TY - JOUR
AU - Whooley,, Mary
AB - BACKGROUND Cystatin C is a promising new biomarker to determine the estimated glomerular filtration. However, the Siemens' cystatin C assay (Siemens), used in many longitudinal studies, has had limited clinical applicability because it requires a specific, dedicated instrument. Other companies, including Gentian and Roche, have developed cystatin C assays that can be used with most routine clinical chemistry analyzers. METHODS We compared the agreement of Gentian and Roche with Siemens assays in 948 participants at the baseline visit of the Heart and Soul Study, a cohort of participants with established coronary artery disease who were followed for an average of 8 years. We then compared associations of all 3 cystatin C measures and eGFR–Modification of Diet in Renal Disease (MDRD) with clinical outcomes. RESULTS The Gentian assay had higher correlation with Siemens (r = 0.96) than did Roche (r = 0.93, P < 0.001). After cross-tabulating quartiles of each cystatin C measure, agreements (κ statistic) were higher for Siemens and Gentian (0.73, 95% CI 0.72–0.75) than for Roche and Siemens (0.64, 0.63–0.66) or for Roche and Gentian (0.69, 0.65–0.71). These differences in agreement had minimal impact on associations with clinical outcomes; the hazard ratios (HRs) for mortality comparing the high vs low quartiles were 3.2 (95% CI 2.1–4.8) for Siemens, 3.1 (CI 2.1–4.7) for Gentian, 3.1 (CI 2.1–4.7) for Roche, and 1.6 (CI 1.1–2.3) for eGFR-MDRD, after multivariate adjustment. CONCLUSIONS In summary, agreement with the Siemens' assay was modestly higher for the Gentian compared with the Roche assay, although all 3 methods for cystatin C measurement had similar utility as predictors of clinical outcomes. Cystatin C is a low molecular weight cysteine protease inhibitor that has been introduced as an alternative glomerular filtration marker (1–4). Many studies have shown that cystatin C more closely approximates actual glomerular filtration rate (GFR)4 than creatinine or the Modification of Diet in Renal Disease (MDRD) equation, and is less influenced by demographic characteristics, nutritional habits, and muscle mass (5). In several population-based studies, cystatin C had much stronger associations than creatinine with longitudinal adverse outcomes, such as cardiovascular disease (CVD), heart failure, and death (6–9). In 1 large clinical trial of people with established chronic kidney disease, the MDRD Study, cystatin C had a closer association with actual GFR and predicted mortality better than creatinine-based estimated GFR (eGFR)-MDRD (10). Despite its potential advantages over creatinine as a clinical indicator of kidney function, cystatin C is rarely used in the US. The major reasons for the lack of incorporation of cystatin C into clinical medicine have been its higher reagent cost and limited availability in hospital clinical laboratories. The primary method for cystatin C measurement in US-based research studies has been via nephelometry, marketed by Siemens (formerly Dade Behring), which was the first cystatin C method to be cleared by the US Food and Drug Administration (FDA). The limitation of the Siemens assay has been that it requires a nephelometer, which is not standard equipment in most hospital laboratories; in addition, some have experienced lack of stable calibration with the Siemens assay. In the Heart and Soul Study, we previously reported that cystatin C (Siemens) was a substantially stronger predictor of CVD and mortality events than creatinine-based eGFR (11). New methods for measuring cystatin C have been developed and/or marketed recently, including products from Gentian and Roche. These assays can be performed by use of most clinical chemistry analyzers. The application of the cystatin C assay on these types of instruments will increase the potential for widespread clinical use, should they prove to be effective. We conducted this study to compare the extent of agreement of the 2 newer assays with the Siemens cystatin C assay among 948 participants from the Heart and Soul Study, a longitudinal study of individuals with established coronary artery disease. We then compared associations of all 3 methods and eGFR-MDRD with risk for death, cardiovascular disease, and heart failure over ≤8 years of follow-up. Materials and Methods PARTICIPANTS The Heart and Soul Study is a prospective cohort examining the associations between psychosocial factors and coronary artery disease progression in the San Francisco Bay area (12, 13). Participants recruited from outpatient clinics in the San Francisco Bay area met 1 of the following inclusion criteria: history of myocardial infarction, angiographic evidence of >50% stenosis in 1 or more coronary vessels, evidence of exercise-induced ischemia by treadmill or nuclear testing, history of coronary revascularization, or documented diagnosis of coronary artery disease by an internist or cardiologist. Methods have been described in detail (12, 13). Between September 2000 and December 2002, 1024 participants were enrolled. Participants underwent a baseline study visit, including a medical history interview, a physical examination, and a comprehensive health questionnaire (11). Fasting blood samples were collected and serum was stored at −70 °C. We included all participants with frozen serum; 948 individuals were included in this analysis. The appropriate institutional review boards approved this study protocol, and all participants provided written, informed consent. MEASUREMENTS Primary predictor variable. Serum cystatin C was initially measured in 2005 from the frozen serum samples collected at the baseline study appointment, by use of a BN II® nephelometer (Siemens AG, www.siemens.com) as described (11). The intraassay CV ranged from 2.0% to 2.8% (12). Outliers for the Siemens assay were Siemens 0.35 and 7.78. In 2010, we conducted measures of cystatin C on the same samples using the Gentian and Roche cystatin C assays. Serum cystatin C was measured on an Architect® ci8200 by use of particle-enhanced cystatin C assay (code 1014) and calibrator (code 1012) and control set (code 1019) from Gentian (www.gentian.no). Uniform polystyrene particles are coated with purified avian antibodies to human cystatin C, and the cystatin C assay reagents are shipped ready for use. The calibrator and controls consist of delipidated human serum spiked with human (nonrecombinant) cystatin C purified from pooled urine. After measurement, the samples were stored frozen at −70 °C for approximately 1 month before being analyzed with Roche cystatin C reagents. The handling of the samples was the same when thawing and running the assays on both the Architect and the Cobas analyzers. Although the samples were frozen and thawed twice, cystatin C has been shown to be stable after freezing and thawing (7). The CV for the Gentian method was 1.55%, calculated from duplicate analysis of patient samples, and outliers were 0.55 and 7.55. We also performed serum cystatin C measurements on a Cobas® 6000 analyzer (Roche Diagnostics, www.roche.com) according to the instructions from the manufacturer. The Roche cystatin C kit (cat. no. 04975774) contains latex particles coated with rabbit antibodies to recombinant cystatin C. The cystatin C calibrator (04975901, Roche Diagnostics) was used for calibration. The cystatin C calibrator is traceable to a Roche Diagnostics in-house reference preparation of pure recombinant human cystatin C. The CV for the Roche method was 2.25%, calculated from duplicate analysis of patient samples; outliers were 0.42, 0.44, and 6.35. Other measurements. At the baseline study visit (2002–2004), eGFR was measured by use of serum creatinine measured by the rate Jaffe method and calculated by use of the MDRD Study formula: eGFR = 186 × (serum creatinine−1.154) × (age−0.203) × (0.742 if female) × (1.21 if black) (11). Serum creatinine was measured on a UniCel® DxC 600/800 System (Beckman Coulter, www.beckmancoulter.com), and the CV was approximately 2% (11). Baseline demographics and medical history variables were recorded at the baseline interview via questionnaire, and included age, sex, race, body mass index (BMI), and presence of diabetes, hypertension, or smoking. Other serum measurements included HDL cholesterol and high-sensitivity C-reactive protein (hsCRP). Clinical outcomes. Clinical outcomes for this analysis were mortality, cardiovascular events (i.e., CVD), and congestive heart failure (CHF). Mortality was captured by report of family or by the National Death Index and verified by review