The present invention relates to the field of cognitive function. More specifically, the present invention provides compositions and methods useful for assessing cognitive dysfunction/function in Alzheimer's disease and other diseases of cognition. In one embodiment, the method comprises the steps of (a) reducing heterocomplexes comprising NPTX1 and NPTX2 present in a biological sample obtained from the patient into NPTX1 and NPTX2 monomers; (b) covalently modifying the thiol groups of the NPTX1 and NPTX2 monomers to prevent re-formation of NPTX1/NPTX2 heterocomplexes; (c) detecting NPTX2 in the sample; and (d) assessing cognitive function in the patient by comparing NPTX2 detected in the sample to a control.