Blog entry by Meguid El Nahas
An article by Ix and colleagues in the January 2011 issue of CJASN describes new equations to calculate urine creatinine excretion rate.
Urine creatinine excretion is increasingly relied upon to calculate the degree of proteinuria/albuminuria through the reporting of urine protein:creatinine ratio (PCR) or albumin:creatinine ratio (ACR). Unfortunately, such formulation has a number of problems including the overestimation of proteinuria/albuminuria in patients whose urine creatinine excretion is low due to a number of factors including muscle wasting and malnutrition. In other words, a wasted individual can have a raised PCR/ACR not due to increase proteinuria/albuminuria but instead due to decreased urine creatinine excretion.
Ix and colleagues attempt to address this issue by developing equation correcting urine creatinine excretion rate to take into account the individual's age, gender, weight and race; all potential confounders of serum and urine creatinine levels. This approach is comparable to that used for estimation of creatinine-derived GFR (eGFR) and makes good sense.
Estimated urine creatinine excretion (eCER) equations:
eCER: 879.89 + 12.51 x weight (kg) - 6.19 x age + (34.51 if black) - (379.42 if female)
eCER: 1115.89 + 11.97 x weight (kg) - 5.83 x age -60.18 x phosphorus (mg/dl) - (52.82 if black) - (368.75 if female)
Correction of urine creatinine excretion rate to account for commonly available variables is an important step towards improving the estimation of PCR and ACR and enhancing the diagnostic accuracy of these ratios.