A worldwide multicentre comparison of assays for cerebrospinal fluid biomarkers in Alzheimer's disease

Diffusion tensor imaging in Alzheimer's disease: insights into the limbic-diencephalic network and methodological considerations

Glucose hypometabolism and gray matter atrophy are well known consequences of Alzheimer's disease (AD). Studies using these measures have shown that the earliest clinical stages, in which memory impairment is a relatively isolated feature, are associated with degeneration in an apparently remote group of areas—mesial temporal lobe (MTL), diencephalic structures such as anterior thalamus and mammillary bodies, and posterior cingulate. These sites are thought to be strongly anatomically inter-connected via a limbic-diencephalic network. Diffusion tensor imaging or DTI—an imaging technique capable of probing white matter tissue microstructure—has recently confirmed degeneration of the white matter connections of the limbic-diencephalic network in AD by way of an unbiased analysis strategy known as tract-based spatial statistics (TBSS). The present review contextualizes the relevance of these findings, in which the fornix is likely to play a fundamental role in linking MTL and diencephalon. An interesting by-product of this work has been in showing that alterations in diffusion behavior are complex in AD—while early studies tended to focus on fractional anisotropy, recent work has highlighted that this measure is not the most sensitive to early changes. Finally, this review will discuss in detail several technical aspects of DTI both in terms of image acquisition and TBSS analysis as both of these factors have important implications to ensure reliable observations are made that inform understanding of neurodegenerative diseases.

Reporting standards for studies of diagnostic test accuracy in dementia: The STARDdem Initiative

OBJECTIVE

To provide guidance on standards for reporting studies of diagnostic test accuracy for dementia disorders.

METHODS

An international consensus process on reporting standards in dementia and cognitive impairment (STARDdem) was established, focusing on studies presenting data from which sensitivity and specificity were reported or could be derived. A working group led the initiative through 4 rounds of consensus work, using a modified Delphi process and culminating in a face-to-face consensus meeting in October 2012. The aim of this process was to agree on how best to supplement the generic standards of the STARD statement to enhance their utility and encourage their use in dementia research.

RESULTS

More than 200 comments were received during the wider consultation rounds. The areas at most risk of inadequate reporting were identified and a set of dementia-specific recommendations to supplement the STARD guidance were developed, including better reporting of patient selection, the reference standard used, avoidance of circularity, and reporting of test-retest reliability.

CONCLUSION

STARDdem is an implementation of the STARD statement in which the original checklist is elaborated and supplemented with guidance pertinent to studies of cognitive disorders. Its adoption is expected to increase transparency, enable more effective evaluation of diagnostic tests in Alzheimer disease and dementia, contribute to greater adherence to methodologic standards, and advance the development of Alzheimer biomarkers.

Advancing research diagnostic criteria for Alzheimer's disease: the IWG-2 criteria

In the past 8 years, both the International Working Group (IWG) and the US National Institute on Aging-Alzheimer's Association have contributed criteria for the diagnosis of Alzheimer's disease (AD) that better define clinical phenotypes and integrate biomarkers into the diagnostic process, covering the full staging of the disease. This Position Paper considers the strengths and limitations of the IWG research diagnostic criteria and proposes advances to improve the diagnostic framework. On the basis of these refinements, the diagnosis of AD can be simplified, requiring the presence of an appropriate clinical AD phenotype (typical or atypical) and a pathophysiological biomarker consistent with the presence of Alzheimer's pathology. We propose that downstream topographical biomarkers of the disease, such as volumetric MRI and fluorodeoxyglucose PET, might better serve in the measurement and monitoring of the course of disease. This paper also elaborates on the specific diagnostic criteria for atypical forms of AD, for mixed AD, and for the preclinical states of AD.

Variability of CSF Alzheimer's disease biomarkers: implications for clinical practice

BACKGROUND

Cerebrospinal fluid (CSF) biomarkers are increasingly being used for diagnosis of Alzheimer's disease (AD).

OBJECTIVE

We investigated the influence of CSF intralaboratory and interlaboratory variability on diagnostic CSF-based AD classification of subjects and identified causes of this variation.

METHODS

We measured CSF amyloid-β (Aβ) 1-42, total tau (t-tau), and phosphorylated tau (p-tau) by INNOTEST enzyme-linked-immunosorbent assays (ELISA) in a memory clinic population (n = 126). Samples were measured twice in a single or two laboratories that served as reference labs for CSF analyses in the Netherlands. Predefined cut-offs were used to classify CSF biomarkers as normal or abnormal/AD pattern.

RESULTS

CSF intralaboratory variability was higher for Aβ1-42 than for t-tau and p-tau. Reanalysis led to a change in biomarker classification (normal vs. abnormal) of 26% of the subjects based on Aβ1-42, 10% based on t-tau, and 29% based on p-tau. The changes in absolute biomarker concentrations were paralleled by a similar change in levels of internal control samples between different assay lots. CSF interlaboratory variability was higher for p-tau than for Aβ1-42 and t-tau, and reanalysis led to a change in biomarker classification of 12% of the subjects based on Aβ1-42, 1% based on t-tau, and 22% based on p-tau.

CONCLUSIONS

Intralaboratory and interlaboratory CSF variability frequently led to change in diagnostic CSF-based AD classification for Aβ1-42 and p-tau. Lot-to-lot variation was a major cause of intralaboratory variability. This will have implications for the use of these biomarkers in clinical practice.

Limited agreement between biomarkers of neuronal injury at different stages of Alzheimer's disease

New diagnostic criteria for Alzheimer's disease (AD) treat different biomarkers of neuronal injury as equivalent. Here, we quantified the degree of agreement between hippocampal volume on structural magnetic resonance imaging, regional glucose metabolism on positron emission tomography, and levels of phosphorylated tau in cerebrospinal fluid (CSF) in 585 subjects from all phases of the AD Neuroimaging Initiative. The overall chance-corrected agreement was poor (Cohen κ, 0.24-0.34), in accord with a high rate of conflicting findings (26%-41%). Neither diagnosis nor APOE ε4 status significantly influenced the distribution of agreement between the biomarkers. The degree of agreement tended to be higher in individuals with abnormal versus normal CSF β-amyloid (Aβ1-42) levels. Prospective diagnostic criteria for AD should address the relative importance of markers of neuronal injury and elaborate a way of dealing with conflicting biomarker findings.

