The total tau to amyloid-β42 ratio from cerebrospinal fluid reliably distinguishes Alzheimer’s disease from frontotemporal degeneration, according to a paper in the April 9 Archives of Neurology online. First author David Irwin and colleagues at the University of Pennsylvania in Philadelphia also showed they could translate results from the older enzyme-linked immunosorbent assays (ELISAs) to match data from their new method, Luminex xMAP technology.

“Can you differentiate AD from FTLD [frontotemporal lobar degeneration] in the memory clinic? This study profoundly suggests, yes, you can,” said author Les Shaw, citing “very compelling” differences between tau:Aβ ratios in the two conditions. The ratio tends to be lower in FTLD cerebrospinal fluid (CSF) than AD samples, the team reported previously (Grossman et al., 2005; Bian et al., 2008). In this study, using a cutoff ratio of 0.34, the researchers obtained a sensitivity of at least 90 percent and specificity of at least 91 percent, depending on the samples used. Doctors at the university are already applying the biomarker test, Shaw told ARF. While not diagnostic on its own, it could help distinguish ambiguous cases, said senior author Murray Grossman.

In addition to comparing AD with FTLD, the team compared ELISA versus xMAP immunoassays. While ELISAs remain the standard in many labs, the newer platform is catching on, Shaw said. “In our experience, it offers the possibility of improved precision and reproducibility,” he added. Like an ELISA, the xMAP assay, using reagents developed by Fujirebio/Innogenetics, relies on antibodies to capture antigens from biofluid samples. But whereas in an ELISA the antibodies line a plastic well, in xMAP studies, the antibodies are conjugated to microbeads. The beads are fluorescently color coded depending on the attached antibody. This allows researchers to mix up to a dozen different sets of beads in the same sample, Shaw estimated, then distinguish them by flow cytometry. The Luminex-based method is "a much better way to ascertain tau and amyloid levels," Grossman said.

In adopting the Luminex system, the researchers did not want to simply discard old ELISA data. Instead, they sought a way to reconcile the two. They compared 75 samples in both assays and worked out an equation to convert ELISA results to Luminex-like numbers. Crucially, Shaw noted, this conversion applies only to the study population analyzed by these UPenn labs in this set of experiments. If scientists wished to translate ELISA results from another group, they would have to perform a similar analysis and come up with their own equation. Thus, the research described here does not solve the ongoing challenge of standardizing biomarker results across many labs, Shaw said (see ARF related news story).—Amber Dance

Comments

Make a Comment

To make a comment you must login or register.

Comments on this content

  1. This paper by Irwin at al. raises some important points.

    For a study on CSF frontotemporal dementia (FTD) biomarkers, they managed to collect a relatively fair number of patients. Also, the postmortem confirmation is a strong point, as FTD is heterogeneous and clinical diagnosis is not simple. However, for the kind of conclusions (very high sensitivity and specificity), the study is not powered enough (although we realize it is not easy to obtain high numbers of characterized, postmortem confirmed FTD patients). In the end, the sensitivity and specificity numbers are based on just 10 FTD patients versus 10 AD patients, and thus every single patient may influence these numbers strongly.

    Their approach to use the natural log to transform ELISA data to Luminex data leads to a good fit. But, on the other hand, there is an overfitting of the data: In the majority of figures of the Ln-transformed data of the test sets in Figure 2, the correlation coefficient is high due to a single outlier, and the trend lines are thus too steep. Since the correlation coefficients of the test sets are therefore much lower than the training set, it is clear that the current transformation model cannot be extrapolated to other cohorts or even single patients. This is clearly also due to the fact that the training set of 40 data points does not provide sufficient power to obtain a generalizable formula.

    Finally, we find that the aim not very clearly stated: Is it to compare two platforms? Or is it to define cutoff points for discriminating AD from FTD? The title may be taken as the aim is a comparison of the two platforms. Instead, what they did is a merging of data obtained from two different platforms, which are not easily interchangeable (Jongbloed et al., in press).

    References:

    . Discriminatory and predictive capabilities of enzyme-linked immunosorbent assay and multiplex platforms in a longitudinal Alzheimer's disease study. Alzheimers Dement. 2013 May;9(3):276-83. Epub 2012 Oct 27 PubMed.

References

News Citations

  1. Paris: Standardization a Hurdle for Spinal Fluid, Imaging Markers

Paper Citations

  1. . Cerebrospinal fluid profile in frontotemporal dementia and Alzheimer's disease. Ann Neurol. 2005 May;57(5):721-9. PubMed.
  2. . CSF biomarkers in frontotemporal lobar degeneration with known pathology. Neurology. 2008 May 6;70(19 Pt 2):1827-35. PubMed.

External Citations

  1. Luminex xMAP technology

Further Reading

Papers

  1. . Biomarkers in frontotemporal lobar degenerations--progress and challenges. Prog Neurobiol. 2011 Dec;95(4):636-48. PubMed.
  2. . Comparison of analytical platforms for cerebrospinal fluid measures of β-amyloid 1-42, total tau, and p-tau181 for identifying Alzheimer disease amyloid plaque pathology. Arch Neurol. 2011 Sep;68(9):1137-44. PubMed.
  3. . Novel CSF biomarkers for frontotemporal lobar degenerations. Neurology. 2010 Dec 7;75(23):2079-86. PubMed.
  4. . Biomarker discovery for Alzheimer's disease, frontotemporal lobar degeneration, and Parkinson's disease. Acta Neuropathol. 2010 Sep;120(3):385-99. PubMed.
  5. . The Alzheimer's Association external quality control program for cerebrospinal fluid biomarkers. Alzheimers Dement. 2011 Jul;7(4):386-395.e6. PubMed.
  6. . Qualification of the analytical and clinical performance of CSF biomarker analyses in ADNI. Acta Neuropathol. 2011 May;121(5):597-609. PubMed.

Primary Papers

  1. . Comparison of Cerebrospinal Fluid Levels of Tau and Aβ 1-42 in Alzheimer Disease and Frontotemporal Degeneration Using 2 Analytical Platforms. Arch Neurol. 2012 Apr 9; PubMed.