Objective: No definitive peripheral biological marker has yet been found to correlate with Alzheimer's disease (AD). This study investigated the role of macrocytosis as a marker which could significantly improve our understanding of AD in the Down's syndrome (DS) population.

Methods: This study investigated the possible association between raised mean corpuscular volume (MCV) and AD in 150 adults with DS over a five year study period.

Results: A raised MCV is common in the DS population. A statistically significant association between a further elevated MCV and clinically diagnosed AD in adults with DS was found.

Conclusions: The findings suggest that further research into the association between macrocytosis and DS could improve our understanding of the underlying biological mechanisms of AD. The measurement of MCV should be undertaken in all DS individuals with a diagnosis of possible or definite dementia. A significant raised MCV (above 97fl for males and 99fl for females) may further improve the clinical diagnosis. A single MCV measurement, however, cannot be recommended as a diagnostic test for AD in adults with DS.