Clinical utility of cerebrospinal fluid biomarkers in the diagnosis of early Alzheimer's disease

Several potential disease-modifying drugs for Alzheimer's disease (AD) have failed to show any effect on disease progression in clinical trials, conceivably because the AD subjects are already too advanced to derive clinical benefit from treatment and because diagnosis based on clinical criteria alone introduces a high misdiagnosis rate. Thus, well-validated biomarkers for early detection and accurate diagnosis are crucial. Low cerebrospinal fluid (CSF) concentrations of the amyloid-β (Aβ1-42) peptide, in combination with high total tau and phosphorylated tau, are sensitive and specific biomarkers highly predictive of progression to AD dementia in patients with mild cognitive impairment. However, interlaboratory variations in the results seen with currently available immunoassays are of concern. Recent worldwide standardization efforts and quality control programs include standard operating procedures for both preanalytical (e.g., lumbar puncture and sample handling) and analytical (e.g., preparation of calibration curve) procedures. Efforts are also ongoing to develop highly reproducible assays on fully automated instruments. These global standardization and harmonization measures will provide the basis for the generalized international application of CSF biomarkers for both clinical trials and routine clinical diagnosis of AD.

The screening of Alzheimer's patients with CSF biomarkers, modulates the distribution of APOE genotype: impact on clinical trials

Polymorphism of the apolipoprotein E gene (APOE) plays a role in the level of neuropathological lesions and in drug response in Alzheimer's disease (AD). The aim of this study was to investigate whether the selection of AD patients based on cerebrospinal fluid (CSF) biomarkers assessment may be biased by their APOE distribution. We studied the relationships between APOE genotype and CSF biomarkers levels in a total of 432 patients (AD, n = 244; non-AD, n = 188) explored for cognitive disorders. We studied the distribution of APOE genotypes among AD patient subgroups selected by various cut-offs of CSF biomarkers. Strategies of screening based on CSF Aβ1-42 lead to overselection of ε4/ε4 patients in the AD group. Screening based on tau levels did not change Apoe4 distribution in the AD group. CSF Aβ1-42 discriminated better AD patients with at least one ε4 than AD patients with no ε4. A strong allele-effect relationship was detected between APOE genotype and CSF amyloid-β (Aβ1-42) in AD patients. Selecting AD patients on CSF amyloid levels only may create an overselection of ε4/ε4 carriers, and might potentially bias the population of patients included in clinical trial studies.

Evaluation of the cerebrospinal fluid amyloid-β1-42/amyloid-β1-40 ratio measured by alpha-LISA to distinguish Alzheimer's disease from other dementia disorders

BACKGROUND

The well-established core biomarkers used to identify Alzheimer's disease (AD) overlap with other dementia disorders such as dementia with Lewy bodies (DLB) and Parkinson's disease with dementia (PDD). This study aimed to evaluate whether the cerebrospinal fluid (CSF) amyloid-β (Aβ)1-42/Aβ1-40 ratio, measured by a novel method, could improve the differential diagnosis of AD, DLB and PDD.

METHOD

CSF levels of Aβ1-40 and Aβ1-42 in patients with PDD, DLB, AD, Parkinson's disease and controls were analyzed using an amplified luminescent proximity homogenous immunoassay along with conventional immunoassays.

RESULTS

The CSF Aβ1-42/Aβ1-40 ratio increased discrimination of AD from PDD and DLB compared with either of the two Aβ biomarkers individually.

CONCLUSION

The use of the Aβ1-42/Aβ1-40 ratio could improve the differentiation of AD from PDD and DLB.

CSF biomarker variability in the Alzheimer's Association quality control program

BACKGROUND

The cerebrospinal fluid (CSF) biomarkers amyloid beta 1-42, total tau, and phosphorylated tau are used increasingly for Alzheimer's disease (AD) research and patient management. However, there are large variations in biomarker measurements among and within laboratories.

METHODS

Data from the first nine rounds of the Alzheimer's Association quality control program was used to define the extent and sources of analytical variability. In each round, three CSF samples prepared at the Clinical Neurochemistry Laboratory (Mölndal, Sweden) were analyzed by single-analyte enzyme-linked immunosorbent assay (ELISA), a multiplexing xMAP assay, or an immunoassay with electrochemoluminescence detection.

RESULTS

A total of 84 laboratories participated. Coefficients of variation (CVs) between laboratories were around 20% to 30%; within-run CVs, less than 5% to 10%; and longitudinal within-laboratory CVs, 5% to 19%. Interestingly, longitudinal within-laboratory CV differed between biomarkers at individual laboratories, suggesting that a component of it was assay dependent. Variability between kit lots and between laboratories both had a major influence on amyloid beta 1-42 measurements, but for total tau and phosphorylated tau, between-kit lot effects were much less than between-laboratory effects. Despite the measurement variability, the between-laboratory consistency in classification of samples (using prehoc-derived cutoffs for AD) was high (>90% in 15 of 18 samples for ELISA and in 12 of 18 samples for xMAP).

CONCLUSIONS

The overall variability remains too high to allow assignment of universal biomarker cutoff values for a specific intended use. Each laboratory must ensure longitudinal stability in its measurements and use internally qualified cutoff levels. Further standardization of laboratory procedures and improvement of kit performance will likely increase the usefulness of CSF AD biomarkers for researchers and clinicians.

Alzheimer disease: from biomarkers to diagnosis

The discovery of biomarkers, considered as surrogate markers of the underlying pathological changes, led an international work group (IWG) to propose a new conceptual framework for AD in 2007 Dubois et al. (2007). According to the IWG, AD is now defined as a dual clinico-biological entity that can be recognized in vivo, prior to the onset of the dementia syndrome, on the basis of: i) a specific core clinical phenotype comprised of an amnestic syndrome of the hippocampal type and ii) supportive evidence from biomarkers reflecting the location or the nature of Alzheimer-type changes. Therefore, AD is diagnosed with the same criteria throughout all symptomatic phases of the disease based on the biologically-based approach to diagnosis independent of clinical expression of disease severity. The definitions were further clarified in 2010 (Dubois et al., 2010). Although the new criteria are proposed for research purposes, we encourage expert centres with adequate resources to begin to use the proposed algorithm in order to move the field forward and facilitate translation into clinical practice.

Clinical utility and analytical challenges in measurement of cerebrospinal fluid amyloid-β(1-42) and τ proteins as Alzheimer disease biomarkers

BACKGROUND

Over the past 2 decades, clinical studies have provided evidence that cerebrospinal fluid (CSF) amyloid β(1-42) (Aβ(1-42)), total τ (t-τ), and τ phosphorylated at Thr181 (p-τ(181)) are reliable biochemical markers of Alzheimer disease (AD) neuropathology.

