Plasma alkaline phosphatase is elevated in Alzheimer's disease and inversely correlates with cognitive function.

Alkaline phosphatase is present on neuronal membranes and plasma alkaline phosphatase activity increases in brain injury and cerebrovascular disease, suggesting that plasma alkaline phosphatase may partly reflect neuronal loss. As neuronal loss occurs in Alzheimer's disease (AD), we hypothesised that alterations in plasma alkaline phosphatase activity may correlate with cognitive impairment. Plasma alkaline phosphatase activity was measured in the longitudinal Oxford Project to Investigate Memory and Aging (OPTIMA) cohort (121 AD patients, 89 mild cognitive impairment (MCI) patients and 180 control subjects). Plasma alkaline phosphatase activity was significantly higher in the AD patients relative to the controls (p<0.001). In the MCI patients, plasma alkaline phosphatase was at a level in between that seen in control and AD subjects, consistent with the clinical status of this group. Furthermore, plasma alkaline phosphatase activity inversely correlated with cognitive function (assessed by the Cambridge Examination for Mental Disorders (CAMC0G)) in controls (z= -2.21 p=0.027), MCI (z= -2.49, p=0.013) and AD patients (z= -3.61, p=0.0003). These data indicate that plasma alkaline phosphatase activity is increased in AD and inversely correlates with cognitive function regardless of diagnostic status.