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Saudi Journal of Kidney Diseases and Transplantation
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Table of Contents   
ORIGINAL ARTICLE  
Year : 2012  |  Volume : 23  |  Issue : 3  |  Page : 477-483
Comparison of C-reactive protein and high-sensitivity C-reactive protein levels in patients on hemodialysis


1 Department of Internal Medicine A and Laboratory of Kidney Pathology 02, Charles Nicolle Hospital, Tunis, Tunisia
2 Biochemical Laboratory, Charles Nicolle Hospital, Tunis, Tunisia
3 Department of Epidemiology, Charles Nicolle Hospital, Tunis, Tunisia

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Date of Web Publication7-May-2012
 

   Abstract 

Chronic inflammation is highly prevalent in patients on hemodialysis (HD), as evidenced by increased levels of C-reactive protein (CRP). We compared CRP to high-sensitivity C-reactive protein (hs-CRP) to determine whether it has any clinical implications and prognostic significance in terms of mortality. CRP was measured using a standard immunoturbidometric assay on the COBAS; INTEGRA system and hs-CRP was measured using the Dade Behring on the Konelab Nephelometer in 50 patients on HD. CRP (≥6 mg/L) and hs-CRP (≥3 mg/L) levels were elevated in 30% and 54% of the patients, respectively. A significant correlation was noted between hs-CRP and CRP levels (r = 0.98, P <0.001). Deming regression analysis showed that the slope was near one (r = 0.90; 0.83-0.94) and that the intercept was small. Multivariate regression confirmed that age above 40 years (RR = 3.69, P = 0.027) and duration on HD greater than five years (RR = 3.71, P = 0.028) remained significant independent predictors of serum hs-CRP. Thirteen patients died during follow-up (26%). Multivariate Cox regression demonstrated that hs-CRP (RR = 1.062, P = 0.03) and CRP levels (RR = 1.057, P = 0.009) and age (RR = 1.078, P = 0.001) were the most powerful predictors of mortality. The CRP standard assay presents a reasonable alternative to the hs-CRP assay in patients on HD. The advantages of the CRP standard assay are its online and real-time availability as well as lower costs, particularly in developing countries.

How to cite this article:
Helal I, Zerelli L, Krid M, ElYounsi F, Maiz HB, Zouari B, Adelmoula J, Kheder A. Comparison of C-reactive protein and high-sensitivity C-reactive protein levels in patients on hemodialysis. Saudi J Kidney Dis Transpl 2012;23:477-83

How to cite this URL:
Helal I, Zerelli L, Krid M, ElYounsi F, Maiz HB, Zouari B, Adelmoula J, Kheder A. Comparison of C-reactive protein and high-sensitivity C-reactive protein levels in patients on hemodialysis. Saudi J Kidney Dis Transpl [serial online] 2012 [cited 2019 May 23];23:477-83. Available from: http://www.sjkdt.org/text.asp?2012/23/3/477/95749

   Introduction Top


Chronic inflammation is an important component in the development and progression of atherosclerosis. [1] C-reactive protein (CRP) has been shown to be both a marker and a mediator of atherosclerosis. [2] In subjects undergoing hemodialysis (HD), CRP is a potent, independent predictor of cardiovascular mortality as well as malnutrition. [3],[4],[5],[6]

In past studies, CRP, a major systemic marker of inflammation, was reported to induce adhesion molecule expression in human endothelial cells, supporting involvement of CRP in the atherosclerotic process. [7] Earlier laboratory assay methods were not sufficiently sensitive to measure blood levels of CRP within the normal range (<10 mg/L); however, the recent development of high-sensitivity assays for CRP (hs-CRP) has permitted detection of even mild elevation of CRP, even within the normal range. Reliable and fully automated high-sensitivity assays for CRP are now widely available. A direct positive association between hs-CRP and future coronary events has been previously reported. [1],[8]

Different commercially available methods for the detection of CRP are used in clinical laboratories, but they may substantially differ in their accuracy. Roberts et al [9],[10] conducted in-depth comparisons of automated high-sensitivity methods of measuring normal to slightly increased plasma CRP levels in healthy subjects. In one of these studies, [3] the authors showed that the results elicited with just four of the nine methods examined were in agreement with those produced by the comparison method. However, similar studies have not yet been carried out in subjects with uremia. To fill this gap, we evaluated the precision and agreement of two commonly used automated methods for the measurement of CRP levels in patients with uremia and its clinical implications.

