P1–178: CSF biomarker variability in the Alzheimer's Association quality control program

Abstract

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 (M€ olndal, Sweden) were analyzed by singleanalyte 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.

Test‐retest variability of plasma biomarkers in Alzheimer's disease and its effects on clinical prediction models

“…The relationship between random error and overlap between normal and abnormal groups is not specific to AD BBMs but is a general problem throughout the field of clinical chemistry. It has therefore been suggested that the gray zones be defined for diagnostic biomarkers, where biomarker results should be interpreted with caution and need to be confirmed with orthogonal methods 24–25,27 …”

“…In this study, we primarily considered only one type of error in the present analysis— random error estimated by collecting and analyzing samples from the same individuals at different occasions but within a short time span. Another source of noise that can greatly affect biomarker values is systematic error caused by assay‐related changes in the analytical performance of the methods such as when changing lots of key materials (such as antibodies or calibrators) 27,28 . However, systematic error is much more difficult to quantify empirically given its unpredictable nature.…”

Characterization of pre‐analytical sample handling effects on a panel of Alzheimer's disease–related blood‐based biomarkers: Results from the Standardization of Alzheimer's Blood Biomarkers (SABB) working group

“…It took decades for cerebrospinal fluid (CSF) Aβ42 and p-tau to be widely accepted as fluid biomarkers for AD, 20,21 among other reasons due to large variability in levels between and within laboratories. 22,23 Variation has been partially mitigated by fully automated analytical instruments, 24,25 and development of Certified Reference Materials to which commercial assays were recalibrated. 26 Different procedures of sample handling in the pre-analytical phase, from body fluid collection until analysis, also caused variation.…”

Robustness of CSF Aβ42/40 and Aβ42/P‐tau181 measured using fully automated immunoassays to detect AD‐related outcomes

“…The total allowable error (TAE) of a biomarker, by contrast, is the maximum total error for which a biomarker still performs well in its intended clinical use. 12 While biological/patient-related factors do not appear to pose a significant problem for CSF AD biomarkers, 13,14 pre-analytical (e.g., sample handling and storage) and analytical (e.g., between/within differences in laboratory procedures) variability is known to affect biomarker values from enzyme-linked immunosorbent assays (ELISA), 15 which are widely used for routine CSF biomarker analysis. This has been shown to result in interlaboratory variation up to 15%.…”

“…While biological/patient‐related factors do not appear to pose a significant problem for CSF AD biomarkers, 13,14 pre‐analytical (e.g., sample handling and storage) and analytical (e.g., between/within differences in laboratory procedures) variability is known to affect biomarker values from enzyme‐linked immunosorbent assays (ELISA), 15 which are widely used for routine CSF biomarker analysis. This has been shown to result in interlaboratory variation up to 15% 15 . This can be addressed through the use of standardized protocols for CSF collection and storage 16 and fully automated platforms such as the Roche Elecsys immunoassays that have high test–retest reliability (<5%) and low laboratory‐ and kit‐associated variability 13 .…”