Epidemiology: February 2012 Archives

Two articles in this week's Neurology consider the significance of biomarkers for Alzheimer disease in the cognitively normal elderly.

Essentially confirming a recently published Mayo Clinic study, investigators from Texas reported the high prevalence of beta amyloid deposits (detected with Avid Pharmaceutical's PET tracer florbetapir) in the brains of a substantial portion (~20%) of healthy elderly subjects (aged 60 years or older). A high burden* of beta amyloid was associated with 1) the presence of the e4 allele of APOE, a genetic risk factor for AD and 2) poorer cognitive performance on more sophisticated** cognitive tests. The authors, two of whom receive financial support or have a financial interest in Avid Pharmaceuticals, suggest that "subtle cognitive changes accrue as amyloid progresses." The implication being that brain amyloid in the so-called healthy elderly may be more predictive than not about the risk of AD (an essential point or spin, if you will, which argues for the clinical utility of Avid's amyloid tracer). However, the authors also concede that "the relative contribution of amyloid vs other well-demonstrated predictors of cognitive aging (eg, regional atrophy, leukoaraiosis) is unclear." Among recommended follow-up studies is the assessment of the long-term consequences of amyloid deposition—ie, Are cognitively normal elderly with high amyloid burdens more like to development AD...that is, if they live long enough?. The question can only be answered with longitudinal follow-up study, which is planned.

Another multi-multi-authored international study assessed the effect of aging on the diagnostic value of CSF biomarkers for AD—specifically the triad of decreased amyloid and increased total and phosphorylated tau—in cognitively normal elderly. This study was distinctive in that, in addition to assessing individuals with AD and healthy elderly controls, a short (2-year) longitudinal assessment was made of subjects with mild cognitive impairment (MCI). The related conclusions (if I'm reading this article correctly and can recall anything useful about specificity, sensitivity, and positive and negative predictive values***) are 1) that there is substantial overlap in the biomarker distributions among these subgroups with advancing age and 2) that (therefore) the specificity of the biomarkers for ruling out AD decreases with age. Nevertheless, the sensitivity (85% cutoff) for the triad in MCI and AD, perhaps not too surprisingly, remained stable. In other words, the biomarker triad lost its strength to identify people with truly negative results as they aged (a decreased negative predictive value), but its ability to recognize truly AD-consistent findings (positive predictive value) remained stable—at least according to the authors and/or how I'm reading the study.

The overall conclusion, therefore, is that positive biomarkers for AD become less and less meaningful (as an indicator of AD) in the general elderly population—which is, ironically, the group in which these tests are most likely to be needed and used.

Addendum: Some would view these study results to mean that amyloid deposits and the triad of CSF changes are suggestive (or even indicative) of age-dependent cognitive dysfunction, albeit very subtle cognitive dysfunction. But the capacity of these tests to support or even confirm a diagnosis of clinical AD—depending on how we're defining the condition (there's the rub, isn't it?)—appears to be eroded in the older, general population (because they're more likely to demonstrate these findings, regardless of whether they demonstrate clinical meaningful cognitive dysfunction).

APOE = apolipoprotein E; SUVR = standardized uptake value ratio.

* At a mean SUVR cutoff of 1.22 or greater.
** Meaning more sophisticated than the Mini-Mental State Examination (MMSE).
*** Damn you, rudimentary biostatistics, when will you leave me alone!

Photograph: Atrophied brain from person with AD from National Institute on Alcohol Abuse and Alcoholism.