OBJECTIVES To allow primary clinicians to detect decline in glomerular filtration rate above 60 milliliters per minute, signaling early kidney impairment before stage-three chronic kidney disease. RESULTS Four patient cases show how serum creatinine referenced to an individual's historical maximum can suggest increased risk, triggering investigation to separate benign processes that alter serum creatinine from true decline in glomerular filtration rate of pre-chronic kidney disease. DISCUSSION/CONCLUSIONS At glomerular filtration rates above 60 milliliters per minute, serial creatinine is more reliable than estimating equations and appears practical for clinical monitoring and early intervention. METHODS We re-examined a standard reference that found low tubular secretion of creatinine at glomerular filtration rates above 80 milliliters per minute and suggested "observation of subtle changes in serum creatinine levels". We used statistics to explain why that method extends to the 80 to 60 milliliters per minute range: expanding the y-axis (to reflect accuracy of modern creatinine assays), fitting a hyperbolic curve, and showing reasonable continuity down to 60 milliliters per minute. We summarized why equations estimating glomerular filtration rate are unsuitable above 60 milliliters per minute. ABBREVIATIONS ASC = adult serum creatinine, CKD = chronic kidney disease, CrCl = creatinine clearance, eGFR = estimated GFR, GFR = glomerular filtration rate, IDMS = isotope-dilution mass spectrometry, KDIGO = Kidney Disease Improving Global Outcomes, m2 = square meters, mGFR = measured GFR, mg/dL = milligrams per deciliter, mL/min = milliliter per minute, umol/L = micromoles per liter, NSAID = nonsteroidal anti-inflammatory drugs, P30 = percentage of estimates within plus or minus 30%, sCr = serum creatinine, sCr-max = maximum sCr to date, sCysC = serum cystatin C, TScr = tubular secretion of creatinine.

The authors have declared no competing interest.

This study did not receive any funding.

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