CONTENT

In this review, we summarize the clinical performance and describe the major challenges for the analytical performance of the most widely used immunoassay platforms [based on ELISA or microbead-based multianalyte profiling (xMAP) technology] for the measurement of CSF AD biomarkers (Aβ(1-42), t-τ, and p-τ(181)). With foundational immunoassay data providing the diagnostic and prognostic values of CSF AD biomarkers, the newly revised criteria for the diagnosis of AD include CSF AD biomarkers for use in research settings. In addition, it has been suggested that the selection of AD patients at the predementia stage by use of CSF AD biomarkers can improve the statistical power of clinical trial design. Owing to the lack of a replenishable and commutable human CSF-based standardized reference material (SRM) and significant differences across different immunoassay platforms, the diagnostic-prognostic cutpoints of CSF AD biomarker concentrations are not universal at this time. These challenges can be effectively met in the future, however, through collaborative ongoing standardization efforts to minimize the sources of analytical variability and to develop reference methods and SRMs.

SUMMARY

Measurements of CSF Aβ(1-42), t-τ, and p-τ(181) with analytically qualified immunoassays reliably reflect the neuropathologic hallmarks of AD in patients at the early predementia stage of the disease and even in presymptomatic patients. Thus these CSF biomarker tests are useful for early diagnosis of AD, prediction of disease progression, and efficient design of drug intervention clinical trials.

Recommendations to standardize preanalytical confounding factors in Alzheimer's and Parkinson's disease cerebrospinal fluid biomarkers: an update

Early diagnosis of neurodegenerative disorders such as Alzheimer's (AD) or Parkinson's disease (PD) is needed to slow down or halt the disease at the earliest stage. Cerebrospinal fluid (CSF) biomarkers can be a good tool for early diagnosis. However, their use in clinical practice is challenging due to the high variability found between centers in the concentrations of both AD CSF biomarkers (Aβ42, total tau and phosphorylated tau) and PD CSF biomarker (α-synuclein). Such a variability has been partially attributed to different preanalytical procedures between laboratories, thus highlighting the need to establish standardized operating procedures. Here, we merge two previous consensus guidelines for preanalytical confounding factors in order to achieve one exhaustive guideline updated with new evidence for Aβ42, total tau and phosphorylated tau, and α-synuclein. The proposed standardized operating procedures are applicable not only to novel CSF biomarkers in AD and PD, but also to biomarkers for other neurodegenerative disorders.

Reference measurement procedures for Alzheimer's disease cerebrospinal fluid biomarkers: definitions and approaches with focus on amyloid β42

Cerebrospinal fluid (CSF) biomarkers for Alzheimer's disease (AD) are increasingly used in clinical settings, research and drug trials. However, their broad-scale use on different technology platforms is hampered by the lack of standardization at the level of sample handling, determination of concentrations of analytes and the absence of well-defined performance criteria for in vitro diagnostic or companion diagnostic assays, which influences the apparent concentration of the analytes measured and the subsequent interpretation of the data. There is a need for harmonization of CSF AD biomarker assays that can reliably, across centers, quantitate CSF biomarkers with high analytical precision, selectivity and stability over long time periods. In this position paper, we discuss reference procedures for the measurement of CSF AD biomarkers, especially amyloid β42 and tau. We describe possible technical approaches, focusing on a selected reaction monitoring mass spectrometry assay as a candidate reference method for quantification of CSF amyloid β42.

Proficiency testing programs for Alzheimer's disease cerebrospinal fluid biomarkers

Cerebrospinal fluid (CSF) biomarkers are increasingly used for diagnosis of Alzheimer's disease in research, clinical trials and clinical settings. As for other biochemical measurements, variability between laboratories for these biomarkers may be monitored by proficiency testing programs, where participating laboratories use their local routine methods to analyze test samples shipped from a central laboratory. In this review, we summarize the results from the last years' pilot proficiency programs and describe the ongoing standardization efforts in this area. Global proficiency testing for CSF biomarkers is now fully established. It will continue to play an important part in the standardization of measurements that is a prerequisite for the broad-scale future implementation of CSF biomarkers.

Comparison of two commercial enzyme-linked immunosorbent assays for cerebrospinal fluid measurement of amyloid β1-42 and total tau

Amyloid β_1-42 (Aβ_1-42), total tau (t-tau), and phosphorylated tau (p-tau) are the main cerebrospinal fluid (CSF) biomarkers for early diagnosis of Alzheimer's disease (AD). Detection of AD is critically important in view of the growing number of potential new drugs that may influence the course of the disease in its early phases. However, cut-off levels for these CSF biomarkers have not yet been established. Variability in absolute concentrations of AD biomarkers is high among studies and significant differences were noticed even within the same datasets. Variability in biomarkers levels in these assays may be due to many aspects of operating procedures. Standardization of pre-analytical and analytical procedures in collection, treatment, and storage of CSF samples is crucial because differences in sample handling can drastically influence results. Multicenter studies showed that usage of ELISA kits from different manufacturers also affects outcome. So far only very few studies tested the efficiency of ELISA kits produced by different vendors. In this study, the performance of Innogenetics (Gent, Belgium) and Invitrogen (Camarillo, CA, USA) ELISA kits for t-tau and Aβ_1-42 was tested. Passing-Bablok analysis showed significant differences between Invitrogen and Innogenetics ELISA methods, making it impossible to use them interchangeably.

The impact of cerebrospinal fluid biomarkers on the diagnosis of Alzheimer's disease

The cerebrospinal fluid (CSF) biomarkers β-amyloid(1-42) (Aβ(1-42)), total tau protein (T-tau), and tau phosphorylated at threonine 181 (P-tau(181P)) are gradually finding their way into routine clinical practice as an affirmative diagnostic tool for Alzheimer's disease (AD). These biomarkers have also been implemented in the revised diagnostic criteria for AD. The combination of the CSF biomarkers Aβ(1-42), T-tau, and P-tau(181P) leads to high (around 80%) levels of sensitivity, specificity, and diagnostic accuracy for discrimination between AD and controls (including psychiatric disorders like depression) and can be applied for diagnosing AD in the predementia phases of the disease (mild cognitive impairment). The added value of CSF biomarkers could lie within those cases in which the clinical diagnostic work-up is not able to discriminate between AD and non-AD dementias. However, their discriminatory power for the differential diagnosis of dementia is suboptimal. Other CSF biomarkers, especially those that are reflective of the pathology of non-AD dementia etiologies, could improve the accuracy of differential dementia diagnosis. CSF biomarkers will be of help to establish a correct and early AD diagnosis, even in the preclinical stages of the disease, which will be of importance once disease-modifying drugs for AD become available. Variation in biomarker measurements still jeopardize the introduction of CSF biomarkers into routine clinical practice and clinical trials, but several national and international standardization initiatives are ongoing.

A prediction model to calculate probability of Alzheimer's disease using cerebrospinal fluid biomarkers

BACKGROUND

We aimed to develop a prediction model based on cerebrospinal fluid (CSF) biomarkers, that would yield a single estimate representing the probability that dementia in a memory clinic patient is due to Alzheimer's disease (AD).

