Multiple studies in the past decade,17
including the most recent from the Alzheimer’s Disease Neuroimaging Initiative,9
have suggested that CSF levels of Aβ1–42, total tau, or p-tau181
could serve as biomarkers for AD risk. The International Working Group for New Research Criteria for the Diagnosis of Alzheimer’s Disease20
has recommended incorporating levels of these biomarkers into routine clinical practice. Indeed, new criteria for AD and mild cognitive impairment have been proposed that rely in part on measurements of levels of these biomarkers in the CSF.
There are many uncertainties regarding the validity of these biomarkers to assess the risk of AD. First, several studies21–23
have shown significant laboratory-to-laboratory variability in CSF levels of Aβ1–42, total tau, or p-tau181
, even when using the same reagents and samples. Second, CSF levels of Aβ1–40 and Aβ1–42 have been shown to fluctuate significantly in young volunteers (2- to 3-fold for Aβ1–42) during 24 hours in a partially noncircadian rhythm.13
If this also occurs in older impaired individuals (ie, the ones most likely to receive this test), this variability could prevent recognition of the underlying disease (30%–40% lower levels of CSF Aβ1–42 in individuals at risk for AD).9
Given that the reported variability in CSF Aβ values was not completely explained by circadian factors, standardizing the time of CSF collection would not necessarily correct this problem.
The lack of significant fluctuations in any of the biomarkers we assayed in our study of older, cognitively impaired individuals shows promise for the real-world application of CSF AD biomarkers, as these people are most likely to undergo testing for these biomarkers. Moreover, it is reassuring that changes in the levels of these biomarkers in a clinically relevant cohort imply a disease- or age-relevant variation rather than fluctuations resulting from preanalytical variables such as time of collection.
There are several differences between our study and that of Bateman and colleagues,13
which showed significant variability in CSF Aβ values. First, our patient population was significantly older. However, Bateman and colleagues examined a group of 5 older individuals and found the same variation in AD biomarkers as in the younger controls, and we found no significant difference in the fluctuation of Aβ1–42 between the youngest and oldest patients in our cohort. Thus, the role of age in the discrepant results is uncertain. A second difference between the 2 studies is the sampling frequency. Bateman and colleagues sampled CSF every hour compared with every 6 hours in our study. However, since the peak-to-peak variability for Aβ followed a 12-hour cycle in the prior study, a significant level of variability would have been apparent in our study, which sampled at twice that frequency, consistent with the Nyquist rate.24
Third, all patients in the prior study were in good general health and without neurologic disease in contrast to our participants, each of whom had some ongoing neurologic abnormality. Our reliance on a clinical cohort without healthy controls is a shortcoming of this study and limits comparison with that of Bateman and colleagues. Finally, some technical aspects of CSF collection were different. Because we did not collect samples as frequently, we could discard the first 10 mL of CSF, ensuring that the collected fluid was fresh. In addition, our CSF was dripped directly into polypropylene tubes and frozen almost immediately to minimize any adherence of Aβ to tubing.
Fluctuating levels of CSF Aβ concentrations have been taken as evidence of circadian- or activity-dependent processing of the amyloid precursor protein that produces Aβ.25
Because of our sampling frequency (6 hours) and duration of sampling (36 hours maximum), we cannot exclude fast or slow oscillations in CSF Aβ. Sampling in the lumbar spine is also a limitation to detecting rapid physiologic fluctuations in Aβ, since the lumbar subarachnoid space is far removed from the generation of most Aβ in the cerebral cortex. However, the absence of variability of these AD biomarkers in the lumbar CSF of a clinically relevant cohort of older individuals with neurologic diseases should provide some reassurance as to the validity of these biomarkers in clinical practice.