Study design
The CDCS is a national, multicenter, non-interventional, 24 month prospective cohort survey in CKD patients on dialysis. The study includes a baseline visit and 4 other visits at 6 month intervals. The estimated enrollment period is 10 months. It is planned to consecutively recruit 1520 patients with ESRD receiving stable hemodialysis (HD) or peritoneal dialysis (PD) for at least 6 months in 24 dialysis centers in China. Participating centers will be selected based on logistics constraints. Consecutive patients who fulfill the inclusion and exclusion criteria and are willing to participate will be included in the study (Table
1). Written informed consents were obtained from all patients before participation. Each site is expected to enroll approximately 30 patients. After that, enrollment will be competitive at all sites. This study will be able to mirror real life management of these patients. All statistical analyses will be performed using SAS 9.2 software (SAS Institute, Cary, NC, USA). All hypothesis tests will use two-sided tests with a significance level of 0.05.
Table 1
Inclusion and exclusion criteria of the CDCS
Male or female patients ≥18 years old | Patients’ life expectancy <6 months |
Patients with ESRD receiving stable HD or PD for at least 6 months | Patients with acute kidney injury, active inflammatory diseases, parathyroidectomy or evident malignancies |
Patient or legally accepted representative willing to sign Data Release Consent Form. | Patients with conditions making arterial calcification measurements technically impossible or unreliable, such as cardiac arrhythmias, amputations or severe peripheral vascular lesions |
Concomitant diseases that affect calcium status and soft tissue calcifications (sarcoidosis, multiple myeloma, HIV, amyloidosis) |
Pregnant or lactating women or women planning to become pregnant in the 6 months following entry into the study |
CKD is defined as abnormalities of kidney structure or function, present for >3 months, with implications for health, according to Kidney Disease: Improving Global Outcomes (KDIGO) 2012 Clinical Practice Guideline for the Evaluation and Management of CKD (Table
2) [
19].
Table 2
Criteria for CKD (either of the following present for >3 months)
Markers of kidney damage (one or more) | Albuminuria (AER ≧30 mg/24 h; ACR ≧30 mg/g) |
Urine sediment abnormalities |
Electrolyte and other abnormalities due to tubular disorders |
Abnormalities detected by histology |
Structural abnormalities detected by imaging |
History of kidney transplantation |
Decreased GFR | GFR <60 ml/min/1.73 m2 (GFR categories G3a–G5) |
Data collection
Visit 1 is at baseline/study entry and the follow-up includes 4 visits at 6 ± 1 month, 12 ± 1 month, 18 ± 1 months and 24 ± 1 month, respectively. The flow chart of detailed data and sample collection can be found in Table
3. At baseline, eligible patients will be evaluated for demographic information, smoking, alcohol drinking status, biochemical data, dialysis variables, concomitant medications, and personal and family disease history. Physical measurements including weight, height, body mass index (BMI) and vital signs (blood pressure and heart rate) will be obtained. Older age (male >55 years, female >65 years), arterial hypertension, family history (mother, father, brother, sister, children) of premature cardiovascular disease (≤55 years male, ≤65 years female), smoking, impaired glucose tolerance and/or impaired fasting glucose, dyslipidemia, and obesity are considered potent cardiovascular risk factors. The baseline biochemical data will be obtained using local routine laboratory methods performed within a week at the start of the study. If laboratory tests of serum 25-hydroxyvitamin D (25-OH Vit D) and FGF23 cannot be conducted in some centers, blood samples for testing 25-OH Vit D and FGF23 will be sent by the site study staff to those centers that can perform the assay. Blood sample collection will be performed according to the laboratory manual. Results from routine samples defined as samples taken within a week at the visit. Serum phosphorus, total calcium and iPTH will be tested at each visit. Target level of serum phosphorus, total calcium and iPTH in CKD stage 5 are defined according to Kidney Disease Outcomes Quality Initiative (K/DOQI) and KDIGO guidelines, respectively (Table
4) [
20,
21]. Total serum calcium is adjusted if serum albumin is <4g/dL to better reflect the free calcium, using the formula proposed by the US Bone Metabolism Association: corrected total calcium (mg/dL) = total calcium (mg/dL) + 0.8 × [4.0 − serum albumin (g/dL)]. Date, time, primary reason and characteristics of death with autopsy results if applicable and any supportive documents that relate to fatal and nonfatal CV events are defined as clinical outcomes, and will be collected during the follow-up.
