Diagnosis of Growth Hormone Deficiency in Adults: Provocative Testing with GHRP6 in Comparison to the Insulin Tolerance Test

The aim of the study was to evaluate the clinical applicability of growth hormone releasing peptide 6 (GHRP6) for the diagnosis of GH deficiency in adults. Forty-nine patients with suspected hypothalamic or pituitary disease underwent both ITT and GHRP6 (1 mu g/kg) testing. In addition, 20 healthy controls were tested by GHRP6 only. Blood samples were analyzed for GH levels. Thirty patients had a GH peak response of less than 3 mu g/l during ITT and were considered growth hormone deficient (GHD). For the GHRP6 test, the GH mean peak was 3.0 mu g/l (+/- 0.8, 0.5-20.9) in the GHD group vs. 14.8 mu g/l (+/- 4.7, 1.8-95.3) in the growth hormone sufficient (GHS) group. Receiver operating characteristics (ROC) analysis suggested an optimal peak GH cut-point of 3.5 mu g/l with 80% sensitivity and 95% specificity. Applying upper (11.3 mu g/l) and lower (3.5 mu g/l) cutoffs with high specificities established the diagnosis in nearly two third of the patients. During administration of GHRP6 no side effects were observed. GHRP6 alone as a provocative test is highly specific, but with limited sensitivity for the diagnosis of GH deficiency in adults. Using upper and lower cutoffs, further testing by ITT may be necessary in only one-third of patients.