Evaluation of glomerular filtration rate and of albuminuria/proteinuria.

Kidney function should be evaluated by procedures including the calculation of glomerular filtration rate (GFR) estimates and the assessment of albuminuria or proteinuria as creatinine-normalized urinary ratios for albumin or total protein. GFR estimates are an approximation of true GFR, which circumvent the limitations of serum creatinine and creatinine clearance without increasing costs and time of diagnostic work-up. Estimates by Cockcroft-Gault equation tend to be higher than true GFR and estimates by other equations, because this equation predicts creatinine clearance, hence true GFR plus creatinine excretion via tubular secretion. The inclusion of a weight coefficient in the equation causes a GFR overestimation in the presence of large adiposity or edema. Estimates by equations of the Modification of Diet in Renal Disease (MDRD) study can be unreliable for high-normal GFR because that study did not enroll individuals without kidney disease. The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) group has reported a new equation to overcome this limitation. GFR estimates can be biased by interassay creatinine differences or unusual levels of creatinine generation (muscle mass) or of renal tubular creatinine secretion. The urinary ratio of albumin (or total protein) to creatinine is measurable in untimed spot urine and reflects the urinary excretion rate of albumin (or total protein). Low muscle mass could imply borderline elevation in the ratio merely because of low urinary creatinine. Vice versa, high muscle mass could imply normal ratios even in the presence of high urinary albumin, because of high urinary creatinine due to high creatinine generation.