Background/Purpose: It has previously been demonstrated that the ESR and CRP correlate in patients with RA but with a high degree of variability.  Reasons for this variability are not completely understood, but anemia, adiposity, and age can play a role. It has previously been suggested that ESR and CRP are inversely related to renal function in a non-RA population; it is felt that underlying inflammation is the explanation for the negative correlation with the CRP, specifically.  The aim of this study is to see if the CRP and/or ESR inversely correlate with renal function in patients with RA.  

Methods: This is a retrospective chart review of RA patients followed at USF Health.  Subjects were identified by ICD-9 code (714.0) and they had to have a CRP, ESR, hemoglobin (Hgb) and eGFR performed on the same day.  The most recent labs were utilized and no patient was included twice.  Patients were excluded if they were <18 years of age or had a history of blood cancers known to affect inflammatory markers.  The co-primary endpoints were the correlation between CRP and eGFR as well as ESR and eGFR.  Secondary endpoints included correlation between CRP and ESR as well as both inflammatory markers with Hgb and the patients’ age.  Spearman correlation coefficients (rho) were utilized to determine significance.   

Results: 158 patients met the inclusion criteria; 136 (86%) were females with a mean age of 58.82 years (+/- 11.44 SD; range 26-84 years).  93/130 (71.5%) of these subjects were seropositive; status was unknown on 28 patients.  The mean CRP and ESR in these subjects was 0.89mg/dL (+/- 1.37mg/dL SD) and 16.07mmHg (+/- 16.61mmHg SD), respectively; the mean eGFR and Hgb was 82.76 mL/min/1.73m² (+/- 20.92 mL/min/1.73m²  SD) and 12.77g/dL (+/- 1.31g/dL SD), respectively.  There was no correlation between the CRP and the eGFR (rho = 0.041; p = 0.60) [Fig 1]; there was also no correlation between the ESR and the eGFR (rho = -0.037; p = 0.64) [Fig 1].  There was a correlation between the CRP and ESR but with a high degree of variability (rho = 0.596; p < 0.0001) [Fig 1].  The ESR correlated weakly with Hgb and it did so in an inverse fashion (rho = -0.348; p<0.0001); the CRP did not appear to correlate with the Hgb (rho = -0.115; p = 0.15).  There was no correlation with either CRP or ESR and age (rho = -0.075 and 0.006 respectively). 

Conclusion: This retrospective chart review of 158 patients with RA suggests there is no correlation between CRP and/or ESR with renal function.  Renal function does not appear to be an explanation for some of the variability that is documented between CRP and ESR in patients with RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/comparison-of-c-reactive-protein-crp-and-erythrocyte-sedimentation-rate-esr-with-estimated-glomerular-filtration-rate-egfr-in-patients-with-rheumatoid-arthritis-ra/