In fact, the determination of CRP concentrations might help to single out individuals with an increased atherothrombotic risk. [11],[12] In addition, it might also have a direct pathogenetic role as well. [13]


   Materials and Methods Top


Study population

Fifty randomly selected patients on HD were studied in November 2005 to investigate the CRP and hs-CRP concentrations. All patients were clinically stable and gave informed consent. Patients with infection, neoplastic proliferation or inflammatory disease were excluded from the study. All patients received regular HD using a cellulose-triacetate dialyzer, three-times per week in sessions lasting 4 h. The dialysate was bicarbonate buffered, the dialysate flow rate was 500 mL/min and blood flow ranged from 250 to 300 mL/min. Clinical characteristics and laboratory data of the study patients are shown in [Table 1] and [Table 2]. Other baseline characteristics besides CRP were considered as risk or confounding factors on multivariable analyses, including age, gender, physician-reported history of diabetes mellitus (fasting blood sugar ≥126 mg/dL or the use of an anti-diabetic medication), hypertension (systolic blood pressure ≥140 mmHg, diastolic blood pressure ≥90 mmHg or the use of antihypertensive medication) or hyperlipidemia (total cholesterol ≥200 mg/dL, low-density lipoprotein cholesterol (LDL-C) ≥130 mg/dL or the use of lipid-lowering medication), smoking (active or >10 pack-years) and self-reported previous myocardial infarction.
Table 1: Demographic data of the study patients.

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Table 2: Biochemical values among the hemodialysis patients studied.

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We prospectively followed-up these 50 patients until July 2008 (30 months) to determine the incidence of death. Twelve patients transferred out of our dialysis facilities during follow-up, but outcome was obtained in all these patients. One patient transferred to peritoneal dialysis for problems with vascular access. Thirteen patients died during follow-up (26%).

Laboratory methods

We compared the CRP standard immunoturbidometric assay on the COBAS® INTEGRA system (normal value is less than 6 mg/L) to the Dade Behring hs-CRP on the Konelab Nephelometer (normal value is less than 3 mg/L) in 50 patients on HD. The correlation between the two assays was also analyzed. We collected blood samples from all patients prior to a HD session from the venous line. All other biochemical parameters were measured with routine laboratory methods. Samples for both CRP and other biochemical parameters were obtained during the same HD session.


   Statistical Analysis Top


Continuous variables are reported as mean ± standard deviation. For selected comparisons between two group means, parametric (t-test) or non-parametric (Mann-Whitney test) methods were used. Correlations were reported as either Pearson correlation coefficients or Spearman rank correlation coefficients. Simple linear regression was used to identify the predictors of CRP and hs-CRP levels. Multiple linear regressions were used to produce a predictive model for CRP level. Observed survival of patients was computed by the Kaplan-Meier method. Survival was also evaluated using the univariate and multivariate Cox proportional hazards models. Logistic regression analysis was performed to assess for significant associations with mortality.

A two-tailed P-value less than 0.05 was considered to be statistically significant. Calculations were performed using SPSS for Windows, version 11.0.


   Results Top


Demographics and patient characteristics

The mean age of the study patients was 44.92 ± 13.85 years. In the group studied, 42% were female, 28% were smokers, 4% had cardiac ischemia, 60% had hypertension and 22% had diabetes. The cause of end-stage renal disease (ESRD) was chronic glomerulonephritis in 34%, diabetic nephropathy in 22%, interstitial nephritis in 16%, nephrosclerosis in 18% and unknown cause in 10% of the patients. The mean duration on dialysis at enrolment was 7.03 ± 5.50 years.

At enrolment, mean and median CRP were 7.44 ± 13.13 mg/L (range: 0.1-88.1 mg/L) and 3 mg/L, respectively. Mean and median hs-CRP were 9.52 ± 12.47 mg/L (range: 0.2-62.6 mg/L) and 3.35 mg/L, respectively. In addition, 84% of the patients had anemia (hemoglobin <11 g/dL), 65.2% had hyperparathyroidism (intact parathormone >400 pg/mL) and the mean dialysis quality (Kt/V) was 1.56 ± 0.30, the mean albumin level was 44.68 ± 6.10 g/L, the mean serum LDL-C level was 1.79 ± 0.98 mmol/L and the mean serum calcium- phosphate ion product (Ca × P) level was 3.36 ± 1.43 mmol/L.

