Abstract: A method of detecting subclinical ketosis in a dairy animal comprises the steps of: obtaining a sample from the dairy animal, the sample containing one of a serum sample or a milk sample; incubating a mixture of the sample and a reactant containing NAD+-functionalized quantum dots to produce an incubated mixture; irradiating the incubated mixture with optical radiation of a first wavelength so as to emit fluorescent radiation of a second wavelength from the incubated mixture; irradiating a photodetector with the fluorescent radiation so as to generate an electrical signal; and performing data acquisition using the photodetector electrical signal to determine a BHBA concentration for the sample.