. Development and advanced validation of an optimized method for the quantitation of aβ(42) in human cerebrospinal fluid. AAPS J. 2012 Sep;14(3):510-8. PubMed.

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  1. This is a very important and useful manuscript by Valerie Cullen and coworkers from Link Medicine, who very thoroughly validated and improved an established ELISA platform for the measurement of Aβ1-42 (Innotest® Aβ42 ELISA). The decrease of CSF Aβ1-42 has been (in combination with total tau protein) helpful for the early and differential diagnosis of Alzheimer’s disease, and was even included in clinical research criteria for Alzheimer’s disease (1). Recently, it was shown to be useful for other neurodegenerative diseases, such as dementia with Lewy bodies and Parkinson’s disease. This ELISA has been routinely used in investigations in Europe, but inter-center variation (by pre-analytical, analytical, and post-analytical factors), and inter-laboratory variation (caused by analytical factors), especially for CSF Aβ1-42, lowered the utility also for clinical multicenter trials (2). This has been addressed by a large, multicenter investigation of various assays across 40 laboratories (3).

    The team at Link Medicine validated and improved the Innotest Aβ42 ELISA, according to Good Laboratory Practice level, by minimizing the required CSF volume by dilution, thereby minimizing matrix effects; they changed the calibration curve, introduced quality control samples, and tested different polypropylene vial types and freeze/thaw cycles. This advanced validation of a method for the improvement of quantification of Aβ1-42 in human CSF is extremely helpful to strengthen the use of CSF biomarkers in future clinical trials and in routine analysis. I do hope that other assays (e.g., for total tau protein) and platforms for CSF proteins get the same attention and thorough validation.

    The validation issue will also be addressed by a recent EU initiative (BIOMARKAPD, coordinated by Bengt Winblad) to troubleshoot and improve assay systems. The aim of BIOMARKAPD is the harmonization and standardization of biomarker in cerebrospinal fluid (CSF) for Alzheimer’s disease (Aβ1-42 and tau protein) and Parkinson’s disease (α-synuclein) (see website).

    References:

    . The diagnosis of dementia due to Alzheimer's disease: recommendations from the National Institute on Aging-Alzheimer's Association workgroups on diagnostic guidelines for Alzheimer's disease. Alzheimers Dement. 2011 May;7(3):263-9. PubMed.

    . Inter-laboratory variation in cerebrospinal fluid biomarkers for Alzheimer's disease: united we stand, divided we fall. Clin Chem Lab Med. 2010 May;48(5):603-7. PubMed.

    . Cerebrospinal fluid and plasma biomarkers in Alzheimer disease. Nat Rev Neurol. 2010 Mar;6(3):131-44. PubMed.

  2. This is an interesting paper. It revisits aspects of standardization of Innogenetics Aβ assays (both ELISA and X-MAP) that have received attention before (e.g., by the Blennow and Shaw laboratories). Previous attempts have defined excellent within-laboratory standardization, but inter-laboratory variability for levels of Aβ42 have persistently varied by about 15-30 percent in the international QC program.
    The main new factors to emerge from this study are:

    1. Addition of Tween when cryotubes are thawed to decrease Aβ adsorption to the polypropylene (which may vary by manufacturer).

    Use of frozen CSF samples that have been stored for three months or longer is an important practical aspect for many laboratories, which have to wait until they have collected enough CSF samples to fill an ELISA plate. And for many research studies, running samples in batches has been used as a way to decrease variability—again pointing to attention to sample collection and storage.

    2. Dilution of 1:6 of CSF, which eliminated a matrix effect and suggested that CSF levels of Aβ42 are higher than typically reported using these assays.

    It will be of interest to see whether applying this new assay methodology changes assay reproducibility (especially between laboratories), and sensitivity and specificity for AD, compared to the ELISA that follows the Innogenetics instructions.

  3. The INNOTEST β amyloid(1-42) was developed many years ago using know-how available at that time (1). Notwithstanding an imperfect accuracy (recovery from samples was an issue), the user-friendly assay format, as well as the clinical value of the assay, was documented in many studies (2) and validated using autopsy-confirmed patient samples (3). The assay already obtained a CE label for diagnostic applications. Assay validation data are also described in the kit Insert of the assay.

    More recently, after integration of the assay in AD therapeutic trials, the issue on matrix interference became more apparent. The accuracy of an assay is linked to recovery, linearity, and parallelism. The group of Valerie Cullen used all critical raw materials and components already available in the commercial kit and modified the test instructions to solve the problem of accuracy by the inclusion of a dilution for the CSF and the addition of detergent to limit the absorption to recipients. However, according to our present understanding of the performance of these types of assays, improvements are especially linked to accuracy, and not necessarily to the other analytical performance characteristics (e.g., precision).