METHODS

All patients suspected of dementia in whom the CSF biomarkers had been analyzed were selected from a memory clinic database. Clinical diagnosis was AD (n = 272) or non-AD (n = 289). The prediction model was developed with logistic regression analysis and included CSF amyloid β42, CSF phosphorylated tau181, and sex. Validation was performed on an independent data set from another memory clinic, containing 334 AD and 157 non-AD patients.

RESULTS

The prediction model estimated the probability that AD is present as follows: p(AD) = 1/(1 + e (- [-0.3315 + score])), where score is calculated from -1.9486 × ln(amyloid β42) + 2.7915 × ln(phosphorylated tau181) + 0.9178 × sex (male = 0, female = 1). When applied to the validation data set, the discriminative ability of the model was very good (area under the receiver operating characteristic curve: 0.85). The agreement between the probability of AD predicted by the model and the observed frequency of AD diagnoses was very good after taking into account the difference in AD prevalence between the two memory clinics.

CONCLUSIONS

We developed a prediction model that can accurately predict the probability of AD in a memory clinic population suspected of dementia based on CSF amyloid β42, CSF phosphorylated tau181, and sex.

Alzheimer's disease biomarker discovery using in silico literature mining and clinical validation

Background

Alzheimer’s Disease (AD) is the most widespread form of dementia in the elderly but despite progress made in recent years towards a mechanistic understanding, there is still an urgent need for disease modification therapy and for early diagnostic tests. Substantial international efforts are being made to discover and validate biomarkers for AD using candidate analytes and various data-driven 'omics' approaches. Cerebrospinal fluid is in many ways the tissue of choice for biomarkers of brain disease but is limited by patient and clinician acceptability, and increasing attention is being paid to the search for blood-based biomarkers. The aim of this study was to use a novel in silico approach to discover a set of candidate biomarkers for AD.

Methods

We used an in silico literature mining approach to identify potential biomarkers by creating a summarized set of assertional metadata derived from relevant legacy information. We then assessed the validity of this approach using direct assays of the identified biomarkers in plasma by immunodetection methods.

Results

Using this in silico approach, we identified 25 biomarker candidates, at least three of which have subsequently been reported to be altered in blood or CSF from AD patients. Two further candidate biomarkers, indicated from the in silico approach, were choline acetyltransferase and urokinase-type plasminogen activator receptor. Using immunodetection, we showed that, in a large sample set, these markers are either altered in disease or correlate with MRI markers of atrophy.

Conclusions

These data support as a proof of concept the use of data mining and in silico analyses to derive valid biomarker candidates for AD and, by extension, for other disorders.

Neurochemical dementia diagnostics for Alzheimer’s disease and other dementias: an ISO 15189 perspective

Dementia is one of the most common causes of health problems in the elderly populations of Western industrialized countries. A combined analysis of cerebrospinal fluid-based neurochemical dementia diagnostics biomarkers (amyloid-β peptides, total tau and phosphorylated forms of tau) provides sensitivity and specificity in the range of 85% for the diagnosis of Alzheimer’s disease, the most common cause of dementia. The alterations occur very early in the course of neurodegeneration, enabling medical follow-up of persons with increased risk of developing dementia. With a growing number of laboratories performing neurochemical dementia diagnostics routinely, it is important to standardize protocols and laboratory performance to enable comparisons of results and their interpretations. Together with the recently published expert guidelines for sample handling and preparation, as well as the interpretation (post-analytical) algorithms developed by experienced centers, ISO 15189 norm provides an extremely useful tool for standardization of neurochemical dementia diagnostics.

Upcoming candidate cerebrospinal fluid biomarkers of Alzheimer's disease

Dementia due to Alzheimer's disease (AD) is estimated to reach epidemic proportions by the year 2030. Given the limited accuracy of current AD clinical diagnosis, biomarkers of AD pathologies are currently being sought. Reductions in cerebrospinal fluid levels of β-amyloid 42 (a marker of amyloid plaques) and elevations in tau species (markers of neurofibrillary tangles and/or neurodegeneration) are well-established as biomarkers useful for AD diagnosis and prognosis. However, novel markers for other features of AD pathophysiology (e.g., β-amyloid processing, neuroinflammation and neuronal stress/dysfunction) and for other non-AD dementias are required to improve the accuracy of AD disease diagnosis, prognosis, staging and therapeutic monitoring (theragnosis). This article discusses the potential of several promising novel cerebrospinal fluid analytes, highlights the next steps critical for advancement in the field, and provides a prediction on how the field may evolve in 5-10 years.

Alzheimer's disease: β-amyloid plaque formation in human brain

Although the precise cause of Alzheimer's disease is not known, the β-amyloid peptide chains of 40-42 amino acids are suspected to contribute to the disease. The β-amyloid precursor protein is found on many types of cell membranes, and the action of secretases (β and γ) on this precursor protein normally releases the β-amyloids at a high rate into the plasma and the cerebrospinal fluid. However, the concentrations of the β-amyloids in the plasma and the spinal fluid vary considerably between laboratories. The β-amyloids adsorb in the nanomolar concentration range to receptors on neuronal and glial cells. The β-amyloids are internalized, become folded in the β-folded or β-pleated shape, and then stack on each other to form long fibrils and aggregates known as plaques. The β-amyloids likely act as monomers, dimers, or multimers on cell membranes to interfere with neurotransmission and memory before the plaques build up. Treatment strategies include inhibitors of β- and γ-secretase, as well as drugs and physiological compounds to prevent aggregation of the amyloids. Several immune approaches and a cholesterol-lowering strategy are also being tested to remove the β-amyloids.

Neurochemical dementia diagnostics in Alzheimer's disease: where are we now and where are we going?

Neurochemical dementia diagnostics (NDD) is a routine laboratory tool used in the diagnostic process for patients with neurodegenerative disorders, such as Alzheimer's disease. Currently, two groups of biomarkers analyzed in the cerebrospinal fluid are considered - namely amyloid-β peptides and Tau proteins - along with the hyperphosphorylated forms of the latter (pTau). Current directions in the development of NDD include the following: search for novel biomarkers with improved analytical or diagnostic performance; optimization of the analysis of the biomarkers already available (e.g., by improved quality control and interlaboratory comparison of results); applications of novel technologies enabling better management of patient samples; and search for biomarkers in the blood. This article presents the state-of-the-art in the field of cerebrospinal fluid-based NDD, and also summarizes some of the hypotheses of how the future development of NDD tools might look.