of death certificates. CVD events were defined as coronary heart disease death, nonfatal myocardial infarction, or stroke, all of which have been defined previously, as has the definition for CHF (11). For the outcome of incident CHF, we excluded 166 patients with a history of CHF and 6 patients with missing information for prior CHF, leaving a sample size of 772 individuals for evaluation. We also excluded some participants from the all-cause mortality and CVD events outcomes because of missing information (n = 3 and n = 5, respectively). Annual telephone interviews were conducted with participants for follow-up to determine interval death or hospitalization. For any reported hospital or mortality event, medical records, electrocardiograms, death certificates, autopsies, and coroner's reports were obtained where applicable. Each potential event was reviewed by 2 independent adjudicators, who resolved disagreements by consensus or with a third blinded adjudicator if necessary (11). STATISTICAL ANALYSIS We began this analysis by describing characteristics of the cohort by quartile of cystatin C and comparing agreement across the 3 measures of cystatin C (Siemens, Gentian, and Roche). We then plotted each individual's cystatin C concentrations with each of the newer methods, compared with the prior Siemens measures. We determined the Spearman rank correlation coefficient, a nonparametric measure of statistical dependence. This method was repeated for comparing the Gentian and Roche assays against one another. As a second step, we categorized each set of cystatin C measures into quartiles and cross-tabulated the quartiles by each pair of cystatin C measures. We calculated the extent of agreement between each pair of cystatin C methods, which is the arithmetic sum of participants classified in the same quartile by each method, divided by the total study sample. We also used the κ statistic as an index of interobserver agreement beyond what could be expected by chance alone (14). Bias-corrected 95% CIs were calculated by use of the bootstrap method for polychotomous variables with 1000 repetitions (15). In the second stage of this analysis, we evaluated prognosis using each of the cystatin C assays and eGFR-MDRD. We first calculated the annual rates of death, CVD, and heart failure across quartiles of each kidney function estimate. As in our previously reported evaluation of the Siemens cystatin C measure (11), we used multivariate proportional hazards models, adjusted for the following variables: age, sex, race, BMI, diabetes, hypertension, smoking, HDL cholesterol, and hsCRP. We also compared risk prediction for eGFR-MDRD and each cystatin C measure by the c statistic in unadjusted and adjusted proportional hazards models. In addition, 95% CIs of these c statistics were calculated by use of the “Somer's D” command developed by Newson (16). We next investigated the prognostic importance of disagreements across cystatin C measures by evaluating participants who were categorized into different quartiles by 2 cystatin C measures. For each cross-tabulation of quartiles (e.g., Siemens vs Gentian), discordant cells were grouped by the cystatin C measure, indicating the higher quartile. We then compared event rates in these 2 groups as an indirect method to evaluate classification accuracy. Analyses were conducted by use of Stata version 10.1 (Stata Corp, www.stata.com). Results In the Heart and Soul Study, 948 participants had all 3 cystatin C measures determined from baseline data. Over a median follow-up time of 84 months, 572 events occurred, including 290 deaths, 187 cardiovascular events, and 95 cases of incident heart failure. When participants were categorized by cystatin C quartile, higher quartiles were associated with older age, male sex, and higher likelihood of diabetes, hypertension, prior myocardial infarction, heart failure, lower HDL and total cholesterol, and higher triglycerides and hsCRP (Table 1). Baseline characteristics of participants by cystatin C quartiles (Gentian). Table 1. Baseline characteristics of participants by cystatin C quartiles (Gentian). . Cystatin C quartile . Pa . I . II . III . IV . Cutpoints, mg/L ≤0.96 0.97–1.12 1.13–1.37 1.38–7.55 n 246 230 238 234 Demographics, n (%)     Age, yearsb 61 (10) 65 (10) 69 (10) 72 (11) <0.001     Male sex 186 (76) 181 (79) 208 (87) 197 (84) 0.004     Race 0.095         White 133 (54) 133 (58) 158 (66) 149 (64)         Black 50 (20) 37 (16) 29 (12) 33 (14)         Other 63 (25) 59 (26) 51 (21) 52 (22) Medical history, n (%)     Diabetes 51 (21) 59 (26) 54 (23) 88 (38) <0.001     Hypertension 148 (60) 161 (70) 175 (74) 180 (77) <0.001     Myocardial infarction 133 (55) 110 (49) 121 (51) 143 (61) 0.034     Coronary revascularization 135 (55) 132 (58) 136 (57) 157 (67) 0.039     Heart failure 24 (10) 38 (17) 39 (16) 66 (28) <0.001     Stroke 34 (14) 23 (10) 34 (14) 41 (18) 0.149     Tobacco use 49 (20) 42 (18) 52 (22) 40 (17) 0.583 Measurements, mean (SD)     Body mass index 28 (5) 29 (5) 29 (5) 28 (6) 0.091     HDL cholesterol 50 (16) 44 (13) 45 (13) 43 (13) <0.001     LDL cholesterol 105 (33) 105 (35) 105 (29) 102 (37) 0.200     Triglyceridesc 98 (69, 149) 107 (72, 169) 117 (79, 169) 114 (76, 178) 0.008     Total cholesterolc 174 (150, 201) 170 (148, 199) 177 (154, 199) 162 (143, 191) 0.007     CRPc 1.4 (0.5, 3.0) 2.0 (0.9, 3.9) 2.5 (1.1, 5.6) 3.2 (1.7, 7.3) <0.001 . Cystatin C quartile . Pa . I . II . III . IV . Cutpoints, mg/L ≤0.96 0.97–1.12 1.13–1.37 1.38–7.55 n 246 230 238 234 Demographics, n (%)     Age, yearsb 61 (10) 65 (10) 69 (10) 72 (11) <0.001     Male sex 186 (76) 181 (79) 208 (87) 197 (84) 0.004     Race 0.095         White 133 (54) 133 (58) 158 (66) 149 (64)         Black 50 (20) 37 (16) 29 (12) 33 (14)         Other 63 (25) 59 (26) 51 (21) 52 (22) Medical history, n (%)     Diabetes 51 (21) 59 (26) 54 (23) 88 (38) <0.001     Hypertension 148 (60) 161 (70) 175 (74) 180 (77) <0.001     Myocardial infarction 133 (55) 110 (49) 121 (51) 143 (61) 0.034     Coronary revascularization 135 (55) 132 (58) 136 (57) 157 (67) 0.039     Heart failure 24 (10) 38 (17) 39 (16) 66 (28) <0.001     Stroke 34 (14) 23 (10) 34 (14) 41 (18) 0.149     Tobacco use 49 (20) 42 (18) 52 (22) 40 (17) 0.583 Measurements, mean (SD)     Body mass index 28 (5) 29 (5) 29 (5) 28 (6) 0.091     HDL cholesterol 50 (16) 44 (13) 45 (13) 43 (13) <0.001     LDL cholesterol 105 (33) 105 (35) 105 (29) 102 (37) 0.200     Triglyceridesc 98 (69, 149) 107 (72, 169) 117 (79, 169) 114 (76, 178) 0.008     Total cholesterolc 174 (150, 201) 170 (148, 199) 177 (154, 199) 162 (143, 191) 0.007     CRPc 1.4 (0.5, 3.0) 2.0 (0.9, 3.9) 2.5 (1.1, 5.6) 3.2 (1.7, 7.3) <0.001 a P values were obtained by use of nonparametric equality-of-median tests. Kruskal–Wallis tests were used for continuous variables and χ2 tests for categorical variables. b Mean (SD). c Median (interquartile range). Open in new tab Table 1. Baseline characteristics of participants by cystatin C quartiles (Gentian). . Cystatin C quartile . Pa . I . II . III . IV . Cutpoints, mg/L ≤0.96 0.97–1.12 1.13–1.37 1.38–7.55 n 246 230 238 234 Demographics, n (%)     Age, yearsb 61 (10) 65 (10) 69 (10) 72 (11) <0.001     Male sex 186 (76) 181 (79) 208 (87) 197 (84) 0.004     Race 0.095         White 133 (54) 133 (58) 158 (66) 149 (64)         Black 50 (20) 37 (16) 29 (12) 33 (14)         Other 63 (25) 59 (26) 51 (21) 52 (22) Medical history, n (%)     Diabetes 51 (21) 59 (26) 54 (23) 88 (38) <0.001     Hypertension 148 (60) 161 (70) 175 (74) 180 (77) <0.001     Myocardial infarction 133 (55) 110 (49) 121 (51) 143 (61) 0.034     Coronary revascularization 135 (55) 132 (58) 136 (57) 157 (67) 0.039     Heart failure 24 (10) 38 (17) 39 (16) 66 (28) <0.001     Stroke 34 (14) 23 (10) 34 (14) 41 (18) 0.149     Tobacco use 49 (20) 42 (18) 52 (22) 40 (17) 0.583 Measurements, mean (SD)     Body mass index 28 (5) 29 (5) 29 (5) 28 (6) 0.091     HDL cholesterol 50 (16) 44 (13) 45 (13) 43 (13) <0.001     LDL cholesterol 105 (33) 105 (35) 105 (29) 102 (37) 0.200     Triglyceridesc 98 (69, 149) 107 (72, 169) 117 (79, 169) 114 (76, 178) 0.008     Total cholesterolc 174 (150, 201) 170 (148, 199) 177 (154, 199) 162 (143, 191) 0.007     CRPc 1.4 (0.5, 3.0) 2.0 (0.9, 3.9) 2.5 (1.1, 5.6) 3.2 (1.7, 7.3) <0.001 . Cystatin C quartile . Pa . I . II . III . IV . Cutpoints, mg/L ≤0.96 0.97–1.12 1.13–1.37 1.38–7.55 n 246 230 238 234 Demographics, n (%)     Age, yearsb 61 (10) 65 (10) 69 (10) 72 (11) <0.001     Male sex 186 (76) 181 (79) 208 (87) 197 (84) 0.004     Race 0.095         White 133 (54) 133 (58) 158 (66) 