Screening for renal disease using serum creatinine: who are we missing?

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Abstract
Background. Appropriate management and timely referral of patients with early renal disease often depend on the identification of renal insufficiency by primary care physicians. Serum creatinine (SCr) levels are frequently used as a screening test for renal dysfunction; however, patients can have significantly decreased glomerular filtration rates (GFR) with normal range SCr values, making the recognition of renal dysfunction more difficult. This study was designed to estimate the prevalence of patients who have significantly reduced GFR as calculated by the Cockcroft–Gault (C‐G) formula, but normal‐range SCr. Methods. The study included 2781 outpatients referred by community physicians to an urban laboratory network for SCr measurement. GFR was estimated using the C‐G formula. Patients were grouped according to the concordance of SCr level abnormalities (abnormal >130 μmol/l) with significantly abnormal C‐G values (abnormal ⩽50 ml/min). The C‐G value of ⩽50 ml/min was chosen to reflect substantial renal impairment in all age groups. A further analysis of historical laboratory data was undertaken to determine if there were previously documented changes in renal function parameters in those patients who had overt renal dysfunction during the study period. Results. Of the 2781 outpatients referred, 2543 (91.4%) had normal SCr levels. Of these patients, 387/2543 (15.2%) had C‐G calculated GFR ⩽50 ml/min, representing substantially impaired renal function. Among patients with normal SCr, abnormal C‐G values were identified in 47.3% ⩾70 years old, 12.6% 60–69 years old, and 1.2% 40–59 years old. Analysis of historical available laboratory data for patients with abnormal SCr and abnormal C‐G values showed that 2 years prior to the study period, 72% of this group had abnormal SCr, while 18% had normal SCr with abnormal C‐G values, and 10% had normal SCr with normal C‐G values. Conclusions. This study documents the substantial prevalence of significantly abnormal renal function among patients identified by laboratories as having normal‐range SCr. Including calculated estimates of GFR in routine laboratory reporting may help to facilitate the early identification of patients with renal impairment.