Cerebrospinal fluid markers for Alzheimer's disease in a cognitively healthy cohort of young and old adults

BACKGROUND

Low amyloid β42 (Aβ42) and high total tau and phosphorylated tau (p-tau) concentrations in the cerebrospinal fluid (CSF) are biomarkers of Alzheimer's disease (AD), reflecting brain deposition of amyloid plaques and tangles. Age and apolipoprotein E allele E4 are two strong risk factors for AD, but few data are still available on their effect on CSF markers in normal aging.

OBJECTIVE

To study the effect of age on CSF Aβ42, total tau, and p-tau levels in a well-characterized group of cognitively normal subjects.

METHODS

CSF Aβ42 levels of 81 subjects (27% female, 53 ± 15.3 years, range: 21-88) were determined with sandwich enzyme-linked immunosorbent assay; of these, total tau and p-tau levels were measured in 61 (75%) and 42 (52%) cases, respectively. A linear regression analysis between age and CSF markers was carried out on the whole sample and separately in apolipoprotein E allele ɛ4 carriers and noncarriers.

RESULTS

The median levels of all markers were significantly different between young (<65 years) and old (≥65 years) subjects (Aβ42: P = .03; tau: P = .02; p-tau: P = .002; tau/Aβ42: P = .004; p-tau/Aβ42: P = .03). The association of marker levels with age was confirmed in linear regression models, where a positive relationship with age was observed for total tau (B = 2.3; 95% confidence interval [CI]: 0.89 to 3.7; P = .002), p-tau (B = 0.5; 95% CI: 0.1 to 0.9; P = .02), and tau/Aβ42 ratio (B = 0.006; 95% CI: 0.002 to 0.01; P = .002). No subjects showed abnormal tau, whereas 19% showed abnormal CSF Aβ42 concentrations.

CONCLUSION

In cognitively normal subjects, the concentrations of CSF biomarkers of AD are associated with age. Further longitudinal studies could clarify whether Aβ42 low levels represent a preclinical AD biomarker.

Monte Carlo feature selection and rule-based models to predict Alzheimer's disease in mild cognitive impairment

The objective of the present study was to evaluate a Monte Carlo feature selection (MCFS) and rough set Rosetta pipeline for generating rule-based models as a tool for comprehensive risk estimates for future Alzheimer's disease (AD) in individual patients with mild cognitive impairment (MCI). Risk estimates were generated on the basis of age, gender, Mini-Mental State Examination scores, apolipoprotein E (APOE) genotype and the cerebrospinal fluid (CSF) biomarkers total tau (T-tau), phospho-tau(181) (P-tau) and the 42 amino acid form of amyloid β (Aβ42) in two sets of longitudinally followed MCI patients (n = 217 in total). The predictive model was created in Rosetta, evaluated with the standard tenfold cross-validation approach and tested on an external set. Features were ranked and selected by the MCFS algorithm. Using the combined pipeline of MCFS and Rosetta, it was possible to predict AD among patients with MCI with an area under the receiver operating characteristics curve of 0.92. Risk estimates were produced for the individual patients and showed good correlation with actual diagnosis in cross validation, and on an external dataset from a new study. Analysis of the importance of attributes showed that the biochemical CSF markers contributed the most to the predictions, and that added value was gained by combining several biochemical markers. Despite a correlation with the biochemical markers, the genetic marker APOE ε4 did not contribute to the predictive power of the model.

Simultaneous analysis of cerebrospinal fluid biomarkers using microsphere-based xMAP multiplex technology for early detection of Alzheimer's disease

The xMAP-Luminex multiplex platform for measurement of Alzheimer's disease (AD) cerebrospinal fluid (CSF) biomarkers using Innogenetics AlzBio3 immunoassay reagents that are for research use only has been shown to be an effective tool for early detection of an AD-like biomarker signature based on concentrations of CSF Aβ(1-42), t-tau and p-tau(181). Among the several advantages of the xMAP-Luminex platform for AD CSF biomarkers are: a wide dynamic range of ready-to-use calibrators, time savings for the simultaneous analyses of three biomarkers in one analytical run, reduction of human error, potential of reduced cost of reagents, and a modest reduction of sample volume as compared to conventional enzyme-linked immunosorbant assay (ELISA) methodology. Recent clinical studies support the use of CSF Aβ(1-42), t-tau and p-tau(181) measurement using the xMAP-Luminex platform for the early detection of AD pathology in cognitively normal individuals, and for prediction of progression to AD dementia in subjects with mild cognitive impairment (MCI). Studies that have shown the prediction of risk for progression to AD dementia by MCI patients provide the basis for the use of CSF Aβ(1-42), t-tau and p-tau(181) testing to assign risk for progression in patients enrolled in therapeutic trials. Furthermore emerging study data suggest that these pathologic changes occur in cognitively normal subjects 20 or more years before the onset of clinically detectable memory changes thus providing an objective measurement for use in the assessment of treatment effects in primary treatment trials. However, numerous previous ELISA and Luminex-based multiplex studies reported a wide range of absolute values of CSF Aβ(1-42), t-tau and p-tau(181) indicative of substantial inter-laboratory variability as well as varying degrees of intra-laboratory imprecision. In order to address these issues a recent inter-laboratory investigation that included a common set of CSF pool aliquots from controls as well as AD patients over a range of normal and pathological Aβ(1-42), t-tau and p-tau(181) values as well as agreed-on standard operating procedures (SOPs) assessed the reproducibility of the multiplex methodology and Innogenetics AlzBio3 immunoassay reagents. This study showed within-center precision values of 5% to a little more than 10% and good inter-laboratory %CV values (10-20%). There are several likely factors influencing the variability of CSF Aβ(1-42), t-tau and p-tau(181) measurements. In this review, we describe the pre-analytical, analytical and post-analytical sources of variability including sources inherent to kits, and describe procedures to decrease the variability. A CSF AD biomarker Quality Control program has been established and funded by the Alzheimer Association, and global efforts are underway to further define optimal pre-analytical SOPs and best practices for the methodologies available or in development including plans for production of a standard reference material that could provide for a common standard against which manufacturers of immunoassay kits would assign calibration standard values.

[The future of biomarkers in dementia diagnostics]

Neurochemical dementia diagnostics (NDD) is a routine laboratory tool in the diagnostic process of patients with neurodegenerative disorders, such as Alzheimer's disease (AD). Currently, two groups of biomarkers analyzed in the cerebrospinal fluid (CSF) are being considered, namely amyloid β (Aβ) peptides and tau proteins, along with the hyperphosphorylated forms of the latter (p-tau). Current directions in the development of NDD include the following: 1. search for novel biomarkers with improved analytical or diagnostic performance; 2. search for biomarkers in the blood; 3. applications of novel technologies enabling better management of patient samples; 4. optimization of the analysis of the biomarkers already available (for example, by improved quality control and inter-laboratory comparison of results). This review presents the state of the art in the field of CSF-based NDD and also summarizes some of the hypotheses of how the future development of NDD tools might look.

