Serum creatinine-based estimating equations and spot urine albumin concentrations have begun to replace the outpatient collection of timed urine specimens for assessment of GFR and proteinuria, respectively. However, timed urine collections are at times necessary to perform as confirmatory tests. These timed urine collections are often over- or under-collected and a useful assessment of the accuracy of the individual collections can be usually accomplished by comparing the measured urinary creatinine excretion rate (CER) to an individual’s expected CER.
The most commonly used equation to estimated creatinine excretion rate (CER) was developed by Imbembo and Walser in 1984. The equation is:
If male: eCERWalser= (28.2 – 0.172 X age)
If female: eCERWalser = (21.9 – 0.115 X age)
However, this equation has such extreme “acceptable” normal ranges that the range can be as great as 65% for men and 100% for women. Due to these large ranges of “acceptable” normal the timed urine collections are typically only deemed inaccurate when grossly under- or over-collected.
In 2011, CKD EPI developed two equations to estimate CER using data from three kidney disease studies and validated them in three separate studies. The equations are:
CKD-EPI eCER1 = 879.89 +12.51 x weight (kg) – 6.19 x age + (34.51 if black) – (379.42 if female)
CKD-EPI eCER2 = 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)
Compared to the Walser and other existing equations, the new equations have similar accuracy, but showed less bias in individuals. The percentage of estimates within 30% of measured creatinine excretion was 78 (75, 80)% for Walser, 79 (76, 81)% for CKD-EPI eCER1 and 79 (76, 81)% for CKD-EPI eCER2. The median difference beween measured and estimates creation excretion rate was – 28 (-52, 5) mg/day for Walser compared to – 10 (-32, 12)mg/day for CKD-EPI eCER1 and 4 (-31, 38) mg./dayfor CKD-EPI eCER2 .