Intended use

The DIASTAT? Anti-Thyroid Peroxidase (anti-TPO) test is a quantitative/qualitative enzymelinked immunosorbent assay (ELISA) for the detection of the IgG class of autoantibodies specific for thyroid peroxidase in human serum or EDTA, lithium heparin, and sodium citrate plasma. It is intended to aid in the diagnosis of autoimmune thyroid disorders and is not definitive in isolation. Autoantibody levels represent one parameter in a multicriterion diagnostic process.

Background

Autoimmune thyroid disorders encompass autoimmune destruction and stimulation; both states are associated with predominantly IgG local and circulating thyroid autoantibodies.

The presence of anti-thyroglobulin (Tg) autoantibodies in patients with Hashimoto's thyroiditis was first demonstrated in 1956 subsequently it was shown that many patients with advanced thyroiditis had antibodies for a thyroid antigen distinct from Tg. This was termed thyroid microsomal antigen (TMA). Considerable evidence indicates that TMA is antigenically related to thyroid peroxidase (TPO), a membrane-bound glycoprotein enzyme with an approximate mass of 101kD, whose in vivo function is the iodination of tyrosine in the synthesis of the thyroid hormones T3 and T4. TMA and TPO may be identical moieties; cloning of human TPO gives further support to their close identity. TPO autoantibodies may play a pathogenic role in destructive autoimmune thyroid diseases as they can fix complement and consequently induce cytolysis. Autoimmune reactivity to TPO is believed to be polyclonal, with autoantibodies recognising a minimum of six distinct determinants. Anti-TPO antibodies are found, often in conjunction with anti-thyroglobulin autoantibodies, in the majority of Hashimoto's Thyroiditis, Graves’ disease and in cases of Primary Myxoedema. The relationship of autoimmune thyroid disease in pregnancy has been the subject of considerable interest, with the demonstration of TPO antibodies in most cases of post-partum thyroiditis and the association of thyroid autoantibodies with increased miscarriage risk. Anti-TPO antibodies are found in other non-thyroid conditions, e.g. pernicious anaemia, diabetes mellitus, rheumatoid arthritis, Addison’s disease and Sjogren’s Syndrome. In addition, anti-TPO antibodies are detectable at low levels in 2-8% of apparently healthy individuals, particularly in the elderly and more often in women than in men, although the clinical significance of this is unclear

Technical information

The wells of the microtitre strips are coated with recombinant human TPO (rTPO) During the first incubation, specific autoantibodies in diluted serum or plasma bind to the antigen-coated surface. The wells are then washed to remove unbound components. In the second incubation, the Conjugate, enzyme-labelled antibodies to human IgG, binds any surface-bound autoantibodies. After further washing, specific autoantibodies are traced by incubation with the Substrate. Addition of Stop Solution terminates the reaction, resulting in a coloured end-product. The amount of Conjugate bound is measured in absorbance units. In the qualitative protocol, the amount of Conjugate bound by the sample is compared with that bound by the Reference Control. In the quantitative protocol, the concentration of anti-TPO autoantibody can be estimated by interpolation from a dose-response curve based on Standards. The Standards are calibrated against NIBSC 66/387 thyroid microsomal antibody reference preparation.