Johns Hopkins Diabetes GuideClinical TestsRenal

Renal Function

Daphne H. Knicely, M.D. , Donna Myers, M.D.
Renal Function is a topic covered in the Johns Hopkins Diabetes Guide.

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DESCRIPTION

  • Serum creatinine (Cr) : Useful and convenient measure to monitor renal status. Serum creatinine in the normal range may not reflect normal glomerular filtration rate (GFR). Age, gender and muscle mass need to be considered.
    • Cr in the normal range may not reflect normal glomerular filtration rate (GFR). Age, gender and muscle mass need to be considered. [17]
  • GFR : Simplest measure of renal function; 24-hour urine collection not required.
    • GFR is equal to the sum of the filtration rates in all functioning nephrons so the GFR gives a rough measure of the number of functioning nephrons.
    • Normal depends on age, sex and body size; approximately 130 mL/min/1.73m 2 for men and 120 mL/min/1.73m 2 for women. [18]
    • Elevated GFR indicates increased filtration (hyperfiltration), an early predictor of subsequent diabetic kidney disease (DKD). There is not an exact correlation between loss of kidney mass and the loss of GFR. The kidney adapts to the loss of some nephrons by compensatory hyperfiltration and/or increasing solute and water reabsorption. GFR declines as renal disease progresses in DKD .
    • The most common methods utilized to estimate the GFR are measurement of Cr clearance and estimation equations. There are multiple equations designed to estimate the GFR which include: [10]
      • The Cockcroft-Gault calculates GFR based on Cr, weight, gender and age. It is of limited value in patients with obesity in whom weight may not reflect muscle mass.
      • The Modification of Diet in Renal Disease (MDRD) study equations, four-variable and six-variable, calculates GFR based on Cr, age, race and gender. The six-variable equation includes blood urea nitrogen (BUN) and serum albumin.
      • The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation calculates GFR based on Cr, age, race and gender.
      • The four-variable MDRD and CKD-EPI equations are increasingly utilized.
    • There is an association between age and decreasing GFR. [2]
  • The reciprocal Cr curve (plotting change in Cr over time) is useful but cumbersome. Sudden acceleration of the slope of deterioration suggests a potentially reversible component such as volume depletion resulting in acute on chronic kidney disease. Similarly, a successful intervention such as blood pressure or glycemic control may flatten the curve over time.
  • BUN : Varies inversely with GFR, but can be affected by factors other than GFR (see "Limitations").
  • Urinary albumin excretion (albuminuria): Earliest evidence of DKD, especially in type I diabetes, and should be monitored routinely in diabetics.
    • The spot urine albumin/Cr ratio to assess albuminuria is preferred for monitoring the impact of therapeutic interventions such as renin-angiotensin blockade on proteinuria. Twenty-four hour urine protein collections rarely required. [16]
  • Microscopic urinalysis : [4] [7]
    • A bland sediment may be seen with either pre-renal or post-renal failure.
    • Glomerulonephritis : Documented proteinuria and hematuria with dysmorphic RBCs, RBC casts, granular casts and lipiduria.
    • Nephrosis : Proteinuria and lipiduria with a bland sediment.
    • Infection : Dipstick positive leukocyte esterase and nitrites and confirmed by the microscopic finding of pyuria, hematuria and bacteriuria with or without WBC casts and a positive urine culture.
    • Acute interstital nephritis : WBCs and WBC casts without infection; finding eosinophiluria confirms suspicion of an allergic reaction.
    • Acute tubular necrosis : Muddy brown granular casts.
  • Radiographic examinations : Renal and bladder ultrasound to detect anatomic changes; functional nuclear medicine scans for split function, GFR and obstruction; and CT angiography, renal MRA or Doppler flow studies for renal artery stenosis. [20] (see Limitations)

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Last updated: October 6, 2015