149 (64)         Black 50 (20) 37 (16) 29 (12) 33 (14)         Other 63 (25) 59 (26) 51 (21) 52 (22) Medical history, n (%)     Diabetes 51 (21) 59 (26) 54 (23) 88 (38) <0.001     Hypertension 148 (60) 161 (70) 175 (74) 180 (77) <0.001     Myocardial infarction 133 (55) 110 (49) 121 (51) 143 (61) 0.034     Coronary revascularization 135 (55) 132 (58) 136 (57) 157 (67) 0.039     Heart failure 24 (10) 38 (17) 39 (16) 66 (28) <0.001     Stroke 34 (14) 23 (10) 34 (14) 41 (18) 0.149     Tobacco use 49 (20) 42 (18) 52 (22) 40 (17) 0.583 Measurements, mean (SD)     Body mass index 28 (5) 29 (5) 29 (5) 28 (6) 0.091     HDL cholesterol 50 (16) 44 (13) 45 (13) 43 (13) <0.001     LDL cholesterol 105 (33) 105 (35) 105 (29) 102 (37) 0.200     Triglyceridesc 98 (69, 149) 107 (72, 169) 117 (79, 169) 114 (76, 178) 0.008     Total cholesterolc 174 (150, 201) 170 (148, 199) 177 (154, 199) 162 (143, 191) 0.007     CRPc 1.4 (0.5, 3.0) 2.0 (0.9, 3.9) 2.5 (1.1, 5.6) 3.2 (1.7, 7.3) <0.001 a P values were obtained by use of nonparametric equality-of-median tests. Kruskal–Wallis tests were used for continuous variables and χ2 tests for categorical variables. b Mean (SD). c Median (interquartile range). Open in new tab All 3 cystatin C measures were highly correlated when compared against each other by use of Spearman rank correlation coefficients; however, Gentian and Siemens had the highest correlation (rs = 0.96; P < 0.001) (Fig. 1A–C). After cross-tabulating quartiles of each cystatin C measure, agreement was 80% between Gentian and Siemens, 73% between Roche and Siemens, and 77% between Gentian and Roche. Corresponding κ statistics were higher for Siemens and Gentian (0.73, 95% CI 0.72–0.75) than for Roche and Siemens (0.64, 0.63–0.66) or for Roche and Gentian (0.69, 0.65–0.71). Correlations between 3 measures of cystatin C: Siemens, Gentian, and Roche. Fig. 1. Open in new tabDownload slide Fig. 1. Open in new tabDownload slide Cystatin C quartiles by each measure appeared to have similar associations with cardiovascular disease, heart failure, and death (Fig. 2, A–C). In multivariate analysis, hazard ratios paralleled event rates, as each high quartile had the highest risk for incident heart failure compared with other outcomes (Table 2). All 3 cystatin C measures had stronger and more linear associations with mortality than eGFR-MDRD in multivariate analysis (Table 2). We then determined the c statistic for univariate and multivariate models for eGFR-MDRD and each cystatin C measure (Table 3). In univariate comparisons, eGFR-MDRD had a substantially lower c statistic with 95% CIs that did not overlap those of any of the 3 cystatin C measures. In fact, the univariate c statistics for all cystatin C measures were within 0.04 of the multivariate model without kidney function for all 3 outcomes. Event rates by quartile of cystatin C. Fig. 2. Open in new tabDownload slide Event rates were found to differ significantly across quartiles for each measurement method and each outcome (log-rank tests for equality of survival functions, P < 0.001). a Cardiovascular events include coronary heart disease death, myocardial infarction, and stroke. Fig. 2. Open in new tabDownload slide Event rates were found to differ significantly across quartiles for each measurement method and each outcome (log-rank tests for equality of survival functions, P < 0.001). a Cardiovascular events include coronary heart disease death, myocardial infarction, and stroke. Adjusted associations of high vs low quartiles of 3 cystatin C measures and eGFR-MDRD with clinical outcomes. Table 2. Adjusted associations of high vs low quartiles of 3 cystatin C measures and eGFR-MDRD with clinical outcomes. . Mortality, hazard ratio (95% CI) . CVD, hazard ratio (95% CI) . Chronic heart failure, hazard ratio (95% CI) . Cystatin C     Siemens 3.2 (2.1–4.8) 2.6 (1.6–4.2) 5.3 (2.4–12.1)     Gentian 3.1 (2.1–4.7) 2.6 (1.6–4.2) 5.2 (2.4–11.3)     Roche 3.1 (2.1–4.7) 2.4 (1.5–3.9) 4.6 (2.0–10.3) eGFR-MDRD 1.6 (1.1–2.3) 1.9 (1.2–2.9) 3.2 (1.6–6.6) . Mortality, hazard ratio (95% CI) . CVD, hazard ratio (95% CI) . Chronic heart failure, hazard ratio (95% CI) . Cystatin C     Siemens 3.2 (2.1–4.8) 2.6 (1.6–4.2) 5.3 (2.4–12.1)     Gentian 3.1 (2.1–4.7) 2.6 (1.6–4.2) 5.2 (2.4–11.3)     Roche 3.1 (2.1–4.7) 2.4 (1.5–3.9) 