The utility of the growth hormone (GH) releasing hormone-arginine test for diagnosing GH deficiency in adults with childhood acute lymphoblastic leukemia treated with cranial irradiation

Context: The insulin tolerance test ( ITT) is the current standard diagnostic test for the diagnosis of adult GH deficiency ( GHD), but alternative tests, such as the GHRH- arginine test, have been proposed. Objective: We investigated the sensitivity and specificity of the GHRH- arginine test using ITT as the gold standard in diagnosing GHD in a group of young adults treated with cranial irradiation ( CRT) for childhood acute lymphoblastic leukemia ( ALL). We estimated the positive and negative predictive values of the GHRH- arginine test among patients as well as a number of individual characteristics and therapy- related factors during both the GHRH- arginine test and ITT. Design: Forty- three young adults, treated for childhood ALL with 18 - 30 Gy CRT and chemotherapy, were studied, and comparison was made with matched controls. Results and Conclusions: We evaluated four different cutoff levels for GHD in the GHRH- arginine test: 5, 7.5, 9, and 16.5 mu g/ liter. Using 7.5 mu g/liter as the cutoff yielded high specificity ( 94%), but at the same time the sensitivity was only 66%, which leads to a low negative predictive value ( 27%). In contrast, a failed GH response to the GHRH- arginine test accurately reflects the presence of radiationinduced GHD, illustrated by a high positive predictive value ( 95% at 7.5 mu g/ liter). Only age at CRT and body mass index remained significant predictors of the peak GH during the GHRH- arginine test. Because a high proportion of GHD patients show a normal response to the GHRH- arginine test, it cannot be used reliably to exclude GHD in these patients. Complementary ITT is also warranted to confirm GHD in obese patients.