Levels of nonphosphorylated and phosphorylated tau in cerebrospinal fluid of Alzheimer's disease patients - An ultrasensitive bienzyme-substrate-recycle enzyme-linked Immunosorbent assay

We have developed an ultrasensitive bienzyme-substrate-recycle enzyme-linked immunosorbent assay for the measurement of Alzheimer's disease (AD) abnormally hyperphosphorylated tau in cerebrospinal fluid (CSF). The assay, which recognizes attomolar amounts of tau, is similar to400 and similar to1300 times more sensitive than conventional enzyme-linked immunosorbent assay in determining die hyperphosphorylated tau and total tau, respectively. With this method, we measured both total tau and tau phosphorylated at Ser-396/Ser-404 in lumbar CSFs from AD and control patients. We found that the total tau was 215 +/- 77 pg/ml in cognitively normal control (n = 56), 234 +/- 92 pg/ml in non-AD neurological (n = 37), 304 +/- 126 pg/ml in vascular dementia (n = 46), and 486 +/- 168 pg/ml (n = 52) in AD patients, respectively. However, a remarkably elevated level in phosphorylated tau was only found in AD (187 +/- 84 pg/ml), as compared with normal controls (54 +/- 33 pg/ml), non-AD (63 +/- 34 pg/ml), and vascular dementia (72 +/- 33 pg/ml) groups. If we used the ratio of hyperphosphorylated tau to total tau of greater than or equal to0.33 as cutoff for AD diagnosis, we could confirm the diagnosis in 96% of the clinically diagnosed patients with a specificity of 95%, 86%, 100%, and 94% against nonneurological, non-AD neurological, vascular dementia, and all of the three control groups combined, respectively. It is suggested that the CSF level of tau phosphorylated at Ser-396/ Ser-404 is a promising diagnostic marker of AD.