 61) or VaD (n = 25) were selected from a large database containing 260 patients with cognitive impairment or dementia of various origins (e.g., degenerative, vascular, hereditary, inflammatory, metabolic) who visited our outpatient clinic between 1992 and 2004. Only patients with a diagnosis of probable AD or VaD, according to accepted criteria (1,2), were included. The standard diagnostic examination protocol included a complete geriatric assessment, neurological examination, neuropsychological testing, laboratory testing, imaging of the brain, and a lumbar puncture. As NC participants, we included 30 persons older than 50 years who visited our outpatient clinic for various reasons but turned out not to suffer from a neurological disorder. Their CSF had normal leukocyte and erythrocyte counts, normal total protein, glucose and lactate concentrations, and no oligoclonal immunoglobulin G (IgG) bands.
CSF Analysis
Lumbar punctures were performed after written informed consent was obtained from the patients and from the patients' legal representatives. CSF from all participants was collected in polypropylene tubes, within 30 minutes transported to the adjacent laboratory at room temperature, centrifuged after routine investigations, and immediately aliquoted and stored at –80°C until analysis. Levels of t-tau, Aß42, and p-tau181 in CSF were measured using enzyme-linked immunosorbent assays (all obtained from Innogenetics NV, Gent, Belgium). In five AD and five VaD patients and 10 NC participants, the amount of CSF was insufficient to measure the p-tau181 concentration.
Statistical Analysis
Statistical procedures were performed using GraphPad Prism (San Diego, CA) software. All data were normally distributed; therefore, one-way analysis of variance (ANOVA) with Bonferroni's post hoc correction was used for multiple comparisons. Cutoff values, sensitivity, and specificity for biomarkers in different groups were calculated using receiver operating characteristic (ROC) curves. Cutoff values with the most optimal combination of sensitivity and specificity to discriminate between these two groups for each biomarker were calculated. Subsequently, positive and negative predictive values (PPV and NPV) were calculated. Correlation analysis was performed by using Pearson's method.
RESULTS
Gender distribution was similar in the NC (47% male, 53% female) and combined dementia patient groups (45% male, 55% female). The mean age of NC participants was significantly lower than that of patients with AD and VaD (p <.001). There was no significant age difference between patients with AD and VaD (Table 1).
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High sensitivity and specificity for the discrimination between AD and VaD were obtained at two optimal cutoff levels of Q Aß42/p-tau (Table 2). At a cutoff level of 12.7, sensitivity was 100% and specificity 85% with a PPV of 95% and an NPV of 100%. At a slightly lower cutoff level (10.95), sensitivity and specificity were 95% and 90%, respectively (PPV 96% and NPV 86%). Sensitivity and specificity of any of the other biomarkers were less than those of QAß42/p-tau181.
Optimal separation of the AD and NC groups using QAß42/t-tau was achieved at a cutoff level of 1.895, with a sensitivity of 95% and specificity of 97%. In addition, very high PPV (95%) and NPV (100%) were reached with QAß42/p-tau at a cutoff level of 13.2. Discrimination between VaD and NC participants using a QAß42/t-tau cutoff value of 3.5 resulted in a combination of 68% sensitivity and 87% specificity.
DISCUSSION
In the present study, we found decreased CSF Aß42 levels and increased CSF t-tau levels in patients with AD compared with those with VaD and normal participants, consistent with the literature (6–10). Also, the levels of Aß42 and tau in patients with VaD are in line with another study (8). Our main finding, however, is that the ratio of Aß42 to p-tau181 (Q Aß42/p-tau) distinguishes between AD and VaD patients with high discriminatory power. Mean p-tau181 levels are doubled in the AD group, but normal in VaD, similar to other observations (11,12). Identical results were found using tau protein phosphorylated at serine 199 (p-tau191) and threonine 231 (p-tau231) (9,11,13).
Q Aß42/p-tau was significantly decreased in the AD group compared to the VaD group. Previously, it has been reported that in AD patients a low Aß42/p-tau181 ratio was observed compared to healthy NC participants, patients with non-AD dementias, and patients with other neurological disorders (14). In this study we demonstrated that Q Aß42/p-tau has excellent diagnostic value in the differentiation of AD from VaD. According to a consensus report (15) a useful biomarker should be reliable, reproducible, and have both a sensitivity and a PPV greater than 80% for detecting AD and a specificity greater than 80% for distinguishing other dementias. Q Aß42/p-tau fulfills these requirements, as sensitivity, specificity, PPV, and NPV are all well above 85%. For the discrimination between AD and NC groups, Q Aß42/t-tau may also be a useful biomarker, because at a cutoff level of 1.895 high sensitivity, specificity, PPV, and NPV (each 91% or higher) were obtained. As all patients' diagnoses were based on clinical criteria and not neuropathologically confirmed, improvement of sensitivity and specificity awaits prospective, neuropathology-supported studies.
A potential confounder of this study is the age difference between patients and NC participants. However, t-tau is the only CSF biomarker known to be positively correlated with age (r = 0.60, p <.001) (16). Thus, differences in mean CSF t-tau levels between the NC and AD groups might be smaller than has been described before. Because the results of our study do not suggest an important role for the analysis of t-tau in the discrimination of AD from VaD, and because there was no significant age difference between the AD and VaD groups, this finding does not affect our main results.
In the VaD group, one patient had exceptional high tau levels (1603 pg/ml) with normal p-tau181 levels (56 pg/ml). This patient underwent the lumbar puncture only 5 weeks after he suffered a stroke—a known cause of a transient (3–5 months) increase in CSF t-tau, but not of p-tau (17).
In many previous studies the focus of the application of CSF biomarkers was the differentiation between AD and NC. Only a few studies addressed the truly relevant discrimination between AD and other dementia disorders, particularly VaD. As recent studies suggested that vascular risk factors, including atherosclerosis, diabetes, and smoking, might significantly contribute to the pathogenesis of AD (5,18), the conventional distinction between AD and VaD has become controversial. Our study suggests that (i) there are biological differences between what we call AD and VaD; and (ii) Q Aß42/p-tau in CSF may detect such differences in relevant clinical situations. The true contribution of Aß42/p-tau CSF assessments to the clinical management of patients with late-onset dementia disorders remains to be established, preferably through a prospective randomized masked validation study with appropriate control populations.
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Supported by Zon-MW (917.46.331), the "Hersenstichting Nederland," the American Health Assistance Foundation (A2001-15), and "Alzheimer Nederland." We thank the technicians of the Laboratory of Pediatrics and Neurology for CSF analysis.
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Decision Editor: Luigi Ferrucci, MD, PhD
Received June 25, 2005
Accepted November 10, 2005
References
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