    • An accurate value in a sample can only be obtained after extensive standardization at the level of the assay, the sample, and the operator in the lab (4). It is an integrated approach.
    • A better understanding of what is being measured (monomers, oligomers) will help to evaluate the impact of assay modification on diagnostic applications.
    • The addition of a detergent is important for labs using recipients with a high Aβ absorption rate or to reduce differences in absorption rates, as described previously by Perret-Liaudet (5). The dilution of CSF will reduce the matrix interference and, more importantly, will result in different analyte concentrations in the CSF as compared to previous clinical studies done with the same assay, following test instructions as provided in the kit insert. This is the result of the release of Aβ from the binding proteins present in the CSF (equals higher amounts of detectable Aβ). The qualification of this new, promising approach has only been qualified on a few CSF samples.
    • The results of the new assay format will only be comparable with previous data if a conversion factor can be determined between the old and the new protocol, which is constant over the whole concentration range and independent of the diagnostic groups.
    • In order to integrate this new approach, a full validation, including round-robin studies as done in the Alzheimer's Association Quality Control program (led by Kaj Blennow, Sweden) (6) is required, as well as a confirmation of its clinical value using samples available from worldwide consortia (e.g., ADNI). This will help us understand whether or not the modification will also improve other aspects of the assay, such as precision in testing among labs.
    • The robustness of the CSF dilution protocol will have to be verified.
    • The integration of this new approach, if proven to be valuable, requires an investment from the vendor of the kit.
    • At present, there is a lack of international reference materials, as well as regulatory documents describing all necessary requirements for an immunoassay, either for using in in-vitro diagnostics or pharma. There is a need for consensus on this. The focus on validation of the assay for pharma might be different from validation for in-vitro diagnostics.

    The AD diagnostic market is at a critical point in the sense that clear performance requirements for the assays need to be described for IVD, as well as for pharma. If needed, modifications of the assays have to be implemented, or assays have to be designed on new technology platforms which do not have these matrix issues. The availability of a reference method and/or reference materials will further help to standardize output among the different assays and protocols. In any case, the clinical performance of the assay will have to be confirmed.

    References:

    . Standardization of measurement of beta-amyloid(1-42) in cerebrospinal fluid and plasma. Amyloid. 2000 Dec;7(4):245-58. PubMed.

    . Improved discrimination of AD patients using beta-amyloid(1-42) and tau levels in CSF. Neurology. 1999 May 12;52(8):1555-62. PubMed.

    . Diagnostic performance of a CSF-biomarker panel in autopsy-confirmed dementia. Neurobiol Aging. 2008 Aug;29(8):1143-59. PubMed.

    . Standardization of preanalytical aspects of cerebrospinal fluid biomarker testing for Alzheimer's disease diagnosis: a consensus paper from the Alzheimer's Biomarkers Standardization Initiative. Alzheimers Dement. 2012 Jan;8(1):65-73. PubMed.

    . Risk of Alzheimer's disease biological misdiagnosis linked to cerebrospinal collection tubes. J Alzheimers Dis. 2012 Jan 1;31(1):13-20. PubMed.

    . The Alzheimer's Association external quality control program for cerebrospinal fluid biomarkers. Alzheimers Dement. 2011 Jul;7(4):386-395.e6. PubMed.

  4. Much prior effort in this field has already gone toward validating this assay. As I see it, the authors of this paper have reached a point where we all have been for a decade; in other words, the “modifications” reported here achieved precision and repeatability that every well-functioning laboratory can achieve by following the manufacturer’s SOPs. My lab's numbers are, for example, 12-15 percent for interassay and about 5-7 percent for intra-assay imprecision. Moreover, it also seems to me that the modifications done by the authors have resulted in the increasing of the lowest detection limit.

    It is not fully accurate to state that this assay has not been validated. After all, it has the CE certificate and is routinely used for human diagnostics. In the interest of full disclosure, I have consulted for Innogenetics for some years, and have seen how much effort its assay scientists have invested in validating the assay.

    I have co-coordinated large national/international projects for inter-laboratory QC, and Neurochemical Dementia Diagnostics in my lab is accredited according to ISO 15189. I agree there are imprecision problems around these diagnostics, not only Aβ1-42 but also Tau, pTau181, and Aβ1-40 (see Zimmermann et al, 2011). However, I respectfully submit that the solution is not modifications to get back to 15 percent imprecision. Rather, the starting point should be simply that all laboratories follow what is recommended in the manufacturer’s protocol and standardize pre-analytical sample handling procedures.

