Methods include determining in a sample an amount of a first isomeric analyte and a second isomeric analyte. A first measurement value and a second measurement value are determined. The first measurement value represents a total amount of the first isomeric analyte and the second isomeric analyte. The second measurement value represents an amount of the second isomeric analyte only. The second measurement value is subtracted from the first measurement value to obtain a resulting value and the resulting value is equated to an amount of the first isomeric analyte in the sample.