4.6 (2.0–10.3) eGFR-MDRD 1.6 (1.1–2.3) 1.9 (1.2–2.9) 3.2 (1.6–6.6) Open in new tab Table 2. Adjusted associations of high vs low quartiles of 3 cystatin C measures and eGFR-MDRD with clinical outcomes. . Mortality, hazard ratio (95% CI) . CVD, hazard ratio (95% CI) . Chronic heart failure, hazard ratio (95% CI) . Cystatin C     Siemens 3.2 (2.1–4.8) 2.6 (1.6–4.2) 5.3 (2.4–12.1)     Gentian 3.1 (2.1–4.7) 2.6 (1.6–4.2) 5.2 (2.4–11.3)     Roche 3.1 (2.1–4.7) 2.4 (1.5–3.9) 4.6 (2.0–10.3) eGFR-MDRD 1.6 (1.1–2.3) 1.9 (1.2–2.9) 3.2 (1.6–6.6) . Mortality, hazard ratio (95% CI) . CVD, hazard ratio (95% CI) . Chronic heart failure, hazard ratio (95% CI) . Cystatin C     Siemens 3.2 (2.1–4.8) 2.6 (1.6–4.2) 5.3 (2.4–12.1)     Gentian 3.1 (2.1–4.7) 2.6 (1.6–4.2) 5.2 (2.4–11.3)     Roche 3.1 (2.1–4.7) 2.4 (1.5–3.9) 4.6 (2.0–10.3) eGFR-MDRD 1.6 (1.1–2.3) 1.9 (1.2–2.9) 3.2 (1.6–6.6) Open in new tab Univariate and multivariate comparisons of risk prediction by the c statistic (95% CI) for 3 cystatin C measures and eGFR-MDRD.a Table 3. Univariate and multivariate comparisons of risk prediction by the c statistic (95% CI) for 3 cystatin C measures and eGFR-MDRD.a . Mortality . Cardiovascular events . Heart failure . Univariate     eGFR-MDRD 0.59 (0.56–0.62) 0.59 (0.55–0.61) 0.62 (0.57–0.67)     Siemens 0.67 (0.64–0.70) 0.65 (0.61–0.69) 0.73 (0.68–0.78)     Gentian 0.67 (0.64–0.70) 0.65 (0.61–0.69) 0.73 (0.68–0.78)     Roche 0.67 (0.64–0.70) 0.64 (0.60–0.68) 0.71 (0.66–0.76) Multivariate     Without kidney function variable 0.69 (0.66–0.73) 0.68 (0.64–0.72) 0.75 (0.71–0.80)     eGFR-MDRD 0.71 (0.68–0.74) 0.69 (0.65–0.73) 0.78 (0.73–0.82)     Siemens 0.72 (0.69–0.75) 0.70 (0.66–0.74) 0.79 (0.75–0.84)     Gentian 0.72 (0.69–0.75) 0.71 (0.66–0.74) 0.80 (0.75–0.84)     Roche 0.72 (0.69–0.75) 0.69 (0.66–0.73) 0.79 (0.74–0.73) . Mortality . Cardiovascular events . Heart failure . Univariate     eGFR-MDRD 0.59 (0.56–0.62) 0.59 (0.55–0.61) 0.62 (0.57–0.67)     Siemens 0.67 (0.64–0.70) 0.65 (0.61–0.69) 0.73 (0.68–0.78)     Gentian 0.67 (0.64–0.70) 0.65 (0.61–0.69) 0.73 (0.68–0.78)     Roche 0.67 (0.64–0.70) 0.64 (0.60–0.68) 0.71 (0.66–0.76) Multivariate     Without kidney function variable 0.69 (0.66–0.73) 0.68 (0.64–0.72) 0.75 (0.71–0.80)     eGFR-MDRD 0.71 (0.68–0.74) 0.69 (0.65–0.73) 0.78 (0.73–0.82)     Siemens 0.72 (0.69–0.75) 0.70 (0.66–0.74) 0.79 (0.75–0.84)     Gentian 0.72 (0.69–0.75) 0.71 (0.66–0.74) 0.80 (0.75–0.84)     Roche 0.72 (0.69–0.75) 0.69 (0.66–0.73) 0.79 (0.74–0.73) a The Harrell C index was obtained by use of the Stata stcox postestimation command “estat concordance”; and 95% CIs were obtained by use of the Newson [Newson (16)] method of comparing predictive power of survival models. Open in new tab Table 3. Univariate and multivariate comparisons of risk prediction by the c statistic (95% CI) for 3 cystatin C measures and eGFR-MDRD.a . Mortality . Cardiovascular events . Heart failure . Univariate     eGFR-MDRD 0.59 (0.56–0.62) 0.59 (0.55–0.61) 0.62 (0.57–0.67)     Siemens 0.67 (0.64–0.70) 0.65 (0.61–0.69) 0.73 (0.68–0.78)     Gentian 0.67 (0.64–0.70) 0.65 (0.61–0.69) 0.73 (0.68–0.78)     Roche 0.67 (0.64–0.70) 0.64 (0.60–0.68) 0.71 (0.66–0.76) Multivariate     Without kidney function variable 0.69 (0.66–0.73) 0.68 (0.64–0.72) 0.75 (0.71–0.80)     eGFR-MDRD 0.71 (0.68–0.74) 0.69 (0.65–0.73) 0.78 (0.73–0.82)     Siemens 0.72 (0.69–0.75) 0.70 (0.66–0.74) 0.79 (0.75–0.84)     Gentian 0.72 (0.69–0.75) 0.71 (0.66–0.74) 0.80 (0.75–0.84)     Roche 0.72 (0.69–0.75) 0.69 (0.66–0.73) 0.79 (0.74–0.73) . Mortality . Cardiovascular events . Heart failure . Univariate     eGFR-MDRD 0.59 (0.56–0.62) 0.59 (0.55–0.61) 0.62 (0.57–0.67)     Siemens 0.67 (0.64–0.70) 0.65 (0.61–0.69) 0.73 (0.68–0.78)     Gentian 0.67 (0.64–0.70) 0.65 (0.61–0.69) 0.73 (0.68–0.78)     Roche 0.67 (0.64–0.70) 0.64 (0.60–0.68) 0.71 (0.66–0.76) Multivariate     Without kidney function variable 0.69 (0.66–0.73) 0.68 (0.64–0.72) 0.75 (0.71–0.80)     eGFR-MDRD 0.71 (0.68–0.74) 0.69 (0.65–0.73) 0.78 (0.73–0.82)     Siemens 0.72 (0.69–0.75) 0.70 (0.66–0.74) 0.79 (0.75–0.84)     Gentian 0.72 (0.69–0.75) 0.71 (0.66–0.74) 0.80 (0.75–0.84)     Roche 0.72 (0.69–0.75) 0.69 (0.66–0.73) 0.79 (0.74–0.73) a The Harrell C index was obtained by use of the Stata stcox postestimation command “estat