Enrolment CRP (≥6 mg/L) and hs-CRP (≥3 mg/L) levels were elevated in 30% and 54% of the patients, respectively. In nine patients (13.6%), the CRP and hs-CRP were highly correlated (r = 0.98, P < 0.001). The results show significant correlation between both methods [Figure 1]. Deming regression analysis showed that the slope was near 1 (r = 0.90; 0.83-0.94) in all and that the intercept was small. CRP was calculated by standard techniques and hs-CRP was calculated by the regression equation as follows:

hs-CRP = 1.583 ×CRP - 0.736

hs-CRP lower bound = 1.49 × CRP - 1.53

hs-CRP upper bound = 1.58 × CRP - 0.06
Figure 1: Scatter plot reflecting the perfect correlation between C-reactive protein and high sensitive- C-reactive protein in the 50 study patents.

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Patients with nephrosclerosis had a significantly higher hs-CRP (P = 0.028) than those with interstitial nephritis.

Predictors of CRP and hs-CRP levels

By univariate analysis, only age above 40 years (r = 0.44, P = 0.01) and duration on dialysis greater than five years (β = -0.26, P = 0.04) had a significant correlation with CRP and hs-CRP levels. Neither CRP nor hs-CRP level were correlated with gender, body mass index (BMI), diabetes, hypertension, anemia, albumin and Kt/V. This result may be related to the small number of our study population.

Stepwise multivariate regression analysis confirmed that, in HD patients, age above 40 years (RR = 3.69, P = 0.027) and duration on HD greater than five years (RR = 3.71, P = 0.028) remained significant independent predictors of serum hs-CRP. However, these two factors remained dependent predictors of serum CRP.

Survival

Data for survival analysis were available in 50 patients. Thirteen patients died during followup (26%). Overall survival in these prevalent patients was 82% after one year and 74% after two years [Figure 2].
Figure 2. Kaplan-Maier plot of overall survival in the 50 study patients

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On univariate analysis using the Cox proportional hazard model, survival was significantly influenced by age (P = 0.02), duration on dialysis (P = 0.001), CRP (P = 0.006) and hs-CRP levels (P = 0.004). Survival was not influenced by gender, BMI, diabetes, hypertension, anemia, Kt/V and albumin. The independent predictors of patient survival were determined using a multivariate Cox proportional hazard analysis, which adjusts for other variables known to influence survival [Table 3]. In the Cox model, adjusting for age, sex and diabetes, serum hs-CRP remained a significant independent predictor of mortality (RR = 1.062, P = 0.03). For every 1 mg/L increase in serum hs-CRP at enrolment, mortality risk increased by 6.2%. However, the predictive significance of hs-CRP for mortality diminished when an additional adjustment was made for BMI. The other variables that significantly influenced survival on multivariate analysis were CRP (RR = 1.057, P = 0.009) and age (RR = 1.078, P = 0.001).
Table 3: Significant variables that predicted mortality in the multivariate Cox proportional hazard model.

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   Discussion Top


CRP has a pentamer structure with a molecular weight of 115 kDa. Its physiological function has not been elucidated definitively, but it may act as a clearance factor for endotoxin and opsonized bacterial products. Serum levels in patients on HD are five- to 10-fold higher than in healthy controls, and multiple factors currently are discussed as leading to this pronounced inflammatory response. [14] The process may not only be called microinflammation but may also bear an underlying infectious stimulus. Despite recent new and spectacular information on the predictive power of CRP, both in the general population [15] and in kidney failure patients, [16] several questions cannot be answered.

