Objective: Patients with traumatic brain injury (TBI) are at moderate risk of GH deficiency (GHD), requiring a diagnostic test with high specificity. The GHRH + arginine (GHRH + ARG) test has been recommended as a reliable alternative to the insulin-tolerance test (ITT) as a standard test with a cutoff level of 9 ng/ml. However, it has recently been questioned for its low specificity in obese subjects, and now BMI-dependent cut-off levels are available. In this study, we compared the ITT and GHRH + ARG test in patients with TBI.
Design: A cross-sectional study
Methods: We performed an ITT and a GHRH + ARG test in 21 patients with TBI (6 women, 15 men; mean age 40.2 ± 12.1 years; BMI 30.7 ± 6.2). The number of patients classified discordantly as GH deficient by the ITT and the GHRH + ARG test with both classical and BMI-dependent cut-off levels was assessed.
Results: Using the GHRH + ARG test with the classical cut-off (≤ 9 ng/ml), we identified 12 patients as GH deficient who had a normal GH response to ITT (> 3 ng/ml), and one patient as GH sufficient who had a blunted GH response to ITT (discordance rate 61.9%). All patients discordantly classified as GH deficient by the GHRH + ARG test had a BMI of ≥ 28. With the BMI-dependent cut-offs (4.2, 8.0, and 11.5 ng/ml in obese, overweight, and lean subjects respectively), only 3 of the 21 patients were discordantly classified (discordance rate 14.3%).
Conclusions: Our results discourage the use of a cut-off level of 9 ng/ml for the GHRH + ARG test in obese subjects. The diagnostic reliability of this test is improved with the BMI-dependent cut-offs.