Variable estimates of serum growth hormone concentrations by different radioassay systems

Many different assays are being used to measure serum GH concentrations in children with disorders of growth. We assessed four readily available methods to determine the comparability of the immunopotency estimates: standard double antibody RIA with pituitary standards from the National Hormone and Pituitary Program (assay 1) and from a commercial source (assay 2), a double antibody RIA with serum standards (assay 4), and a commercial immunoradiometric assay (assay 3). There was a high degree of relative correlation between assays (r = 0.95-0.98), but absolute potency estimates differed. Assays 1 and 2 were almost identical. Assay 3 yielded serum GH levels about 65% those of assay 1 or 2 and 80% those of assay 4. Assay 4 gave intermediate values between the low readings in assay 3 and higher values in assay 1 and 2. We conclude that substantial variation occurs in potency estimates in different GH assays. Such differences can affect the interpretation of many GH provocative and sampling studies.