Swader: переводы

→The specificity provided by monoclonal antibodies has introduced additional problems for the analyst. Many monoclonal antibodies have patents associated with them. Whilst different companies may claim that they measure a particular hormone, different assays will pick up different parts of the molecule which may lead to different results between methods. Tumour markers and troponin I are good examples where different monoclonal antibodies can produce different results making standardisation difficult. The use of specific monoclonal antibodies compounds the variations already present due to molecular heterogeneity.

→Antibody specificity

→Many of the hormones of biological interest, e.g. hormones like Thyroid Stimulating Hormone (TSH) and human Chorionic Gonadotropin (hCG) have been defined in terms of their biological function rather than their molecular structure and have subsequently been found to be molecularly heterogeneous [5]. Measurement of a mixture of substances of different molecular structure (and function) is difficult to standardise and difficult to interpret. Molecular heterogeneity will affect antibody recognition in immunoassays and cause problems with the appropriate choice of standard. This predominantly applies to the peptide hormones.

→Molecular heterogeneity

→The problem for the clinical chemist is that heterogeneity of the substance being measured is only one of several significant criticisms of current immunoassays.

→Concerns were raised by Roger Ekins in 1990 in his paper “Is Immunoassay Standardisation Possible?” He argued that “Insofar as the antigenic substances present in standards or test samples are dissimilar and/or molecularly heterogeneous, an immunoassay is invalid and the results it yields have no universal significance. Attempts to standardise “analytically invalid” immunoassays inevitably fail” [5].

→Limitations of immunoassays

→It is easy to look back at these tests and ask whether they were fit for purpose. It was unclear exactly what they were measuring, what standards they were using, what their cut-offs were, the impact of matrix effects and the reproducibility of different tests. None of these historical assays could be classified as standardised and this lack of standardisation led to further developments in the field of hormone measurement. When immunoassays were introduced in the 1970s it was thought that assays with the specificity and reproducibility required to measure almost all hormones had finally arrived. Since then there have been significant developments in immunoassays such as in immunoassay design, the introduction of monoclonal antibodies, and non-isotopic signalling. Most hormone assays used today rely on the principles introduced 30-40 years ago and are based on the recognition of antigenic substances using antibodies. However, if we compare current assays to historical assays, it is clear that they suffer from many of the same limitations as early assays.

→References

→Preston, UK

→The authors

→Clinical Biochemistry

→The measurement of hormones is an art that has been practised by clinical chemists for many thousands of years. One of the earliest recorded “hormone” assays is described in the Berlin Medical Papyrus where, in an attempt to determine whether a woman was pregnant, the clinical chemists of the time added the patient’s urine to a bag containing wheat and barley seeds. If the seeds germinated the woman was pregnant. If the barley seeds germinated first then this would imply that the unborn child was male, if the wheat seeds germinated first then this would indicate that the woman would give birth to a female child [1, 2, 3]. The use of urine in assessing a patient’s clinical state has continued since then and uromancy was common in medieval Europe, as described in 1637 by Thomas Bryant in his critical pamphlet “the Pisse Prophets”. In an attempt to assess pregnancy one test involved the mixing of wine with urine and observing the results [3, 4]. Over the centuries pregnancy testing became more sophisticated and in 1928 German scientists Aschheim and Zondek developed the first bioassay for hCG in urine. This involved injecting a woman’s urine into an immature rat and looking for an estrous reaction. This bioassay was modified with the use of rabbits, while African clawed toads were used in the Hogben test [1, 4].

→By Dr Martin Myers and Rebecca Allcock

→Whilst immunoassays have improved the clinical diagnostic service in the assessment of hormone disorders, differences in standardisation between assays can cause uncertainties in the clinical use of the results. There is therefore an urgent need to harmonise assays to ensure that evidence-based diagnostic cut-offs can be used. Due to the differences between immunoassays, harmonisation of results can only begin to occur if consensus is reached on assay characteristics.