CRP has recently emerged as a useful biomarker for clinical purposes in asymptomatic individuals. [17] This biomarker is accurate, reliable and effective for the early detection of individuals at risk for future vascular events, [18],[19],[20] and explains why most laboratories use high-sensitivity assays (normal concentration <3 mg/L). In fact, these concentrations are not taken any more as "normal" values as it is clear that even individuals with CRP concentrations <3 mg/L have an atherothrombotic disease [21] or are predisposed to future events. [22]

In the field of development of immunological methods, one of the great achievements may be latex particle-enhanced immunoassay of CRP. Traditionally, serum CRP assay was measured by rate immunonephelometry for patients with an acute phase of infection. The nephelometric assay had a sensitivity of 6-10 mg/L. With the particle-enhanced immunonephelometric assay, without labelling or adding any extra reagent and simply by linkage of the CRP antibody onto latex particles, the sensitivity immediately increased from 6 to 10 mg/L to 0.15 to 0.18 mg/L. This more sensitive assay has contributed greatly to monitoring the therapy of cardiovascular diseases. The assay can be performed easily by automated analyzers. Commercial laboratories and industry coined a new name, "high-sensitivity C-reactive protein assay" (hs-CRP) to distinguish between the latex particle-enhanced CRP assay and the traditional "serum CRP" assay.

The results of the present study show that a significant proportion of our HD patients, 30% by routine CRP test (≥6 mg/L) and 54% by hs-CRP test (≥3 mg/L) have elevated levels of CRP.

We found a significant correlation between CRP and hs-CRP. The CRP standard assay performed presents a reasonable alternative to the hs-CRP assay. The advantages of the CRP standard assay are its online and real-time availability as well as lower costs.

Our data also show that in patients on HD, age above 40 years and duration on dialysis greater than five years remained significant independent predictors of serum hs-CRP and CRP. Patients who died during follow-up had higher CRP and hs-CRP levels and were older than those who survived. One of the most important findings in the present study is the association of elevated levels of hs-CRP and CRP with increased risk of all-cause mortality in patients on HD. That finding agrees with previously published reports on the predictive power of CRP in non-ESRD patients, the general population and HD patients. [15],[16] This possible link needs to be further investigated in large-scale randomized clinical trials.

The standard CRP assay that can be performed 24-h a day, seven days a week at relatively low cost, raises the question of how it correlates with the widely used immunoassay that uses the Nephelometer. We were especially interested in individuals with relatively normal CRP concentrations. This interest stems from the following reasons. First, the possibility to perform the test at any time period during the day without additional costs or laboratory personnel and, second, the possibility to use a sensitive assay that covers a standard range of concentrations for individuals who appear in the emergency room with an acute ischemic event including myocardial infarction or stroke. This enables us to use this biomarker for a real-time risk stratification strategy. Thirdly, it offers the possibility to perform the test out of the primary tube at a relatively low cost and on a unified platform, for all biochemical and endo-crinology tests.

The present results are significant in that they show that the CRP assay correlates significantly with the hs-CRP. In fact, the similar correlation between these two assays further supports the notion that the information obtained from both of them is almost interchangeable.

In our study, we found a strong correlation between the two methods. This good correlation was seen in different clinical conditions. Thus, the standard CRP assay could be a useful alternative to the hs-CRP assay for risk evaluation and stratification in developing countries. For several reasons, CRP may be a more sensitive indicator of morbidity than hs-CRP in HD patients.

 
   References Top

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2.Lagrand LK, Visser CA, Hermens WT, et al. C-reactive-protein as a cardiovascular risk more than an epiphenomenon. Circulation, 1999;100:96-102.  Back to cited text no. 2
    
3.Wanner C, Metzger T. C-reactive protein a marker for all-cause and cardiovascular mortality in haemodialysis patients. Nephrol Dial Transplant 2002;17:29-32.  Back to cited text no. 3
    
4.Himmelfarb J, Stenvinkel P, Ikizler A. The elephant of uremia oxidative stress as a unifying concept of cardiovascular disease in uremia. Kidney Int 2002;62:1524-38.  Back to cited text no. 4
    
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15.Danesh J, Whincup P, Walker M, et al. Low grade inflammation and coronary heart disease: Prospective study and updated meta analyses. Br Med J 2000;321:199-204.  Back to cited text no. 15
    
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Correspondence Address:
Imed Helal
Department of Internal Medicine A, and Laboratory of Kidney Pathology 02, Charles Nicolle Hospital, Tunis
Tunisia
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    Figures

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    Tables

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