New Immunoelectrochemiluminometric Assay to Measure Serum Thyrotropin,

Abstract

The log/linear relation between serum thyrotropin (TSH) and free thyroxine (FT4) makes TSH, when measured by a highly sensitive method, a more sensitive indicator of thyroid disfunction than FT4 (1). Nonisotopic immunometric assays are the methods of choice to measure serum TSH, as they are at least equally sensitive to IRMAs, can be automated, are generally faster and more convenient than IRMAs, and do not use radioisotopes. Within the former group of methods, the immunochemiluminometric assay is increasingly popular because of its inherent sensitivity, but instrumental and triggering modes may jeopardize it. A new immunoelectrochemiluminometric assay (IECMA) has recently come on the market (2). It is based on electrogenerated chemiluminescence that, aiming to avoid the instability of luminophores, does not proceed in free solution but on to the surface of a platinum electrode (3)(4). We describe the evaluation of such a method using the random access analyzer Elecsys® 2010 (Boehringer Mannheim Diagnostics). The method consists of two immunological “sandwich” reactions linking, at one side, sample TSH to a solid phase of paramagnetic streptavidin-coated microparticles by means of a biotinylated monoclonal TSH-specific antibody and, at the other side, a Ru-labeled TSH-specific antibody. Separation of bound and unbound TSH is made by a magnet, and luminescence from a luminophore is triggered electrochemically. The immunological reaction consists of a first incubation where 50 μL of sample, the biotinylated antibody, and the Ru-labeled antibody react for 9 min, when antibodies capture the TSH present in the sample to form a sandwich complex. In a second step, streptavidin-coated paramagnetic microparticles are added, and during a second 9-min incubation time the biotinylated antibody attaches to the streptavidin-coated surface of the microparticles. The reaction mixture …