Study participants meeting the LP and PIB interval inclusion criteria included 89 who were cognitively normal (CDR 0), 11 who were very mildly demented (CDR 0.5), one who was mildly demented (CDR 1) and two who were moderately demented (CDR 2) (). Participants spanned a wide age range (46–89 years), with a mean ± SD of 67.8 ± 9.9 years. Sixty-eight percent were female, and 44% had at least one APOE ε4 allele. The median interval between LP and PIB scan was 97 days (3.2 months). The shortest interval was zero days and the longest was 719 days (1.97 years). The vast majority (80%) of participants had LP prior to PIB, including the 12 with the longest LP-PIB test intervals (ranging from 401 to 719 days).
According to data provided by the kit manufacturer, the intra-assay variability should be <6% with the INNOTEST®
ELISAs and <4% with the INNO-BIA platform 25
. In the present study, we observed a mean (± SD) coefficient of variation (% CV) of 4.2 ± 3.8%, 4.5 ± 4.8% and 1.7 ± 1.7% for Aβ1-42
, total tau and p-tau181
, respectively, using INNOTEST®
(). Similar CVs were observed for INNO-BIA, with 3.7 ± 2.8%, 3.6 ± 3.1% and 3.9 ± 3.1% for Aβ1-42
, total tau and p-tau181
, respectively ().
Within-assay performance (% coefficient of variability)
We first assessed the relationship between the analyte values obtained with the two platforms. Although the absolute values for Aβ1-42, total tau and p-tau181 obtained with the two platforms were different, the values correlated well with each other (Aβ1-42, r = 0.7715, p<0.0001; total tau, r = 0.9446, p<0.0001; p-tau181, r = 0.8017, p<0.0001) (), as did the ratios of total tau/Aβ1-42 (r = 0.8746, p<0.0001) and p-tau181/Aβ1-42 (r = 0.8586, p<0.0001) ().
Relationship between the assay platforms for the various CSF analytes. Analytes include A) Aβ1-42; B) total tau; C) P-tau181; D) total tau/Aβ1-42; and E) p-tau181/Aβ1-42. r, Spearman rho correlation coefficient.
Given that a low level of CSF Aβ1-42 has been shown by several groups to be a marker of brain amyloid (as assessed by PET PIB), we next compared head-to-head the amount of cortical PIB binding (MCBP) with CSF Aβ1-42 as determined by the two platforms. Overall, increased MCBP, indicating the presence of cortical amyloid, was associated with low levels of CSF Aβ1-42 as determined by both assays (). MCBP was positively correlated with levels of CSF total tau and p-tau181 (), as well as the tau/Aβ1-42 and p-tau181/Aβ1-42 ratios (). When comparing PIB-CSF correlation coefficients between the two assay platforms, the correlation between MCBP and INNO-BIA Aβ1-42 (r=−0.7068) was stronger than that for MCBP and INNOTEST® Aβ1-42 (r=−0.5673, p=0.00006); INNOTEST® total tau (r=0.5961) was slightly stronger than that for INNO-BIA total tau (r=0.5346, p=0.0045); INNO- BIA p-tau181 (r=0.6417) was stronger than that for INNOTEST® p-tau181 (r=0.5326, p=0.0053); INNO-BIA p-tau181/Aβ1-42 (r=0.7477) was stronger than that for INNOTEST® p-tau181/Aβ1-42 (r=0.6784, p<0.00001); whereas the correlation between MCBP and the ratio of total tau/Aβ1-42 did not differ between the two platforms (INNOTEST® r=0.7205, INNO-BIA r=0.7264, p=0.4674). These relationships were not driven by different distributions between the CDR 0 and CDR>0 groups; similar correlative relationships were observed within the individual clinical groups (data not shown).
Figure 2 Relationship between CSF analytes and cortical amyloid load. Analytes include A, B) Aβ1-42; C, D) total tau; E, F) P-tau181; G, H) total tau/Aβ1-42; and I, J) p-tau181/Aβ1-42. Left panels, INNOTEST®. Right panels, INNO-BIA. (more ...)
We next employed ROC analyses to determine the sensitivity and specificity of the CSF measures for discriminating amyloid+ from amyloid− individuals using a dichotomous PIB cutoff (MCBP ≥ 0.18) 19, 21
. All analytes, regardless of assay platform, performed well in discriminating between the two amyloid groups (n=38 amyloid+, n=65 amyloid−), with AUCs ranging from 0.82 (95% confidence intervals [CI]: 0.73, 0.90) to 0.97 (CI: 0.95, 0.99) (). However, the tau(s)/Aβ1-42
ratios performed the best, even better than Aβ1-42
alone, with sensitivities of 92% (INNO-BIA) and 95% (INNOTEST®
) for total tau/Aβ1-42
and 94% (INNOTEST®
) and 98% (INNO-BIA) for p-tau181
(compared to 85% [INNOTEST®
] and 89% [INNO-BIA ] for Aβ1-42
), all at 80% specificity ( and ). Whereas the AUCs of Aβ1-42
and the tau(s)/Aβ1-42
ratios did not differ statistically as a function of assay platform, the INNOTEST®
total tau values yielded better PIB group discrimination than the INNO-BIA total tau (76% vs 65% sensitivity, p=0.0067), and the INNO-BIA p-tau181
values performed better than the INNOTEST®
(86% vs 69% sensitivity, p=0.016) (). The results were the same when using a MCBP cut-off of 0.20 since the 0.18 and 0.20 dichotomized cohorts were identical in this study.
Comparison of the accuracy of the various CSF analytes to discriminate amyloid-positive from amyloid-negative participants (MCBP cut-off = 0.18)
Figure 3 Receiver operating characteristic (ROC) curves and area under the curve (AUC) describing CSF biomarker sensitivity and specificity for discriminating amyloid-positive (MCBP≥0.18) from amyloid-negative (MCBP<0.18) participants. CSF values (more ...)