The value of cerebrospinal fluid biomarkers for the differential diagnosis of dementia

SUMMARY The diagnostic markers for Alzheimer’s disease (AD) (β-amyloid1–42, total tau protein and hyperphosphorylated tau) are gradually finding their way into routine clinical practice as an affirmative diagnostic tool, rather than an exclusionary one. Their discriminatory power for the differential diagnosis of dementia is, however, suboptimal and other cerebrospinal fluid biomarkers, especially those that are reflective of the pathology of the non-AD dementia etiologies, could improve the differential dementia diagnosis either by their individual use (diagnostic value) or in combination with the established AD biomarkers (added diagnostic value). Unfortunately, validated biomarkers for non-AD dementias do not yet exist, but promising candidates have been identified over recent years. This review summarizes the current literature on cerebrospinal fluid biomarkers for the diagnosis of AD and other dementias.

Cerebrospinal fluid β-amyloid 1-42 correlates with rate of progression in Alzheimer's disease

Emerging treatment options targeting the pathogenetic mechanisms in Alzheimer's disease (AD) and the need to monitor efficacy during treatment trials necessitate the use of biomarkers, which not only may facilitate early and reliable diagnosis, but may also assist in the stratification of patient populations according to their rate of progression. The objective of the present study is to examine whether demographic and cerebrospinal fluid (CSF) parameters at initial evaluation [total tau, tau phosphorylated at threonine-181 and amyloid-beta(1-42) (Aβ42)] can be used to discriminate between slow and rapid progressors in patients with AD. A total of 74 AD patients were included in the study. Patients recruited were divided into slow and rapid progressors according to their Mini-Mental Status Examination (MMSE) score decline before evaluation. Patients with a drop rate of >4/year were considered rapid progressors. Commercially available ELISA kits were used for measuring CSF biomarkers. Comparisons were performed using analysis of covariance. Significantly lower Aβ42 levels in the CSF were found in rapid (mean 392 pg/ml) as compared to slow progressors (mean 453 pg/ml), with a p value of 0.042. The results of the present study suggest that levels of the Aβ42 peptide may be related to the rate of disease progression. Further studies with a prospective design are needed in order to test the possible predictive value of CSF Aβ42 analysis.

Summary metrics to assess Alzheimer disease-related hypometabolic pattern with 18F-FDG PET: head-to-head comparison

In the recently revised diagnostic criteria for Alzheimer disease (AD), the National Institute on Aging and Alzheimer Association suggested that confidence in diagnosing dementia due to AD and mild cognitive impairment (MCI) due to AD could be improved by the use of certain biomarkers, such as (18)F-FDG PET evidence of hypometabolism in AD-affected brain regions. Three groups have developed automated data analysis techniques to characterize the AD-related pattern of hypometabolism in a single measurement. In this study, we sought to directly compare the ability of these three (18)F-FDG PET data analysis techniques--the PMOD Alzheimer discrimination analysis tool, the hypometabolic convergence index, and a set of meta-analytically derived regions of interest reflecting AD hypometabolism pattern (metaROI)--to distinguish moderate or mild AD dementia patients and MCI patients who subsequently converted to AD dementia from cognitively normal older adults.

METHODS

One hundred sixty-six (18)F-FDG PET patients from the AD Neuroimaging Initiative, 308 from the Network for Efficiency and Standardization of Dementia Diagnosis, and 176 from the European Alzheimer Disease Consortium PET study were categorized, with masking of group classification, as AD, MCI, or healthy control. For each AD-related (18)F-FDG PET index, receiver-operating-characteristic curves were used to characterize and compare subject group classifications.

RESULTS

The 3 techniques were roughly comparable in their ability to distinguish each of the clinical groups from cognitively normal older adults with high sensitivity and specificity. Accuracy of classification (in terms of area under the curve) in each clinical group varied more as a function of dataset than by technique. All techniques were differentially sensitive to disease severity, with the classification accuracy for MCI due to AD to moderate AD varying from 0.800 to 0.949 (PMOD Alzheimer tool), from 0.774 to 0.967 (metaROI), and from 0.801 to 0.983 (hypometabolic convergence index).

CONCLUSION

The 3 tested techniques have the potential to help detect AD in research and clinical settings. Additional efforts are needed to clarify their ability to address particular scientific and clinical questions. Their incremental diagnostic value over other imaging and biologic markers makes them easier to implement by other groups for these purposes.

[Multicenter study on lumbar puncture indication, clinical practice and feasibility]

INTRODUCTION

Cerebrospinal fluid (CSF) biomarkers have been extensively studied as diagnostic markers for Alzheimer's disease (AD). However, results are variable probably due to lumbar puncture (LP) procedure, CSF collection and transport. This intercenter variability highlights the need for an efficient standardization of clinical and technical procedures. The aims of this study were firstly to compare the LP procedure and CSF transport process in all French Memory Centers and secondly to evaluate the incidence of LP side effects in 100 patients with cognitive disturbances.

METHODS

LP practice and side effect prospective questionnaires were sent to all French Memory Centers in May 2010. Memory Centers were asked about their LP procedure. The prospective study over a three-week-period has evaluated the LP feasibility and side effects. All data were collected until the end of July 2010.

RESULTS

The answers of 18 out of 26 Memory Centers were collected. Although, these centers did not have the same LP procedure and CSF transport, the majority of them proceeded according to Innogenetics's advices concerning the use of polypropylene tubes and transport duration but not sample conditioning. Polypropylene tubes were different from one center to the other. CSF volume, pharmacological premedication and prevention of post-LP syndrome were variable in all responding centers. The prospective study carried out in 100 patients revealed a very good LP acceptability (93/100 patients). LP feasibility was 97 % (90/93) and failed LP were consequently performed with success using radiological scopes. Three minor complications were observed.

DISCUSSION AND CONCLUSION

All French Centers complied with Innogenetics' recommendations for pretechnical CSF procedures; however each Center put in place its own procedure that was different one center to the other. It will be very interesting to compare cut-off and result values for Aβ, tau and phosphorylated tau protein on threonine 181 between several centers that used their own procedures. Acceptability and safety were very good in our short but significant prospective study. These results confirm the data of Zetterberg et al., 2010.

