The challenges associated with performing surface plasmon resonance (SPR) based measurements in serum and other biofluids have continued to limit the applicability of this valuable sensing technology for sensitive bioaffinity measurements of proteins in clinically relevant samples. In this paper, a new sandwich assay is introduced for the quantitative SPR analysis of α-1 antitrypsin (AAT), which is a recognized biomarker for Alzheimer's disease. Detection was performed via the specific adsorption of AAT onto a gold chip surface modified with a DNA aptamer. The measurement dynamic range and also sensitivity in serum were improved with the subsequent surface binding of antiAAT. A methodology was established to measure the target protein in serum, albumin and immunoglobulin G (IgG) solutions with the results correlated with measurements in buffer only. A comparison between SPR and enzyme-linked immunosorbent assay (ELISA) measurements was also made. The detection of AAT in serum at clinically relevant concentrations was demonstrated with target concentrations as low as 10 fM readily achievable.