1. Supplemental Data Fig. 1. Results from commutability study for hGH. Plotted are the measurement results for clinical samples, hGH preparations, and control samples, as well as the regression line (in grey) and the 95% prediction interval (in red) for the clinical samples. Patient samples are shown as black circles (○), control samples in red, hGH reconstitution into MilliQ water is shown in blue, reconstitution in PBS in green, reconstitution in PBS-BSA in orange. hGH spiking into MilliQ water is represented by (●), into PBS (○), into PBS-BSA (□), into hGH depleted serum (■), into human charcoal stripped serum (♦), into SRM 971 male (◊) and into sheep serum (▲). [hGH] Roche - Cobas e411, µg/L [hGH] Siemens - IMMULITE 2000, µg/L [hGH] Mediagnost - hGH sensitive ELISA, µg/L [hGH] Siemens - IMMULITE 2000, µg/L [hGH] Diasorin - Liaison, µg/L [hGH] Siemens - IMMULITE 2000, µg/L [hGH] IDS - iSYS, µg/L [hGH] Siemens - IMMULITE 2000, µg/L[hGH] Roche - Cobas e411, µg/L [hGH] Mediagnost - hGH sensitive ELISA, µg/L [hGH] Roche - Cobas e411, µg/L [hGH] Diasorin - Liaison, µg/L [hGH] Roche - Cobas e411, µg/L [hGH] IDS - iSYS, µg/L [hGH] Mediagnost - hGH sensitive ELISA, µg/L [hGH] Diasorin - Liaison, µg/L [hGH] Mediagnost - hGH sensitive ELISA, µg/L [hGH] IDS - iSYS, µg/L [hGH] Diasorin - Liaison, µg/L [hGH] IDS - iSYS, µg/L ABC DEF GHI J

2. [hGH] Roche - Cobas e411, µg/L [hGH] Siemens - IMMULITE 2000, µg/L [hGH] Mediagnost - hGH sensitive ELISA, µg/L [hGH] Siemens - IMMULITE 2000, µg/L [hGH] Diasorin - Liaison, µg/L [hGH] Siemens - IMMULITE 2000, µg/L [hGH] IDS - iSYS, µg/L [hGH] Siemens - IMMULITE 2000, µg/L Supplemental Data Fig. 2. Results from recovery study for hGH. Plotted are the measurement results for clinical samples, hGH preparations, and control samples, as well as the regression line (in grey) and the 95% prediction interval (in red) for the clinical samples. Patient samples are shown as black circles (○), control samples in red, hGH reconstitution into PBS is shown in in green, reconstitution in PBS-BSA in orange. hGH spiking into PBS is represented by (○), into PBS-BSA (□), into hGH depleted serum (■), into human charcoal stripped serum (♦), into SRM 971 male (◊) and into sheep serum (▲). [hGH] Roche - Cobas e411, µg/L [hGH] Mediagnost - hGH sensitive ELISA, µg/L [hGH] Roche - Cobas e411, µg/L [hGH] Diasorin - Liaison, µg/L [hGH] Roche - Cobas e411, µg/L [hGH] IDS - iSYS, µg/L [hGH] Mediagnost - hGH sensitive ELISA, µg/L [hGH] Diasorin - Liaison, µg/L [hGH] Mediagnost - hGH sensitive ELISA, µg/L [hGH] IDS - iSYS, µg/L [hGH] Diasorin - Liaison, µg/L [hGH] IDS - iSYS, µg/L 123 456 789 10

3. Supplemental Data Fig. 3. Effect of increasing amount of GHBP on hGH detection by ELISA. GHBP was suplemented at 0, 10, 75, 250 and 500 ng/mL into clinical samples (black circles), and into hGH preparations (orange squares). The different spiked solutions were then measured for their content of hGH by ELISA (Mediagnost – hGH sensitive ELISA). Error bars represent the standard deviation.