i) eluting a dried sample of physiological fluid in a diluent to form a liquid sample, wherein the physiological fluid obtained is from a human subject, and wherein the diluent comprises one or more immunoglobulins, a detergent, urea, and one or more blocking agents;

ii) contacting at least a portion of the liquid sample with a first capture antibody capable of binding tau protein (Tau);

iii) contacting the mixture of step ii) with a plurality of detectable agents, wherein the plurality of detectable agents comprises a first detectable agent comprising a biotinylated first detection antibody capable of binding Tau, and wherein the contacting comprises the formation of an immunocomplex of first capture antibody-Tau-first detection antibody;

iv) washing the mixture of step iii);

v) contacting the combination of step iv) with a streptavidin-bound label, wherein the streptavidin-bound label interacts with a biotin of the biotinylated first detection antibody of the immunocomplex;

vi) washing the mixture of step v);

vii) generating a detectable signal from the streptavidin-bound label of step vi);

viii) detecting the detectable signal of step vii), wherein the limit of detection (LOD) for Tau is less than about 250 pg/mL and/or the limit of quantitation (LOQ) for Tau is less than about 250 pg/mL; and

ix) obtaining a concentration of the Tau in the liquid sample based upon the detectable signal.