Comparability of adrenocorticotropic hormone measured in spray-dried K2-EDTA and spray-dried K3-EDTA tubes

INTRODUCTION: Blood samples for adrenocorticotropic hormone (ACTH) tests are collected in dipotassium or tripotassi-um ethylenediaminetetraacetic acid (K2-EDTA or K3-EDTA) tubes. The interchangeable use of spray-dried K2-EDTA and K3-EDTA tubes may occasionally occur in practice and is appropriate only if both tube types yield comparable analytical results. Therefore, this study aimed to evaluate the interchangeability of these tubes for ACTH measurement.
METHODS: Seventy-two volunteers were included in the study. From each participant, K2-EDTA and K3-EDTA blood samples were collected simultaneously. ACTH was measured by chemiluminescent immunoassay on a Maglumi X3 analyzer (SNIBE, Shenzhen, China). The Wilcoxon signed-rank test, Passing-Bablok regression, Mountain plot, and Bland-Altman analyses were used to evaluate differences between measurements.
RESULTS: The median ACTH concentration in K2-EDTA tubes was 35.1 pg/mL (10.35–107.16) [7.73 pmol/L (2.28–23.60)] and in K3-EDTA tubes was 32.96 pg/mL (9.85–99.90) [7.26 pmol/L (2.17–22.00)]. The Wilcoxon signed-rank test (z=-7.14, p<0.001) demonstrated a statistically significant difference between the results. In the Bland-Altman analysis, the mean difference between the two samples was determined to be 1.49 pg/mL (0.33 pmol/L). Lin’s concordance correlation coefficient was 0.989 (95% CI: 0.984–0.993), indicating good agreement between ACTH results obtained from the two tubes. Mountain plot analysis demonstrated that most differences clustered around zero, with an asymmetric right tail suggesting the presence of larger differences in a limited number of samples.
DISCUSSION AND CONCLUSION: ACTH concentrations measured in both tubes are proportional, and the results obtained from K2-ED-TA and K3-EDTA tubes support analytical comparability within the ACTH concentration range evaluated in this study. However, complete interchangeability between the results of the two tube types cannot be assumed in all clinical scenarios; further studies are needed in subjects with higher ACTH levels.

Keywords: Adrenocorticotropic hormone, bias, K2-EDTA, K3-EDTA, pre-analytical phase, test tube comparison