Increased cerebrospinal fluid A beta38/A beta42 ratio in Alzheimer disease.

We quantitated amyloid beta peptide (A beta) 38, A beta40 and A beta42 levels in matched CSF and plasma from Alzheimer disease (AD) patients and controls. CSF A beta38 and A beta40 levels were similar in AD patients and in controls; however, they were higher in controls with APOE upsilon4 allele than those without. CSF A beta42 levels were lower in AD patients than in controls. The CSF A beta38/42 ratio was higher in AD patients than controls, consistent with the previously reported higher A beta40/42 ratio. A beta38, A beta40 and A beta42 levels in plasma were similar in AD patients and in controls and showed no relationship to levels in CSF. Our findings suggest that the increased CSF A beta38/42 ratio found in AD patients is derived entirely from reduction of CSF A beta42 levels.