Kidney Function, Proteinuria, and Adverse Outcomes

To the Editor: Dr Hemmelgarn and colleagues¹ analyzed the relationship between kidney function and proteinuria and the risks of mortality, myocardial infarction, and progression to kidney failure using a community-based cohort study with 920 985 adults. They pointed out the problem of the current staging of chronic kidney disease (CKD) without consideration of the severity of proteinuria.

Although the conclusion of the importance of proteinuria is reasonable, we are concerned about the presentation of doubling of serum creatinine for the comparison of the decline of renal function among the groups. Doubling of serum creatinine should be used for comparison among patients with similar renal function. The reciprocal of serum creatinine is useful for the estimation of renal function.² Doubling of serum creatinine means the loss of 50% of renal function. It does not seem correct to consider as comparable the time for increase in serum creatinine from 1 to 2 mg/dL and from 4 to 8 mg/dL (to convert to μmol/L, multiply by 88.4). If the decline of renal function is stable over time and baseline serum creatinine is 1 mg/dL, the increase in serum creatinine from 1 to 2 mg/dL and from 4 to 8 mg/dL means a loss to 50% and 12.5% of baseline renal function, respectively. Thus, the time for increase of serum creatinine from 4 to 8 mg/dL would be expected to be one-quarter that from 1 to 2 mg/dL in a patient with stable decline of renal function.