concordance”; and 95% CIs were obtained by use of the Newson [Newson (16)] method of comparing predictive power of survival models. Open in new tab We next investigated disagreements between quartiles defined by each pair of cystatin C measures. The proportion of participants that were classified into 2 different quartiles were 0.27 (Roche vs Siemens), 0.20 (Gentian vs Siemens), and 0.23 (Gentian vs Roche). We next compared risks for adverse outcomes in these participants, grouped by the assay designating the higher quartile. Comparing the discordant groups by Gentian and Siemens, we found similar absolute risks for each event (Fig. 3A). When we compared discordant groups between Roche and Siemens, risks appeared qualitatively higher for the participants classified into higher cystatin C quartiles by Siemens, but were not statistically different (Fig. 3B). Similarly, among participants discordantly classified into quartiles by Gentian and Roche, risks were statistically equivalent despite appearing higher for participants classified into higher quartiles by Gentian (Fig. 3C). Comparison of risk among individuals discordantly classified by 2 cystatin C measurement methods. Fig. 3. Open in new tabDownload slide Participants in these analyses were those classified into different quartiles based on the 2 cystatin C measures. Each bar represents the event rate for those classified higher by the referenced cystatin C method. Fig. 3. Open in new tabDownload slide Participants in these analyses were those classified into different quartiles based on the 2 cystatin C measures. Each bar represents the event rate for those classified higher by the referenced cystatin C method. Discussion In this large, contemporary cohort from the Heart and Soul Study, we compared the agreement of 3 methods of cystatin C measurement (Siemens, Gentian, and Roche) and assessed their associations with clinical outcomes for death, cardiovascular events, and incident heart failure. In the absence of an international standard for cystatin C measures, we considered Siemens to be the referent for comparison with the 2 newer assays, Gentian and Roche. Although all 3 cystatin C measures were highly correlated, Gentian had a higher level of agreement with Siemens than did the Roche method, as determined by their correlations and by the κ statistic. As prognostic markers, the 3 cystatin C measures had similar associations with each clinical outcome. We conclude that these newer methods of cystatin C measurement can reproduce the risk stratification of the Siemens cystatin C assay. This study builds on numerous prior studies that found cystatin C to be a powerful prognostic marker, particularly relative to creatinine-based estimates. An unanswered question has been whether different cystatin C measures yield similar estimates. Prior articles that used the Dade/Siemens method found thresholds for moderate cardiovascular risk at approximately 1.0 mg/L and high cardiovascular risk at thresholds between 1.23 and 1.30 mg/L (7, 8, 11). In this article, the thresholds for the high quartile of cystatin C differed slightly across methods: Siemens 1.31 mg/L, Gentian 1.38 mg/L, and Roche 1.32 mg/L. In these analyses, the high quartile by each measure delineated a group at high cardiovascular risk. It is interesting that the markers were so similar for prognosis despite some disagreement. The precision required in a filtration marker, such as creatinine and cystatin C, for reliable GFR estimation may be higher than for risk prediction because they are incorporated into equations which then exponentiate the measure. Thus, small differences in actual cystatin C (mg/L) values will be expanded to large differences in eGFR [mL · min−1 · (1.73 m2)−1]. In most prognostic analyses, the filtration marker is split into broad categories (e.g., quartiles) so that it has to be precise only within the study, not calibrated to an external standard. For the purposes of risk prediction and broad epidemiological surveillance, we concluded that these 3 methods appear generally equivalent, although they may have different clinical performances. For widespread clinical use as a filtration marker, the optimal cystatin C assay will need to demonstrate consistent accuracy and calibration against an international