    References:

    . Preanalytical sample handling and sample stability testing for the neurochemical dementia diagnostics. J Alzheimers Dis. 2011;25(4):739-45. PubMed.

  5. 1. Minimal required dilutions (MRD): It is a fact that the CSF matrix interferes with the Aβ42 peptides/oligomers. When dilutions are performed with the CSF, a better linearity is seen in this paper. In this way, dilutions seem to be the solution, and I agree on that. However, one problem arises: Due to the dilutions, the Lower Limit Of Quantitation increases to LLOQ = 375 pg/ml. This is not a problem per se, since all results increase, but as a consequence, a new reference range has to be set and a transformation formula for historically collected results has to be established. To set reference ranges and cut-offs, there must be linearity between the diluted CSF (new method) and the undiluted CSF (old method) in a large set of samples.

    2. Aβ42 adsorbs significantly; I agree on that as well. Aβ42 is sticky, and we have to take care of this problem. Everything that decreases the adherence of Aβ42 (without interfering with the assay) has to be implemented, and the effects of adding Tween-20 after thawing are striking and convincing in the current paper. This finding needs replication using other tubes that are currently in use (see Perret-Liaudet et al., 2012).

    3. I am interested in how many analysts performed the different assays in this paper. Is there any difference in the results if different people perform the same assay? In Amsterdam, we organized an international workshop to test this. During this workshop, analysts from 17 international centers performed the same assay under the same conditions, and at least 23 differences between analysts in performing the assay were noted (Teunissen et al., 2010). This indicates that it is very important to follow the exact procedures.

    4. What are the lot numbers of the assays that are used in this study? In our lab, we have experienced differences in results using the INNOTEST assay with different lot numbers. It is something that we have to take into consideration. Ideally, we have to have an assay (plates, diluents, standard, and QC samples) that never changes over time (Mulder et al., 2010; Verwey et al., 2008).

    5. One of the questions we have been asking ourselves for the last five years is, "What is the exact composition of the 'INNOTEST sample diluent'?” And how is the Aβ42 (standard included in the INNOTEST) processed? How is this Aβ42 synthesized, and by whom? I have experienced a lot of differences in synthetic Aβ42 made by different companies. Also, within the same company, different batches of Aβ42 can react differently when dissolved or used to spike. This is the same for Aβ40, although this peptide is more hydrophilic. Maybe this is one of the most important points. How exactly is the INNOTEST made?

    6. One of the big problems the authors touch upon is that the INNOTEST does not deliver QC samples. It would be a great step forward for quality if QC samples are being delivered together with the assay. Now we are forced to make our own QC samples to garantee quality over time. To implement an assay worldwide, QC samples are a necessity. This is the focus of the Global Biomarkers Standardization Consortium (GBSC), initiated by the Alzheimer's Association (Mattsson et al., 2010; Mattsson et al., 2011; Verwey et al., 2009).

    In summary, Aβ42 is influenced by the CSF matrix, and Aβ42 is a sticky peptide. At this moment, the suggestions of this paper to implement a minimal dilution and to add Tween-20 are interesting ones that need to be validated by independent studies before implementing in clinical practice. We believe that it is time to modify the INNOTEST assay to reach the goal of a solid assay that can be reliably used and yields uniform results worldwide.

    References:

    . Cerebrospinal fluid collection tubes: a critical issue for Alzheimer disease diagnosis. Clin Chem. 2012 Apr;58(4):787-9. PubMed.

    . Standardization of Assay Procedures for Analysis of the CSF Biomarkers Amyloid β((1-42)), Tau, and Phosphorylated Tau in Alzheimer's Disease: Report of an International Workshop. Int J Alzheimers Dis. 2010;2010 PubMed.

    . Amyloid-beta(1-42), total tau, and phosphorylated tau as cerebrospinal fluid biomarkers for the diagnosis of Alzheimer disease. Clin Chem. 2010 Feb;56(2):248-53. PubMed.

    . Variability in longitudinal cerebrospinal fluid tau and phosphorylated tau measurements. Clin Chem Lab Med. 2008;46(9):1300-4. PubMed.

    . Lessons from Multicenter Studies on CSF Biomarkers for Alzheimer's Disease. Int J Alzheimers Dis. 2010;2010 PubMed.

    . The Alzheimer's Association external quality control program for cerebrospinal fluid biomarkers. Alzheimers Dement. 2011 Jul;7(4):386-395.e6. PubMed.

    . A worldwide multicentre comparison of assays for cerebrospinal fluid biomarkers in Alzheimer's disease. Ann Clin Biochem. 2009 May;46(Pt 3):235-40. PubMed.

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