Material and methods
Patients
From January 2014 to June 2014, we included 80 patients diagnosed with RA (62 females, 18 males) in the outpatient department of the Division of Rheumatology in Gyeongsang National University Hospital located in Jinju, Korea. RA diagnosis was based on the 1987 and/or 2010 American College of Rheumatology criteria/European League against Rheumatism collaboration. All patients were treated by the same group of rheumatologists.
A serum sample was obtained from each patient and was stored at − 80 ℃ until analysis. The institutional review board of the abovementioned hospital approved this study (2013-03-006), which conformed to the Declaration of Helsinki II. All patients were informed and gave their written consent.
Clinical data and measurement of inflammatory markers
All patients were followed up to obtain the following clinical data: age, sex, smoking history, ESR, C-reactive protein (CRP), rheumatoid factor (RF), anti-cyclic citrullinated peptide (anti-CCP) antibody, and the disease activity score in 28 joints (DAS28) with ESR/CRP (DAS28-ESR/CRP).
DAS28 was calculated as described in this link: http://www.4s-dawn.com/DAS28/. We then classified the patients into low-, moderate-, and high-disease-activity groups according to the DAS28 results (low, < 3.2; moderate, 3.2–5.1; high > 5.1) [12].
Measurement of IGF-1 and IGFBP3
Serum IGF-1 and IGFBP-3 levels were measured by enzyme-linked immunosorbent assay (ELISA) using a commercial kit (R&D systems) according to the manufacturer’s instructions.
Statistical analysis
All statistical data were analyzed using SPSS 21.0 (SPSS Inc., Chicago, IL, USA). In addition, p < 0.05 indicated a significant difference. Using the Kruskal–Wallis test, we compared the serum cytokine levels among the patient groups. Furthermore, the correlations of serum cytokine levels with the DAS28-ESR/CRP were investigated by Spearman’s rank correlation analysis.