Table 3
Study flow chart of the CDCS
Data release consent form | ● | | | | |
Demography data | ● | | | | |
Inclusion/exclusion criteria | ● | | | | |
Physical measurements (vital signs) | ● | | | | |
Smoking and alcohol drink status | ● | | | | |
Cardiovascular risk factors | ● | | | | |
Disease history | ● | | | | |
Concomitant medications | ● | ● | ● | ● | ● |
Hemoglobin, hematocrit | ● | | | | |
Serum total calcium, phosphorus, iPTH | ● | ● | ● | ● | ● |
25-OH Vit D, FGF23 | ● | | ● | | ● |
Glucose, HDL-C, LDL-C, TC and TG | ● | | | | |
hs-CRP | ● | | | | |
Serum albumin, ferritin, alkaline phosphatase | ● | | | | |
Urea, uric acid, creatinine | ● | | | | |
ABI | ● | | | | |
Kt/V, dialysate calcium concentration | ● | | | | |
Dialysis vintage, dialysis frequency and modality, membrane type | ● | | | | |
EBCT or MDCT scan | ● | | | | ● |
Plain lateral lumbar radiograph | ● | | | | ● |
Echocardiography, LVMI/LVH | ● | | | | ● |
Clinical outcomes | | ● | ● | ● | ● |
Table 4
Target level of serum phosphorus, calcium and iPTH in CKD stage 5
Serum phosphorus | 3.5–5.5 mg/dL (1.13–1.78 mmol/L) | Toward Normal [2.5–4.5 mg/dL (0.81–1.45 mmol/L)] |
Corrected total calcium | 8.4–9.5 mg/dL (2.10–2.38 mmol/L) | Maintain Normal [Healthy: 8.5–10.5 mg/dL (2.13–2.63 mmol/L)] |
Serum iPTH | 150–300 pg/mL (16.5–33.0 pmmol/L) | 2–9 times the upper normal limit (65 pg/mL, 6.9 pmmol/L) |
Imaging methods
VC images will be taken within a week of enrolment and the final visit of the study. Coronary artery calcification (CAC), abdominal aortic calcification (AAC) and cardiac valve calcification will be assessed by electron beam computed tomography (EBCT) or multi-detector computed tomography (MDCT), plain lateral lumbar radiograph and echocardiography, respectively. In addition, results of left ventricular mass index (LVMI) and left ventricular hypertrophy (LVH) through echocardiography will be taken within a week of enrolment and the final visit of the study as well.
CAC is measured by EBCT or MDCT as originally described by Agatston et al. [
22]. Thirty to forty contiguous tomographic slices will be obtained at 3-mm intervals beginning 1 cm below the carina and progressing caudally to include the entire coronary tree. An attenuation threshold of 130 Hounsfield units (HU) and a minimum of 3 contiguous pixels will be used for identification of a calcific lesion. The density factor is assigned in the following manner: 1 for lesions with peak attenuation of 130–199 HU, 2 for lesions with peak attenuation of 200–299 HU, 3 for lesions with peak attenuation of 300–399 HU, and 4 for lesions with peak attenuation >400 HU [
22]. Total CACS is determined by summing individual lesion scores from each of four anatomic sites (left main, left anterior descending, left circumflex, and right coronary artery). The CACS will be divided into approximate tertiles (0, <100, ≥100, ≥400 and ≥1000). CAC progression is evaluated as a continuous variable by calculating the difference in Agatston score between the two CT scans divided by the time between imaging. The 2-year progression of CAC is categorized as follows: absent (≤25th percentile), moderate (25th-75th percentiles), and accelerated (≥75th percentile).
AAC is assessed by semiquantitative scoring of a plain lateral lumbar radiograph using previously validated 24-point aortic calcification scale as originally described by Kauppila et al. [
23]. Midpoint of the intervertebral space above and below the vertebrae will be used to assess calcific deposits in the aorta adjacent to each lumbar vertebra (L1-L4), and lesion grading is based on a 0–3 scale: 0, no aortic calcific deposits; 1, small scattered calcific deposits (<1/3 of the longitudinal wall of the aorta); 2, calcific deposits ≥1/3 and <2/3; 3, calcific deposits ≥2/3 [
23]. The scores, obtained separately for the anterior and posterior wall, result in a range from 0 to 6 for each vertebral level and 0 to 24 for the total AAC score. The AAC will be divided into approximate tertiles (0, <5, and ≥5, ≥16).
