Methods

Systematic data were obtained for every patient using a standardized questionnaire, which included traditional cardiovascular risk factors such as age, gender, body mass index (BMI), smoking status, hypertension (defined as a blood pressure ≥ 140/90 mm Hg in non-diabetics, ≥ 130/80 mm Hg in diabetics or use of anti-hypertensive medication), diabetes mellitus (defined as a plasma fasting glucose > 126 mg/dl or use of anti-diabetic medication), lipid profile (cholesterol, triglycerides) and non-traditional cardiovascular risk factors – CKD (etiology, eGFR, proteinuria, albuminuria), calcium-phosphate metabolism parameters (serum parathyroid hormone, 25(OH)D, total calcium, phosphate, total alkaline phosphatase), inflammation (CRP, serum albumin), and serum uric acid.

Serum vitamin D was measured in fresh blood samples using a 25(OH)D electrochemiluminescence immunoassay. Patients were categorized into two groups based on the commonly used cut-off value of 15 ng/ml: vitamin D deficient or sufficient [12]. To convert nanograms per milliliter to nanomolar values, one should multiply by 2.496.

The severity of cardiovascular damage was assessed by measurements of intima-media thickness (IMT), abdominal aortic calcifications score (AAC), ankle-brachial index (ABI) – as markers of atherosclerotic disease – cardio-ankle vascular index (CAVI) as a marker of arterial stiffness, and interventricular septum thickness (IVS) as an index of left ventricular hypertrophy.

Intima-media thickness was determined by B-mode ultrasonography of the carotid artery with a transducer frequency of 7 MHz. Up to 4 cm of the common carotid artery, the carotid bifurcation and the internal carotid 2 cm distally from the bifurcation were scanned bilaterally using longitudinal and transverse sections. Intima-media thickness was defined as the distance between the leading edge of the first echogenic line (lumen-intima interface) and the second echogenic line (media-adventitia interface) in plaque-free arterial segments. Abdominal aortic calcifications score was evaluated on a lateral lumbar X-ray (acquired in the standing position), as described by Kauppila et al. [13]. Ankle-brachial index and CAVI were measured with the subjects in supine position at rest for at least 10 minutes by trained technicians using the VaSera VS-1000 screening device (Fukuda Denshi, Tokyo, Japan) as described by the manufacturer. Interventricular septum thickness thickness was assessed in diastole using two-dimensional M-mode echocardiography with a 2.5 MHz transducer. All measurements were performed under blind conditions.