Evidence for Elevated Cerebrospinal Fluid ERK1/2 Levels in Alzheimer Dementia

Cerebrospinal fluid (CSF) samples from 33 patients with Alzheimer dementia (AD), 21 patients with mild cognitive impairment who converted to AD during followup (MCI-AD), 25 patients with stable mild cognitive impairment (MCI-stable), and 16 nondemented subjects (ND) were analyzed with a chemiluminescence immunoassay to assess the levels of the mitogen-activated protein kinase ERK1/2 (extracellular signal-regulated kinase 1/2). The results were evaluated in relation to total Tau (tTau), phosphorylated Tau (pTau), and beta-amyloid 42 peptide (Aβ42). CSF-ERK1/2 was significantly increased in the AD group as compared to stable MCI patients and the ND group. Western blot analysis of a pooled cerebrospinal fluid sample revealed that both isoforms, ERK1 and ERK2, and low amounts of doubly phosphorylated ERK2 were detectable. As a predictive diagnostic AD biomarker, CSF-ERK1/2 was inferior to tTau, pTau, and Aβ42.

How golden is the gold standard of neuropathology in dementia?

Current Alzheimer's disease (AD) criteria state that a definite diagnosis can only be made by postmortem examination. The neuropathological confirmation is often referred to as the "gold standard." In this article, we review what constitutes a gold standard and how the neuropathological examination of AD lives up to that standard. We conclude that there is no evidence for this notion because results between different laboratories differ to an important extent, especially when the clinical picture is in doubt, for example, when the dementia is mild. As an alternative, we propose to abandon thinking in standards and value neuropathology as any other biomarker, and to strive to use and integrate multiple sources of information to make the diagnosis of AD in all its complexity.

Nuclear medicine imaging in dementia: a practical overview for hospitalists

Dementia is a clinical syndrome with diverse presentation, a challenging differential diagnosis, and time-sensitive therapy. The most common cause of dementia in patients aged > 65 years is Alzheimer's disease, which now affects 4 million people in the United States, but is often underrecognized, especially in the inpatient population. The hospitalist may have the opportunity to evaluate a patient's initial presentation of dementia. Addressing the inpatient's dementia symptoms can improve overall care and outcomes, so it is imperative that the hospitalist is abreast of recent developments in the dementia workup. The focus of this article is to overview how nuclear medicine imaging of the brain can aid in this process, with perfusion single-photon emission computed tomography (SPECT) and fludeoxyglucose F 18 ((18)F-FDG) positron emission tomography (PET) as the 2 most common modalities. Our discussion focuses on Alzheimer's disease, as this the most common etiology of dementia in patients aged > 65 years; however, we also touch on the other common neurodegenerative dementias (eg, dementia with Lewy bodies, vascular dementia, and frontotemporal dementia) for completeness. We begin with a summary of the most recent published guidelines for each of these neurodegenerative diseases, and then expand on the role that nuclear imaging plays in each. We provide a basic overview of the principles of these nuclear medicine techniques, and then illustrate findings in perfusion SPECT and (18)F-FDG PET for typical patterns of dementia, with emphasis on evidence regarding diagnostic accuracy of each modality, in comparison with accepted gold standards. Finally, we outline some future research topics within the field of nuclear medicine in dementia, including amyloid plaque imaging and dopamine transporter imaging.

Role of CSF biomarkers in the diagnosis of prodromal Alzheimer’s disease

Alzheimer’s disease (AD) core biomarkers (Aβ1-42, total tau and phosphorylated tau) have proven to be useful in the clinical practice to evaluate patients with mild cognitive impairment in order to predict progression to Alzheimer’s disease. Multicenter studies have shown an good overall performance of Aβ1-42, total tau and phosphorylated tau in the diagnosis of early AD; however, they also evidenced some possible weakness in terms of variability among centers, which generates some concern about their use in routine clinical practice. Therefore, the need for a joint effort of academia, companies and government agencies is evident. In this article we will provide the state of art of AD biomarkers application for the diagnosis of early AD, also describing some of the most promising new putative biomarkers currently studied. The final aim is to introduce a panel of AD biomarkers that is able to describe the preclinical phases of AD, as fully as possible paving the way to a routine early diagnosis in view of treatment by means of disease-modifying drugs.

Comparison of immunosorbent assays for the quantification of biomarkers for Alzheimer's disease in human cerebrospinal fluid

BACKGROUND

The clinical diagnosis of Alzheimer's disease in early stages may be substantiated by the quantification of the biomarkers Abeta42, Abeta40 and total-Tau (t-Tau) in cerebrospinal fluid (CSF). Different commercially available immunosorbent assays yield reliable results, yet the absolute values obtained may differ in between tests.

METHODS

We used CSF samples from patients that reported to our memory clinic. Enzyme-linked immunosorbent assays obtained from Innogenetics were used for the quantification of Abeta42 and t-Tau, test kits from IBL International were used to determine Abeta42 and Abeta40 concentrations. The multiplex assay system obtained from Mesoscale Discovery (MSD) Systems was used for the quantification of all three biomarkers.

RESULTS

For all biomarkers, the absolute values obtained with different test systems differ. However, the data sets highly correlate for all comparisons, with the MSD test system proving to be slightly more sensitive. Correlation coefficients (c) for the Abeta42 and Abeta40 quantifications lie between c = 0.80 and c = 0.87, and for the t-Tau quantifications we determined c = 0.99.

CONCLUSION

We conclude that all assays evaluated give reliable results, yet absolute values obtained have to be assessed differently within the framework of diagnostic procedures, depending on the system used.

The Alzheimer's Association external quality control program for cerebrospinal fluid biomarkers

BACKGROUND

The cerebrospinal fluid (CSF) biomarkers amyloid β (Aβ)-42, total-tau (T-tau), and phosphorylated-tau (P-tau) demonstrate good diagnostic accuracy for Alzheimer's disease (AD). However, there are large variations in biomarker measurements between studies, and between and within laboratories. The Alzheimer's Association has initiated a global quality control program to estimate and monitor variability of measurements, quantify batch-to-batch assay variations, and identify sources of variability. In this article, we present the results from the first two rounds of the program.

METHODS

The program is open for laboratories using commercially available kits for Aβ, T-tau, or P-tau. CSF samples (aliquots of pooled CSF) are sent for analysis several times a year from the Clinical Neurochemistry Laboratory at the Mölndal campus of the University of Gothenburg, Sweden. Each round consists of three quality control samples.

RESULTS

Forty laboratories participated. Twenty-six used INNOTEST enzyme-linked immunosorbent assay kits, 14 used Luminex xMAP with the INNO-BIA AlzBio3 kit (both measure Aβ-(1-42), P-tau(181P), and T-tau), and 5 used Meso Scale Discovery with the Aβ triplex (AβN-42, AβN-40, and AβN-38) or T-tau kits. The total coefficients of variation between the laboratories were 13% to 36%. Five laboratories analyzed the samples six times on different occasions. Within-laboratory precisions differed considerably between biomarkers within individual laboratories.

