Report CSF Ab42 and tau in relation to amyloid in humans amyloid-b42 (Ab42), the primary constituent of brain amyloid plaques (Fagan et al, 2006, 2007). Individuals with cortical amyloid (as detected by PET PIB) had low CSF Ab42 whereas those without cortical amyloid had high CSF Ab42. This relationship was observed independent of clinical status; several cognitively normal individuals had a CSF Ab42/PIB proﬁle indistinguishable from that of other individuals diagnosed clinically with early stage dementia of the Alzheimer type (DAT). These observations are consistent with the idea of preclinical AD and suggest that these measures may have clinical utility as antecedent biomarkers of the disease. We have now obtained CSF and PIB data from 189 cognitively normal individuals ranging in age from 43 to 89 years. We explored the relationship between in vivo brain amyloid and CSF markers of proteins present in neurons and constituents of neuroﬁbrillary tangles (tau and ptau181), as well as Ab species in plasma, and investigated whether the CSF Ab42/PIB relationship remains robust in this large cohort of non-demented individuals. The data we obtained shed further light on the potential utility of these measures as antecedent biomarkers of AD, and also provide insight into the normal time course of the pathophy- siology of the disease as reﬂected in CSF, information that should aid in the design and evaluation of secondary prevention trials. RESULTS One hundred and eighty-nine research participants with a clinical dementia rating of 0 (CDR 0, indicating cognitively normal) (Morris, 1993) met the selection criterion of having a PIB scan within 2 years of CSF collection by lumbar puncture (LP). Combined PIB and biomarker data from 25 of these participants have been reported by us in previous studies (Fagan et al, 2006, 2009), whereas the remaining 164 are unique to the present study. The demographic characteristics of the present cohort are similar to what we have previously published with the exception of age (Table 1). By design, we have included a wide range of ages in the present study (43–89 years, normally distributed) so as to better capture the various biomarker correlations during the potential preclinical stage of AD (which is estimated to begin 10–20 years prior to cognitive symptoms). Therefore, the mean age of our cohort is younger (64.7 10.4) than what we have reported on previously (71.41 8.62; Fagan et al, 2009). In keeping with this age difference, CSF Ab42 levels in the present study (652 235 pg/ml) are higher than that reported by us in our studies of older individuals (572 208 pg/ml) and, as expected, CSF tau (285 151 pg/ml vs. 334 180 pg/ml), and ptau181 (52 23 pg/ml vs. 61 27 pg/ ml) levels are lower (Fagan et al, 2009). The absolute plasma Ab values cannot be compared between our various studies because different methods were used to measure these analytes. Given the wide range of ages in the present cohort, we ﬁrst investigated whether any of the biomarker measures correlated with age at the time of LP. As shown in Fig. 1, positive correlations were observed between age and cortical amyloid (represented by mean cortical PIB binding potential (MCBP); Table 1. Participant demographic characteristics, psychometric perform- ance and biomarker values N Age at LP (year) Age range (year) M/F (%F) APOE e4–/e4þ (% e4þ) Selective remembering (possible score, 0–48) Animal naming Trailmaking A (# of connections/s) Trailmaking B (# of connections/s) CSF Ab38 CSF Ab40 CSF Ab42 CSF tau CSF ptau181 Plasma Ab1–40 Plasma Abx–40 Plasma Ab1–42 Plasma Abx–42 PIB MCBP CDR 0 participants 189 64.7 (10.4) 43–89 62/127 (67%) 125/64 (34%) 31.2 (6.3) 21.6 (5.3) 0.916 (0.303) 0.384 (0.151) 1228 (514) 8958 (4464) 652 (235) 285 (151) 52 (23) 217 (60) 37 (11) 193 (44) 25 (9) 0.0931 (0.213) Fluid psychometric and PET PIB (MCBP) values are represented as means (standard deviations). CSF and plasma values are in pg/ml. PIB MCBP values are in arbitrary units (generated by Logan graphical analyses). APOE, apolipoprotein E; CDR, clinical dementia rating; CSF, cerebrospinal ﬂuid; LP, lumbar puncture; MCBP, mean cortical binding potential; PIB, Pittsburgh compound B. Mintun et al, 2006), CSF tau and ptau181 and plasma Ab1–40. An inverse correlation was observed between age and CSF Ab42, and no relationships between age and CSF Ab38 or Ab40 were found. Due to these age effects, all subsequent analyses were corrected for age. We next investigated whether the inverse relationship between CSF Ab42 and cortical PIB binding that we had reported previously in a mixed cohort of mildly demented and non-demented individuals was observed in this cohort of cognitively normal individuals. Overall, 29 participants in this cohort had MCBP values greater than or equal to 0.18 whereas 160 participants had MCBPs below 0.18 (Fig 2A). In individuals with MCBP values greater than 0.18, PIB retention is visualized in the neocortex and appears qualitatively greater than back- ground levels. We continued to observe a robust and linear relationship between CSF Ab42 and cortical amyloid in this group of cognitively normal individuals (Fig 2B). Every partici- pant with high PIB binding had CSF Ab42 values <582 pg/ml; 86% had CSF Ab42 values <500 pg/ml. A large majority (84%) of participants with low PIB binding had CSF Ab42 values >500 pg/ml (Fig 2A). Consistent with our previous ﬁndings (Fagan et al, 2006, 2007), many of the CDR 0 participants within this broad age range had little or no cortical amyloid and high mean CSF Ab42 levels ( 500 pg/ml) (Fig 2B). Twenty-ﬁve of the 189 CDR 0 participants displayed the typical AD biomarker phenotype in relation to Ab, with high PIB binding and low CSF Ab42 (Fig 2B). In many cases their PET PIB scans were indistinguishable from demented individuals with DAT (CDR > 0) (Fig 2C). In contrast to CSF Ab42, CSF Ab40 was 372 2009 EMBO Molecular Medicine EMBO Mol Med 1, 371–380 www.embomolmed.org