Association of cystatin C and estimated GFR with inflammatory biomarkers: the Heart and Soul Study

David Singh 2 3 5 Mary A. Whooley 0 2 3 4 5 Joachim H. Ix 1 2 3 5 Sadia Ali 0 2 5 Michael G. Shlipak 0 2 3 4 5 0 Department of Veterans Affairs Medical Center 1 Department of Nephrology 2 MPH, 4150 Clement Street (111A1), San Francisco, CA 94121 3 Department of Medicine 4 Department of Epidemiology and Biostatistics, University of California , San Francisco, CA, USA 5 Acknowledgements. This work was supported by grants from the Department of Veterans Affairs (Epidemiology Merit Review Program) , Washington, DC ; the Robert Wood Johnson Foundation (Generalist Physician Faculty Scholars Program) , Princeton, New Jersey ; the American Federation for Aging Research (Paul Beeson Faculty Scholars in Aging Research Program) , New York, New York ; the University of California Academic Senate (JHI) , and Dade Behring, Inc., Deerfield, IL , USA . M.G.S. was funded by the American Federation for Aging Research and National Institute on Aging (Paul Beeson Scholars Program) , the Robert Wood Johnson Foundation (Generalist Faculty Scholars Program) and by R01 DK066488 Background. Cystatin C is a marker of kidney function that may also be associated with inflammation. In this study, we compared the relative strengths of association of cystatin C and estimated glomerular filtration rate (eGFR) with inflammatory biomarkers. Methods. We measured serum cystatin C and creatinine in 990 outpatients with coronary artery disease enrolled in the Heart and Soul Study. GFR was estimated (eGFR) by the abbreviated Modification of Diet in Renal Disease (MDRD) equation. We compared the associations of serum cystatin C and eGFR with C-reactive protein (CRP) and fibrinogen, after adjustment for 24 h creatinine clearance. Results. Cystatin C concentrations had moderate correlations with CRP (r 0.15, P < 0.001) and fibrinogen (r 0.26, P < 0.0001); eGFR had similar correlations with CRP (r 0.17, P 0.01) and fibrinogen (r 0.25, P < 0.001) among persons with eGFR 60 ml/min, but had no association with either biomarker among those with eGFR > 60 ml/min (r 0.04, P 0.32; r 0.03, P 0.38). Quartiles of cystatin C were strongly and directly associated with CRP (P 0.02) and fibrinogen (P < 0.007) after multivariate adjustment. However, these associations disappeared after adjustment for creatinine clearance (P 0.26 and 0.23, respectively). Conclusions. Cystatin C concentrations have moderate associations with CRP and fibrinogen that are not independent of creatinine clearance. Although a gold standard of kidney function is lacking, this analysis suggests that cystatin C captures an association of mildly impaired kidney function with increased inflammation. Introduction The use of serum creatinine as a marker of kidney function is limited by factors that influence creatinine concentrations such as age, gender, race and weight. While an improvement over serum creatinine in predicting glomerular filtration rate (GFR), formulae such as the CockcroftGault or the Modification of Diet in Renal Disease (MDRD) to calculate estimated GFR (eGFR), also have limitations and may be unreliable in certain populations [1,2]. The shortcomings of creatinine and creatinine-derived equations have prompted a search for a more reliable endogenous marker of kidney function. Cystatin C is a non-glycosylated cysteine protease inhibitor with a molecular weight of 13 kDa that appears to be produced at a constant rate by all nucleated cells. It is freely filtered by the renal glomerulus and metabolized by the proximal tubule [3,4]. Some studies have suggested that cystatin C may be superior to serum creatinine or creatinine-based estimating equations as a marker of kidney function, whereas others have found no difference [36]. A recent study from the Prevention of Renal and Vascular End-Stage Renal Disease (PREVEND) cohort found that cystatin C was significantly associated with C-reactive protein (CRP), smoking and body mass index even after adjustment for creatinine clearance (CRCL) levels [17]. The authors concluded that cystatin C levels were influenced by these factors in addition to kidney function. Recent longitudinal studies have shown that cystatin C has a stronger and more linear association with cardiovascular disease and mortality outcomes compared with creatinine-based measures [710]. These findings led to the hypothesis that cystatin Cs link to inflammation could explain its advantage over creatinine as a prognostic marker [11]. In the Heart and Soul Study, we compared the relative strengths of association of cystatin C and eGFR with two inflammatory biomarkers, CRP and fibrinogen, and we determined whether these measured CRCL. Methods Study participants were independent The Heart and Soul Study is a prospective cohort study that was designed to examine the influence of psychosocial factors on coronary artery disease (CAD) progression. Details of our methods have been published previously [12]. Between September 2000 and December 2002, patients with CAD were identified from administrative databases at the Department of Veterans Affairs Medical Centers in Palo Alto and San Francisco, from the University of California San Francisco Medical Center, and from nine public health clinics in San Francisco. Patients were eligible to participate if their medical history contained at least one of the following: myocardial infarction, coronary revascularization, angiographic evidence of >50% stenosis in one or more coronary vessels, evidence of exercise-induced ischaemia by treadmill or nuclear testing, or a diagnosis of CAD by their internist or cardiologist. Patients were excluded if they were unable to walk one block, had a myocardial infarction in the last 6 months, or were planning to move out of the region within 3 years. A total of 1024 participants enrolled in the study and completed a day-long examination, of whom 990 provided adequate serum samples for measurement of cystatin C. The study protocol was approved by the appropriate Institutional Review Boards, and all participants provided written informed consent. Kidney function Cystatin C was measured from fasting serum samples collected between September 2000 and December 2002 and stored at 708C until October 2004. Concentrations were measured using a BNII nephelometer (Dade Behring Inc., Deerfield, IL, USA) and a particle-enhanced immunonephelometric assay (N Latex cystatin C, Dade Behring Inc.). The detection limit of the assay is 0.05 mg/l, the analytical sensitivity is 0.005 mg/l and the reference range is 0.530.95 mg/l. Between and within-run coefficients of variation are both <3.6%. We used the four-variable MDRD to calculate eGFR. The following formula was used: eGFR 186 [SCR/88.4] 1.154) (age) 0.203 (0.742, if female) (1.210, if African-American). Creatinine clearance was determined by 24 h urine collection, as previously described [13], using the following formula: urine creatinine (mg/dl) 24 h urine volume (dl)/serum creatinine (mg/dl) 1440 (min/d (...truncated)