A method for the differential diagnosis and sub-classification of Alzheimer's disease into different disease stages by direct analysis of the secondary structure distribution of a soluble Amyloid-beta (Aβ) peptide fraction and a soluble Tau protein fraction in bodily fluids, comprising the steps (a) conducting, in a first IR cell comprising a first infrared sensor element having an internal reflection element with a core of an infrared transparent material and at least one receptor for the Aβ peptide directly grafted to at least one surface of said core, said at least one receptor for the Aβ peptide being antibodies capable of specific and conformationally independent binding to the Aβ peptide, at least one flux of a body fluid with soluble Aβ peptide, submitting an IR beam through said first IR cell, and obtaining an infrared spectrum therefrom; (b) conducting, in a second IR cell comprising a second infrared sensor element having an internal reflection element with a core of an infrared transparent material and at least one receptor for the Tau protein directly grafted to at least one surface of said core, said at least one receptor for the Tau protein being antibodies capable of specific and conformationally independent binding to the Tau protein, at least one flux of a body fluid with soluble Tau protein, submitting an IR beam through said second IR cell, and obtaining an infrared spectrum therefrom; and (c) analyzing the obtained infrared spectra to determine the secondary structure distribution of the soluble Aβ peptide and of the soluble Tau protein in the bodily fluids for the differential diagnosis, wherein a downshift of the amide I band of the Aβ peptide and of the Tau protein is indicative for the disease stage. (a) conducting, in a first IR cell comprising a first infrared sensor element having an internal reflection element with a core of an infrared transparent material and at least one receptor for the Aβ peptide directly grafted to at least one surface of said core, said at least one receptor for the Aβ peptide being antibodies capable of specific and conformationally independent binding to the Aβ peptide, at least one flux of a body fluid with soluble Aβ peptide, submitting an IR beam through said first IR cell, and obtaining an infrared spectrum therefrom; (b) conducting, in a second IR cell comprising a second infrared sensor element having an internal reflection element with a core of an infrared transparent material and at least one receptor for the Tau protein directly grafted to at least one surface of said core, said at least one receptor for the Tau protein being antibodies capable of specific and conformationally independent binding to the Tau protein, at least one flux of a body fluid with soluble Tau protein, submitting an IR beam through said second IR cell, and obtaining an infrared spectrum therefrom; and (c) analyzing the obtained infrared spectra to determine the secondary structure distribution of the soluble Aβ peptide and of the soluble Tau protein in the bodily fluids for the differential diagnosis, wherein a downshift of the amide I band of the Aβ peptide and of the Tau protein is indicative for the disease stage.