So whats the difference between biochemical assays and histological ones. I am reading a paper and this is what the sentences says: "...the degree of dementia correlates much better with Abeta assayed biochemically than with histologically determined plaque counts"

 

So histologically means counting the plaques...but then what will the biochemical assay give me?

The precise procedure should be indicated in the paper. One assay I am familiar with amyloid beta deposits (if that is what you are referring to) involves the extraction in organic solvent and subsequent Western or MS identification/quantification. Histological assays are generally more involved and are more dependent on the expertise of the experimenter.