Myxovirus protein A (MxA) is a biomarker that can be used to distinguish between viral and bacterial infections. While MxA lateral flow assays (LFAs) have been successfully used for viral vs. bacterial differential diagnosis for children, the clinically relevant level of MxA for adults has been reported to be 100 times lower, which is too low for traditional LFAs. We present results applying the use of surface enhanced Raman spectroscopy (SERS) to detect MxA. AuAg nanoshells (AuAg NSs) were used to enhance the Raman signal of mercaptobenzoic acid (4-MBA), enabling readout by SERS. The AuAg NSs were conjugated to antibodies for the biomarker of interest, resulting in a "nanotag", that could be used in a dipstick immunoassay for detection. We first optimized the nanotag parameters using anti-human IgG/human IgG as a model antibody/antigen system, and then demonstrated detection of MxA using anti-MxA antibodies. We show that SERS readout of immunoassays for MxA can quantify MxA levels at clinically relevant levels for adult viral infection.