CONCLUSIONS

Measurements of CSF AD biomarkers show large between-laboratory variability, likely caused by factors related to analytical procedures and the analytical kits. Standardization of laboratory procedures and efforts by kit vendors to increase kit performance might lower variability, and will likely increase the usefulness of CSF AD biomarkers.

EFNS guidelines for the diagnosis and management of Alzheimer's disease

BACKGROUND AND OBJECTIVES

In 2008 a task force was set up to develop a revision of the European Federation of the Neurological Societies (EFNS) guideline for the diagnosis and management of Alzheimer's disease (AD) and other disorders associated with dementia, published in early 2007. The aim of this revised international guideline was to present a peer-reviewed evidence-based statement for the guidance of practice for clinical neurologists, geriatricians, psychiatrists, and other specialist physicians responsible for the care of patients with AD. Mild cognitive impairment and non-Alzheimer dementias are not included in this guideline.

METHODS

The task force working group reviewed evidence from original research articles, meta-analysis, and systematic reviews, published before May 2009. The evidence was classified and consensus recommendations graded (A, B, or C) according to the EFNS guidance. Where there was a lack of evidence, but clear consensus, good practice points were provided.

RESULTS

The recommendations for clinical diagnosis, blood tests, neuropsychology, neuroimaging, electroencephalography, cerebrospinal fluid (CSF) analysis, genetic testing, disclosure of diagnosis, treatment of AD, behavioural and psychological symptoms in dementia, legal issues, counselling and support for caregivers were all revised as compared with the previous EFNS guideline.

CONCLUSION

A number of new recommendations and good practice points are made, namely in CSF, neuropsychology, neuroimaging and reviewing non-evidence based therapies. The assessment, interpretation, and treatment of symptoms, disability, needs, and caregiver stress during the course of AD require the contribution of many different professionals. These professionals should adhere to these guideline to improve the diagnosis and management of AD.

Polymeric complements to the Alzheimer's disease biomarker β-amyloid isoforms Aβ1-40 and Aβ1-42 for blood serum analysis under denaturing conditions

Treatment of Alzheimer's disease (AD) is plagued by a lack of practical and reliable methods allowing early diagnosis of the disease. We here demonstrate that robust receptors prepared by molecular imprinting successfully address current limitations of biologically derived receptors in displaying affinity for hydrophobic peptide biomarkers for AD under denaturing conditions. C-terminal epitope-imprinted polymers showing enhanced binding affinity for Aβ1-42 were first identified from a 96-polymer combinatorial library. This information was then used to synthesize molecularly imprinted polymers for both of the β-amyloid (Aβ) isoforms and a corresponding nonimprinted polymer. A solid-phase extraction method was developed to be compatible with sample loading under conditions of complete protein denaturation. This resulted in a method capable of quantitatively and selectively enriching a shorter C-terminal peptide corresponding to the sequences Aβ33-40 and Aβ33-42 as well as the full-length sequence Aβ1-40 and Aβ1-42 from a 4 M guanidinum chloride solution. Application of the method to serum allowed selective, high-recovery extraction of both biomarkers at spiking levels marginally higher than clinically relevant concentrations found in cerebrospinal fluid.

Genetic counseling and testing for Alzheimer disease: joint practice guidelines of the American College of Medical Genetics and the National Society of Genetic Counselors

Alzheimer disease is the most common cause of dementia. It occurs worldwide and affects all ethnic groups. The incidence of Alzheimer disease is increasing due, in part, to increased life expectancy and the aging baby boomer generation. The average lifetime risk of developing Alzheimer disease is 10-12%. This risk at least doubles with the presence of a first-degree relative with the disorder. Despite its limited utility, patients express concern over their risk and, in some instances, request testing. Furthermore, research has demonstrated that testing individuals for apolipoprotein E can be valuable and safe in certain contexts. However, because of the complicated genetic nature of the disorder, few clinicians are prepared to address the genetic risks of Alzheimer disease with their patients. Given the increased awareness in family history thanks to family history campaigns, the increasing incidence of Alzheimer disease, and the availability of direct to consumer testing, patient requests for information is increasing. This practice guideline provides clinicians with a framework for assessing their patients' genetic risk for Alzheimer disease, identifying which individuals may benefit from genetic testing, and providing the key elements of genetic counseling for AD.

Qualification of the analytical and clinical performance of CSF biomarker analyses in ADNI

The close correlation between abnormally low pre-mortem cerebrospinal fluid (CSF) concentrations of amyloid-β1-42 (Aβ(1-42)) and plaque burden measured by amyloid imaging as well as between pathologically increased levels of CSF tau and the extent of neurodegeneration measured by MRI has led to growing interest in using these biomarkers to predict the presence of AD plaque and tangle pathology. A challenge for the widespread use of these CSF biomarkers is the high variability in the assays used to measure these analytes which has been ascribed to multiple pre-analytical and analytical test performance factors. To address this challenge, we conducted a seven-center inter-laboratory standardization study for CSF total tau (t-tau), phospho-tau (p-tau(181)) and Aβ(1-42) as part of the Alzheimer's Disease Neuroimaging Initiative (ADNI). Aliquots prepared from five CSF pools assembled from multiple elderly controls (n = 3) and AD patients (n = 2) were the primary test samples analyzed in each of three analytical runs by the participating laboratories using a common batch of research use only immunoassay reagents (INNO-BIA AlzBio3, xMAP technology, from Innogenetics) on the Luminex analytical platform. To account for the combined effects on overall precision of CSF samples (fixed effect), different laboratories and analytical runs (random effects), these data were analyzed by mixed-effects modeling with the following results: within center %CV 95% CI values (mean) of 4.0-6.0% (5.3%) for CSF Aβ(1-42); 6.4-6.8% (6.7%) for t-tau and 5.5-18.0% (10.8%) for p-tau(181) and inter-center %CV 95% CI range of 15.9-19.8% (17.9%) for Aβ(1-42), 9.6-15.2% (13.1%) for t-tau and 11.3-18.2% (14.6%) for p-tau(181). Long-term experience by the ADNI biomarker core laboratory replicated this degree of within-center precision. Diagnostic threshold CSF concentrations for Aβ(1-42) and for the ratio t-tau/Aβ(1-42) were determined in an ADNI independent, autopsy-confirmed AD cohort from whom ante-mortem CSF was obtained, and a clinically defined group of cognitively normal controls (NCs) provides statistically significant separation of those who progressed from MCI to AD in the ADNI study. These data suggest that interrogation of ante-mortem CSF in cognitively impaired individuals to determine levels of t-tau, p-tau(181) and Aβ(1-42), together with MRI and amyloid imaging biomarkers, could replace autopsy confirmation of AD plaque and tangle pathology as the "gold standard" for the diagnosis of definite AD in the near future.