Determination of IGF-I, IGF-II, IGFBP-2, and IGFBP-3 levels in serum and plasma: comparisons using the Bland-Altman method.

The measurement of circulating insulin-like growth factors (IGFs) and IGF binding proteins (IGFBPs) have significant implications in the risk assessment of various diseases (e.g. cancer) and growth abnormalities. It is often assumed that values measured in serum and plasma are interchangeable. This study challenges this assumption by comparing determinants using the Bland-Altman method. Blood was obtained from 47 healthy volunteers (age 21-72 years) in serum, heparin plasma and EDTA plasma, and IGF-I, IGF-II, IGFBP-2, and IGFBP-3 measured, and results compared. Mean values for IGF-I, IGF-II, IGFBP-2 and IGFBP-3 determined in all three media were generally comparable; correlations were generally strong and significant (P<0.001). However, the Bland-Altman plots revealed significant lack of agreement for many analytes measured in EDTA plasma compared with serum and heparin plasma. Additionally, the ranges of the limits of agreement were consistently greater for EDTA plasma compared with the other two methods. These findings emphasize the need to standardize methods of collecting blood samples in future epidemiological studies and trials.