Clinical utility of a 22-kDa growth hormone-specific assay

Human growth hormone (hGH) circulates in different molecular forms, with the 22-kDa monomer being the predominant one and the 20-kDa variant corresponding to 5 to 15% of the serum hGH on a weight basis. Using monoclonal antibodies with different specificities we developed two immunoenzymometric assays, one with 22 + 20-kDa specificity and the other specific only for the 22-kDa form. Both assays used microtiter plates as solid phase and streptavidin-peroxidase for color development; intra-assay CV was less than 10% in the range of 1 to 100 mIU/l for the 22 + 20-kDa assay and in the range of 3 to 100 for the 22-kDa assay, with an inter-assay CV of less than 14% for both assays; sensitivity was 0.2 mIU/l for the 22 + 20-kDa assay and 0.5 mIU/l for the 22-kDa assay. The two assays were compared by measuring 200 serum samples with detectable hGH levels by both assays. Higher values were obtained with the 22 + 20-kDa assay (62.1 +/- 5.9 vs 59.2 +/- 6.1 mIU/l, mean +/- SD) with a correlation coefficient (r) of 0.99. In no clinical condition (28 patients with growth retardation and 14 acromegalics) did the two assays give discrepant values. We conclude that there was no practical advantage in using an assay with specificity restricted to the 22-kDa form for measuring hGH in clinical serum samples.