Gold standard test for beta-HCG levels? (AIPG 2011)
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a. Gold standard test for beta-HCG levels is Radioimmunoassay.
b. hCG is composed of ↑and ↓subunits and can be produced as intact hormone, which is biologically active, or as uncombined biologically inert subunits. Ectopic production of intact hCG occurs most often in association with testicular embryonal tumors, germ cell tumors, ex- tragonadal germinomas, lung cancer, hepatoma, and pancreatic islet tumors.
c. Eutopic production of hCG occurs with trophoblastic malig- nancies. Low levels of hCG or its uncombined ↑ or ↓subunits have been reported in a wide array of tumors. hCG ↑subunit production is particularly common in lung cancer and pancreatic islet cancer. In men, high hCG levels stimulate steroidogenesis and aromatase activity in testicular Leydig cells, resulting in increased estrogen production and the development of gynecomastia.
d. Precocious puberty in boys or gynecomastia in men should prompt measurement of hCG and con- sideration of a testicular tumor or another source of ectopic hCG pro- duction. Most women are asymptomatiC. hCG is easily measured.
e. Treatment should be directed at the underlying malignancy.
Radioimmuno- assays are the most important diagnostic tool in endocrinology, as they allow sensitive, specific, and quantitative determination of steady-state and dynamic changes in hormone concentra- tions.
f. Radioimmunoassays use antibodies to detect specific hor- mones.
g. For many peptide hormones, these measurements are now configured to use two different antibodies to increase bind- ing affinity and specificity.
h. There are many variations of these assays; a common format involves using one antibody to cap- ture the antigen (hormone) onto an immobilized surface and a second antibody, coupled to a chemilumenescent (ICMA) or ra- dioactive (IRMA) signal to detect the antigen.
i. These assays are sensitive enough to detect plasma hormone concentrations in the picomolar to nanomolar range, and they can readily distin- guish structurally related proteins, such as PTH from PTHrP.
j. A variety of other techniques are used to measure specific hor- mones, including mass spectroscopy, various forms of chroma- tography, and enzymatic methods; bioassays are now rarely used.
k. Most hormone measurements are based on plasma or serum samples. However, urinary hormone determinations remain use- ful for the evaluation of some conditions.