Table 3.

Primary Use of Estimated GFR using Creatinine or Cystatin C and Urine Albumin-to-Creatinine Ratio and Sources of Error in Interpretation

	eGFRcr	eGFRcys	Urine ACR
Primary use*	Initial test for assessment of GFR	Confirmatory test for assessment of GFR	Initial test for assessment of albuminuria
Non-steady state (AKI)	Change in eGFR lags behind the change in mGFR (eGFR overestimates mGFR when mGFR is declining and underestimates mGFR when mGFR is rising)	Change in eGFR lags behind the change in mGFR (eGFR overestimates mGFR when mGFR is declining and underestimates mGFR when mGFR is rising)	ACR overestimates AER when mGFR is declining (creatinine excretion is decreased) and underestimates AER when mGFR is rising (creatinine excretion is increased)
Non-GFR factors**	Directly measured in clinical studies	Hypothesized from clinical observations and epidemiologic studies	NA
Factors affecting generation	Decreased by large muscle mass, high protein diet, ingestion of cooked meat and creatine supplements Increased by small muscle mass, limb amputation, muscle wasting diseases	Decreased in hyperthyroidism, glucocorticoid excess, and possibly obesity, inflammation and smoking Increased in hypothyroidism	Decreased by large muscle mass (higher urinary creatinine concentration) Increased by small muscle mass (lower urinary creatinine excretion).
Factors affecting tubular reabsorption of secretion	Decreased by drug-induced inhibition of secretion (trimethoprim, cimetidine, fenofibrate)	NA	NA
Factors affecting extra-renal elimination	Decreased by inhibition of gut creatininase by antibiotics Increased by dialysis, large losses of extracellular fluid (drainage of pleural fluid or ascites)	Increased by large losses of extracellular fluid (drainage of pleural fluid or ascites)	NA
Range	Less precise at higher GFR, due to higher biological variability in non-GFR determinants relative to GFR, and larger measurement error in SCr and GFR	Less precise at higher GFR, due to higher biological variability in non-GFR determinants relative to GFR, and larger measurement error in SCysC and GFR	Less precise at lower ACR, due to higher biologic variability in AER, and larger measurement error in urine albumin concentration
Interference with assays	Spectral interferences (bilirubin, some drugs) Chemical interferences (glucose, ketones, bilirubin, some drugs)	NA	Very high urine albumin concentration (“prozone effect”)
Interfering conditions	NA	NA	Contamination with albumin in menstrual blood and lower urinary tract inflammation

^*Reference test for GFR is measured GFR (mGFR) using clearance methods; reference test for albuminuria is albumin excretion rate (AER) in timed urine collection.

^**Effects of factors affecting non-GFR determinants refer to effects on eGFR

Abbreviations: GFR, glomerular filtration rate; ACR, albumin-to-creatinine ratio; AER, albumin excretion rate; SCr, serum creatinine; SCysC, serum cystatin C; AKI, acute kidney injury