reference material. Hence, there are several steps necessary for cystatin C to become widely adopted and valuable as a diagnostic test. The first of these steps is international standardization, which is already underway by the IFCC. All 3 assays are in the process of being calibrated to this international reference material (17, 18). Another important step will be an equation for cystatin C that is broadly applicable in the general population across the range of age, GFR, and race/ethnicity. The IFCC cystatin C working group's aim is to produce 1 general equation to convert the cystatin C (mg/L) measure to a GFR estimation [eGFR, mL · min−1 · (1.73 m2)−1] for all cystatin C assays, regardless of analytical platform. An additional priority will be to characterize fully the non-GFR determinants of cystatin C; it is clearly less affected by muscle mass and health status than creatinine, but it is still critical to uncover and quantify all relevant factors that influence the association between cystatin C and kidney function, such as thyroid disorders, corticosteroid use, and treatment with cytotoxic drugs. For practical use, the test's turnaround time is an important consideration, and cystatin C will need to have a turnaround time similar to that of creatinine to be useful for clinicians. Rapid turnaround time will be more likely if the reagent is available on the same clinical chemistry analyzer as creatinine, such as with the Gentian and Roche reagents. Cystatin C measures must maintain precision and calibration over time to allow for a consistent, unbiased method of GFR estimation. Finally, a critical issue is the cost of the assay, and a frequently used assay should be affordable for widespread use to be justified. STRENGTHS AND LIMITATIONS The relatively large size of the cohort, the length of follow-up time, and the adjudication for each outcome event were all strengths of this study. There were also several important limitations, however. Most importantly, this study lacked measured GFR, so we cannot compare the cystatin C measures as GFR estimates directly. The other major issue is that the cohort is comprised of primarily male veterans with coronary heart disease, so our results may not be generalizable to women, nonveterans, or populations without heart disease. In summary, we compared 3 measures of cystatin C: Siemens, Gentian, and Roche, and found that Gentian has closer correlation and higher agreement with Siemens than the Roche assay. However, based on risk prediction, all performed similarly. 4 Nonstandard abbreviations: GFR glomerular filtration rate MDRD Modification of Diet in Renal Disease CVD cardiovascular disease eGFR estimated GFR FDA US Food and Drug Administration BMI body mass index hsCRP high-sensitivity C-reactive protein CHF congestive heart failure. " Author Contributions:All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article. " Authors' Disclosures or Potential Conflicts of Interest:Upon manuscript submission, all authors completed the Disclosures of Potential Conflict of Interest form. Potential conflicts of interest: " Employment or Leadership: None declared. " Consultant or Advisory Role: None declared. " Stock Ownership: None declared. " Honoraria: None declared. " Research Funding: M.G. Shlipak, NIH (R01 DK066488-01) and the American Heart Association Established Investigator Award. Siemens cystatin C measures were supported by a grant by Siemens (Dade-Behring) to the Heart and Soul Study. The Gentian and Roche measures were funded by a grant from Gentian and Roche, respectively, to the Heart and Soul Study. These funding sources had no involvement in the design or execution of this study. " Expert Testimony: None declared. 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Google Scholar Crossref Search ADS PubMed WorldCat © 2011 The American Association for Clinical Chemistry This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
TI - Comparison of Cardiovascular Prognosis by 3 Serum Cystatin C Methods in the Heart and Soul Study
JO - Clinical Chemistry
DO - 10.1373/clinchem.2010.158915
DA - 2011-05-01
UR - https://www.deepdyve.com/lp/oxford-university-press/comparison-of-cardiovascular-prognosis-by-3-serum-cystatin-c-methods-z6tl0PU09E
SP - 737
VL - 57
IS - 5
DP - DeepDyve
ER -