Renal Function

DESCRIPTION

• Serum creatinine: 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.
• GFR: simplest measure of renal function; 24 hour urine collection not required. Elevated GFR indicates hyperfiltration, an early predictor of subsequent diabetic renal disease. GFR then declines as renal disease progresses. The four variable MDRD equation calculates GFR based on serum creatinine, age, race and gender. The six variable expanded MDRD equation also includes blood urea nitrogen (BUN) and serum albumin. The Cockcroft-Gault equation calculates GFR based on serum creatinine, weight, gender and age. It is of limited value in patients with obesity in whom weight may not reflect muscle mass.
• The reciprocal serum creatinine curve useful but cumbersome, plotting change in creatinine over time. Sudden acceleration of the slope of deterioration suggests a potentially reversible component such as volume depletion resulting in acute on chronic kidney injury. 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").
• Microalbuminuria: earliest evidence of diabetic nephropathy, especially in type I diabetes, and should be monitored routinely in diabetes
• The spot urine albumin/creatinine ratio to assess microalbuminuria preferred for monitoring the impact of therapeutic interventions such as renin-angiotensin blockade on proteinuria. Twenty-four hour urine protein collections rarely required.
• Microscopic urinalysis: a bland sediment may be seen with either pre-renal or post-renal failure. Glomerulonephritis: documented proteinuria and hematuria with dysmorphic rbc’s, 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: include 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. (see Limitations)