The study examined the accuracy of equations for estimating glomerular filtration rate (GFR) in patients with moderate chronic kidney disease over three years. Median measured GFR decreased from 48.1 to 43.6 mL/min/1.73 m², and all equations matched the change in measured GFR in more than 72.5% of participants. Equations combining creatinine and cystatin C (dual biomarker equations) showed better agreement with change in measured GFR—for example, CKD-EPI creatinine-cystatin achieved agreement in 78.6% of subjects compared with 73.1% for an equation based on creatinine alone. All equations had low sensitivity (less than 54.1%) in identifying progression of renal disease, but good specificity (greater than 90.4%). Progression of kidney disease was observed in 139 individuals (15.9%). The results suggest that equations including both biomarkers more accurately monitor GFR change than equations based on a single biomarker.