| Summary: | <p><strong>Background:</strong> Monitoring is the mainstay of chronic kidney disease management in primary care. There is little evidence on how best to monitor.</p>
<p><strong>Aim:</strong> To compare the effectiveness of eGFR derived from creatinine or cystatin C, to predict renal function decline among those with a recent eGFR of 30-89 ml/min/1.73m².</p>
<p><strong>Design and setting:</strong> Observational cohort study in UK primary care.</p>
<p><strong>Method:</strong> In 750 adult patients with a recent estimated glomerular filtration rate (eGFR) of 30-89 ml/min/1.73m² both creatinine and cystatin C were measured at seven study visits over two years. The primary outcome was change in eGFR derived from creatinine or cystatin C between 6 and 24 months.</p>
<p><strong>Results:</strong> Average change in eGFR was 0.51 ml/min/1.73m²/year or -2.35 ml/min/1.73m²/year when estimated by creatinine or cystatin C respectively. The c-statistic for predicting renal decline using creatinine-derived eGFR was 0.495 (95% CI 0.471 to 0.519). The equivalent c-statistic using cystatin C-derived eGFR was 0.497 (95% CI 0.468 to 0.525). Similar results were obtained when restricting analyses to those over or under 75 years, or with eGFR above 60 ml/min/1.73m2. In those with eGFR below 60 ml/min/1.73m2 cystatin C-derived eGFR was more predictive than creatinine-derived eGFR for future decline.</p>
<p><strong>Conclusion:</strong> In the primary analysis neither eGFR estimated from creatinine nor cystatin C predicted future change in kidney function, partly due to small changes during two years. In some secondary analyses there was a suggestion that cystatin C to estimate eGFR was a more useful biomarker, especially in those with baseline eGFR < 60 ml/min/1.73m2.</p>
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