Plasma Tau Levels in Cognitively Normal Middle-Aged and Older Adults

Using an ultra-sensitive technique, an immunomagnetic reduction assay, the plasma tau level can be measured to a limit of quantification of pg/ml. In total 126 cognitively normal middle-aged and older adults (45-95 years old) were recruited. The plasma tau levels were significantly higher in the older group (aged 65-95 years) 18.14 +/- 7.33 pg/ml than those in the middle-aged group (aged 45-64 years) 14.35 +/- 6.49 pg/ml when controlled gender and ApoE epsilon 4 carrier status (F = 3.102, P = 0.029). The ApoE epsilon 4 carriers had higher plasma tau levels than the non-carriers when controlled age and gender (F = 6.149, P = 0.001). Men had higher plasma tau levels than their women counterparts when controlled ApoE epsilon 4 carrier status and gender (F = 6.149, P = 0.001). The plasma tau levels were found to be positively associated with their ages (r = 0.359, P < 0.001). Regression analysis showed that age explained approximately 13% of the variance in the plasma tau levels, and explained more than 10% of the variance in the volumes of the hippocampus and white matter hypodensity (R-2 change 0.123 similar to 0.167, all P < 0.001), and explained less than 10% of the variance in the volume of the amygdala, and central part of the corpus callosum (R-2 change 0.085 similar to 0.097, all P = 0.001). However, the plasma tau levels do not further explain any residual variance in the volume of brain structures. In conclusion, the effect of age on the plasma tau levels should always be considered in clinical applications of this surrogate biomarker to middle-aged and elderly subjects.