What is the initial workup for a patient presenting with impaired renal function?

Initial Workup for Renal Failure

Begin with serum creatinine, blood urea nitrogen, complete blood count, urinalysis with microscopy, and renal ultrasound to categorize the cause as prerenal, intrinsic renal, or postrenal. 1

Essential Laboratory Tests

Blood Tests

• Serum creatinine with calculated eGFR using the 2009 CKD-EPI equation is the primary assessment of kidney function 2, 1
• Complete blood count to evaluate for anemia and infection 2
• Serum electrolytes including sodium, potassium, calcium, magnesium, chloride, and phosphorus to identify life-threatening imbalances 2, 1
• Blood urea nitrogen (BUN) with calculation of BUN-to-creatinine ratio (>20:1 suggests prerenal cause, <10:1 suggests intrinsic renal disease) 2, 3
• Glucose and fasting lipid profile 2
• Liver function tests 2
• Thyroid-stimulating hormone 2

Urine Tests

• Urinalysis with microscopy to detect cells, casts, and crystals—this is critical for differentiating causes 2, 1, 4
  • Muddy brown casts or renal tubular epithelial cell casts indicate acute tubular necrosis 5, 4
  • Dysmorphic red blood cells and red cell casts suggest glomerulonephritis 5
  • White blood cell casts suggest acute interstitial nephritis or pyelonephritis 4
• Urine albumin-to-creatinine ratio (ACR) from a spot urine sample to quantify proteinuria 2, 1
• Fractional excretion of sodium (FENa) to distinguish prerenal (<1%) from intrinsic renal (>1%) causes 5, 3, 4

Imaging

• Renal ultrasound is the initial imaging study of choice to assess kidney size, echogenicity, and rule out obstruction 5, 1, 4
• Small, echogenic kidneys suggest chronic kidney disease, while normal-sized kidneys are typical in acute kidney injury 2, 4
• Unenhanced CT of abdomen/pelvis if ultrasound shows hydronephrosis to determine level and cause of obstruction 1

Clinical Assessment

History

• Medication review focusing on nephrotoxins (NSAIDs, aminoglycosides, ACE inhibitors, ARBs, contrast agents) 4
• Recent procedures involving contrast media 2
• Symptoms of volume depletion (vomiting, diarrhea, decreased oral intake) or volume overload (dyspnea, edema) 4
• Systemic symptoms suggesting vasculitis or infection (fever, rash, arthralgias) 2, 4
• Urinary symptoms (dysuria, frequency, hematuria, decreased urine output) 4

Physical Examination

• Volume status assessment including jugular venous pressure, orthostatic vital signs, mucous membranes, skin turgor, and presence of edema 2, 4
• Weight measurement for serial monitoring 2
• Skin examination for rashes suggesting systemic disease 2
• Cardiovascular and pulmonary examination for signs of fluid overload 2

Diagnostic Algorithm

1. Measure serum creatinine and calculate eGFR to confirm renal dysfunction 1
2. Obtain urinalysis with microscopy and calculate FENa to categorize as prerenal, intrinsic, or postrenal 5, 3, 4
3. Perform renal ultrasound to rule out obstruction and assess kidney size 5, 1
4. Check electrolytes, particularly potassium, as hyperkalemia >5.7 mEq/L requires urgent management 2, 4
5. If FENa <1% with bland urine sediment, suspect prerenal azotemia and optimize volume status 3, 4
6. If FENa >1% with abnormal sediment, suspect intrinsic renal disease and consider nephrology referral 4
7. If hydronephrosis present, obtain CT to identify obstruction level and consider urology referral 1

Additional Testing in Select Cases

• Cystatin C when eGFR based on creatinine may be inaccurate (extremes of muscle mass, malnutrition) 1
• Screening for hemochromatosis or HIV in selected patients with unexplained renal failure 2
• Tests for rheumatologic diseases, amyloidosis, or pheochromocytoma when clinically suspected 2
• Renal biopsy when glomerular disease is suspected, particularly with significant proteinuria, red cell casts, or unexplained acute kidney injury 2

Critical Monitoring

• Serial monitoring of serum electrolytes and renal function is essential, particularly when initiating diuretics or renin-angiotensin-aldosterone system inhibitors 2
• Assess creatinine up to day 3 after contrast exposure to detect contrast-induced nephropathy 2
• Patients with progressive renal insufficiency should be referred to nephrology, particularly when eGFR <30 mL/min/1.73m² 2

Important Caveats

• A single abnormal result is insufficient for chronic kidney disease diagnosis; abnormalities must persist >3 months 2, 1
• Certain medications interfere with creatinine measurements (trimethoprim, cimetidine), affecting eGFR accuracy 1
• Avoid iodinated contrast in acute kidney injury unless absolutely necessary for diagnosis 2, 1
• Normal kidney size does not exclude chronic kidney disease in diabetic nephropathy or infiltrative disorders 1