A4 quantifies aggregated Aβ in plasma and brains of Alzheimer disease mouse models and in cell culture media. A4 incorporates a concentration matrix that captures soluble and insoluble Aβ aggregates at the onset of aggregation. The immunoassay component quantifies Aβ with femtogram sensitivity, thereby allowing researchers to detect Aβ aggregation quantitatively and sensitively several months before plaque formation.