Cardiac valve calcification is measured by a standard two-dimensional M-mode color Doppler echocardiography. Cardiac valve calcification will be considered present when bright echoes of more than 1-mm thickness are seen on 1 or more cusps of the aortic valve, mitral valve, or mitral annulus. Thereafter, patients are divided into 3 groups according to number of calcified valves: patients without valve calcification, patients with a single (aortic or mitral) valve calcification, and patients with calcifications in both valves.
LVMI is calculated with echocardiography and normalized to body height as an index in g/m2. LVH is diagnosed when LVMI is >125 g/m2 in males or >120 g/m2 in females.
CCI
Points of CCI are assigned for patients age (60–69 and 70 years, 1 and 2 points, respectively), dialysis vintage 2 years (1 point), aortic and mitral valve calcification (3 and 1 points, respectively), and abdominal aorta X-ray scores of 1–6 and 7 (2 and 4 points, respectively). Race, sex and pulse pressure do not contribute to the CCI. Therefore, the CCI ranges from 0 to 11 points.
Demographic and baseline data analysis
Categorical data will be presented in contingency tables along with frequencies and percentages. Continuous data will be summarized with frequency (n), mean, standard deviation, median, 1st quartile, 3rd quartile, minimum and maximum. Descriptive analyses will be presented by groups (with VC vs. without VC, with CAC vs. without VC, with AAC vs. without VC, with cardiac valve calcification vs. without VC). For categorical variables, group comparison will be based upon the results of the Chi-square test (for 2 × 2 tables) or the Cochran-Mantel-Haenszel test (for r × c tables, where r or c is greater than 2). Chi-square test will be replaced by Fisher’s exact test if the expected frequency in any of the cells of the contingency table is less than 5. For continuous variables, group comparison will be based upon the results of the t-test or the Wilcoxon rank-sum test, depending on the distribution.
Secondary analysis
The key secondary analysis will be a comparison of survival (based on all-cause mortality) between patients with and without VC based upon the log-rank test performed at a significance level of 0.05. Hazard ratios and 95 % CIs will be calculated by a Cox proportional hazard regression model. In addition, the multivariable Cox proportional hazard regression model will be performed, using the stepwise method (p <0.05 as the inclusion criteria and p <0.10 as the exclusion criteria), considering with or without VC as the independent variable and the baseline variables with p <0.25 in univariate analysis as covariates, and hazard ratios and 95 % CIs for each factor will be presented. The same analyses will also be done for survival (based on CVD mortality) and non-fatal CV event free survival. For each of the above endpoints, the separate comparisons of patients with CAC, with AAC, with cardiac valve calcification to patients without VC will be made at a significance level of 0.05.
Association between disease management (including serum level of calcium, phosphorus and iPTH, and medications), 25-OH Vit D, FGF23, and all-cause/CVD mortality, non-fatal CV events will be identified with Cox proportional hazard regression model, considering disease management as the independent variables. Associations will be tested using the Wald test and described through the hazard ratios and 95 % CIs.
Association between disease management, 25-OH Vit D, FGF23, ankle-brachial index (ABI), and LVMI/LVH and the occurrence of CAC, AAC and cardiac valve calcification at baseline will be identified with the logistic regression model. The univariate logistic regression model will be performed separately, considering disease management, ABI, and LVMI/LVH as independent variables. Associations will be tested using the Wald test and described through the odds ratios and 95 % CIs.
Association between disease management, 25-OH Vit D, FGF23, ABI, and LVMI/LVH and severities (scores) of CAC, AAC and cardiac valve calcification at baseline will be identified with the linear regression model, considering disease management, ABI, and LVMI/LVH as independent variables.
Association between CAC progression and disease management, 25-OH Vit D, FGF23, ABI, as well as LVMI/LVH will be evaluated by ANCOVA as continuous variables and evaluated by ordinal multinomial logistic regression as categorical variables.
Receiver operating characteristic curve (ROC curve) will be performed to assess the ability of the CCI to predict the presence of CAC (lesion score ≥100, ≥400 or ≥1000), with the area under the curve (AUC), positive and negative likelihood ratios calculated.