Abstract: Provided herein are methods for determining the level of muscarinic acetylcholine receptor subtype-1 (M1 receptor) anticholinergic activity in a blood serum sample. The methods include radioactive methods and non-radioactive methods. The method comprises the steps of removing protein from the blood serum sample by treatment with perchloric acid (PCA) to produce a PCA-treated serum sample, incubating the PCA-treated serum sample with a membrane preparation from cultured cells expressing the M1 receptor and an M1 receptor ligand; detecting an amount of binding of the M1 receptor ligand to the M1 receptor and comparing the amount of binding to a standard to determine the level of M1 receptor anticholinergic activity in the blood serum sample. Alternatively, the method may comprise loading calcium sensitive dye into the cells and testing serum from a subject in a fluorescence assay to determine the anticholinergic activity relative to a sample known to contain little to no anticholinergic activity.