Sandwich ELISA is the most frequently used immunoassay format for the detection of high molecular-weight antigens (e.g. proteins and peptides). An antibody pair of the capture antibody (immobilized) and detection antibody (in solution) forms a “sandwich“ with a target antigen in between. The main features of those immunoassays are high sensitivity and specificity. The use of a biotin-labeled detection antibody in combination with a streptavidin-peroxidase conjugate is a way how to improve the ELISA sensitivity reaching as low detection limit as sub pg/ml level.