Quantification of day-to-day variability in growth hormone levels in acromegaly

Growth hormone (GH) measurements are routinely used for important treatment decisions in patients with acromegaly, yet their reliability is affected by numerous factors including assay precision and variability, sampling intensity, and hormone pulsatility. The day-to-day variation in GH in acromegaly has not been studied. This study quantified the magnitude of day-to-day GH variability in patients with acromegaly by performing an analysis of previously obtained plasma GH profiles. The analysis was performed at the Michigan Clinical Research Unit at the University of Michigan. A total of nine 48 h Q10 min GH profiles obtained in nine patients with active acromegaly were examined. The study was planned after data collection and analysis was conducted using Altman-Bland methods. Day 1 vs. Day 2 values were examined. 95% confidence intervals of the D2 vs. D1 ratios were calculated on all individual subject data as well as on a single 0800 h GH sample and composite mean data for 2-, 5-, 9-, and 24-h sampling protocols. Confidence interval range was 0.66-1.50 for the 0800 h sample and was similar for all sampling protocols except somewhat more narrow for the 24-h sampling (0.75-1.32). Daily variations in GH levels introduce an additional confounding element when using a single GH level or even daily GH curves to assess a patient's GH milieu. It may have an impact on result interpretation and subsequent treatment decisions especially when GH results are considered borderline.