How is urinary protein excretion determined during the workup for chronic glomerulonephritis?

Updated: Feb 24, 2020
  • Author: Moro O Salifu, MD, MPH, FACP; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

Urinary protein excretion can be estimated by calculating the protein-to-creatinine ratio on a spot morning urine sample. The ratio of urinary protein concentration (in mg/dL) to urinary creatinine (in mg/dL) reflects 24-hour protein excretion in grams. For instance, if the spot urine protein value is 300 mg/dL and the creatinine value is 150 mg/dL, the protein-to-creatinine ratio is 2. Thus, in this example, the 24-hour urine protein excretion is 2 g. [11]

The estimated creatinine clearance rate is used to assess and monitor the glomerular filtration rate (GFR). The following 3 formulas are available for calculation of the GFR:

  • Cockcroft-Gault formula
  • Modification of Diet in Renal Disease (MDRD) Study formula
  • Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) Creatinine Equation

The Cockcroft-Gault formula is simple to use but overestimates the GFR by 10-15% because creatinine is both filtered and secreted. The MDRD formula is much more complex and has been found to underestimate GFR by 6.2% in patients with CKD and by 29% in healthy persons. [28] The CKD-EPI is based on the same four variables as the MDRD Study formula but uses a 2-slope “spline” to model the relationship between estimated GFR and serum creatinine, and a different relationship for age, sex, and race. The National Kidney Foundation (NKF) recommends using the CKD-EPI Creatinine Equation to estimate GFR; a CKD-EPI calculator is available on the NKF Web site.

The estimated creatinine clearance rate is also used to monitor response to therapy and to initiate an early transition to renal replacement therapy (eg, dialysis access placement and transplantation evaluation). The degree of proteinuria, especially albuminuria, helps predict the renal prognosis in patients with chronic glomerulonephritis. Patients with proteinuria exceeding 1 g/day have an increased risk of progression to end-stage renal failure.


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