Evaluation of growth hormone assays using ratio plots

To permit comparison between growth hormone (GH) results obtained using the Pharmacia polyclonal assay and the Delfia monoclonal assay, we used both methods to measure GH concentrations in peak GH responses to the pyridostigmine-growth-hormone-releasing-hormone (PD-GHRH) test and in unstimulated samples from 40 healthy adults and 31 patients with suspected GH deficiency. Ratio plots were used for the comparison, and acceptability criteria were based on inherent analytical imprecision and on analytical quality specifications. The mean ratio (r; Pharmacia/Delfia) for the peak GH responses in 40 healthy adults was calculated to be 1.59, and the 95% prediction interval for ratios fulfilling the imprecision criterion was applied. For GH values >1 mIU/L, the peak and unstimulated GH ratios in healthy adults and patients were within the 95% prediction interval, and fulfilled the biological criterion as well. Therefore, the conversion factor of 1.59 is applicable for the evaluation of GH-stimulation tests.