Double isotope dilution surface-enhanced Raman scattering (double IDSERS) is qualified as a method for accurate and precise determination of biomarkers in human blood serum. Providing a full evaluation of the measurement uncertainty as well as traceability to a reference material sets the procedure in line with the requirements of a primary ratio method. Data evaluation is based on a partial least squares (PLS) model, whose prediction ability is validated from quantifying the uric acid concentration in both an artificial reference solution and a real human blood serum sample. With the proposed approach, the uric acid serum concentration can be determined with an uncertainty of 1.6% at a confidence level of 95%.