What are the initial physical assessment and laboratory tests for a patient reporting kidney pain?

Initial Assessment for Patient-Reported Kidney Pain

For a patient presenting with kidney pain, immediately perform urinalysis with microscopy and obtain serum creatinine with eGFR calculation, followed by renal ultrasound to rule out obstruction or structural abnormalities. 1, 2

Physical Examination Priorities

Focused physical examination should assess:

• Hemodynamic status: Blood pressure, heart rate, and signs of volume depletion (dry mucous membranes, decreased skin turgor) or volume overload (peripheral edema, jugular venous distension, pulmonary crackles) 3
• Abdominal examination: Palpate for masses, bladder distension, or costovertebral angle tenderness to identify potential obstructive causes 1, 3
• Urethral meatus inspection: Check for blood, particularly if trauma is suspected, as this indicates potential urethral injury requiring immediate evaluation before catheterization 1

The physical examination alone has significant limitations for differentiating between prerenal and intrarenal causes of kidney dysfunction, particularly when both conditions coexist. 3

Essential Laboratory Tests

Blood tests (obtain immediately):

• Serum creatinine with eGFR calculation using the 2009 CKD-EPI equation—this is the primary marker for kidney function assessment 1, 2
• Complete electrolyte panel: Sodium, potassium, calcium, chloride, phosphorus, and magnesium 2, 4
• Blood urea nitrogen (BUN) to assess severity and calculate BUN/creatinine ratio 4
• Complete blood count (CBC) to evaluate for anemia or infection 1
• C-reactive protein (CRP) if infection or inflammation is suspected 1
• Uric acid and ionized calcium for metabolic stone risk assessment 1

Urine tests (obtain immediately):

• Urinalysis with microscopy: Essential for detecting hematuria, pyuria, casts, crystals, and cells that differentiate causes 1, 2, 4
• Urine albumin-to-creatinine ratio (ACR) from untimed sample to quantify proteinuria 2, 4
• Urine sodium concentration and fractional excretion of sodium (FENa): FENa <1% suggests prerenal cause, while >1% indicates intrarenal tubular damage 2, 3

Important caveat: The diagnostic value of FENa has been questioned in sepsis and other complex clinical scenarios, and the traditional "prerenal" classification can be misleading as it is often misinterpreted as simply "hypovolemic." 3

Imaging Studies

Ultrasound is the primary initial imaging modality and should not delay emergency care if obstruction or severe pathology is suspected. 1

• Renal ultrasound: Assess kidney size, echogenicity, hydronephrosis, and rule out obstruction (sensitivity 45%, specificity 94% for ureteral stones and 88% for renal stones) 1, 2, 4
• Doppler examination: Consider if renovascular causes are suspected based on risk factors 4

Contrast-enhanced CT with delayed urographic phase is the gold standard when:

• Ultrasound is inconclusive or suggests significant pathology 1
• Urolithiasis is strongly suspected (93.1% sensitivity, 96.6% specificity with low-dose protocol) 1
• Trauma is involved with gross hematuria or high-energy mechanism 1
• Collecting system injury or urinary extravasation is suspected 1

Critical pitfall: Avoid iodinated contrast in acute kidney injury unless there is an overriding clinical question that cannot be answered with alternative imaging. 4

Interpretation Framework for Acute Presentations

If hematuria is present:

• Gross hematuria with trauma: Requires immediate contrast-enhanced CT with delayed phase 1
• Microscopic hematuria in children with blunt trauma: Imaging only indicated if ≥50 RBCs/HPF, hypotension present, or high-energy mechanism 1
• Hematuria with suspected stones: Low-dose non-contrast CT is preferred over plain radiography (KUB has only 44-77% sensitivity) 1

If obstruction is suspected:

• Ultrasound showing hydronephrosis warrants CT to determine level and cause of obstruction 2
• Blood at urethral meatus, especially with pelvic fracture, requires retrograde urethrography before catheterization 1

If infection is suspected:

• Fever with flank pain requires immediate evaluation with blood cultures, urinalysis, and imaging to rule out pyelonephritis or perinephric abscess 1

Differentiating Acute from Chronic Kidney Disease

A single abnormal test is insufficient—persistence of abnormalities for >3 months is required to diagnose chronic kidney disease. 1, 2, 4

• Review past measurements of creatinine and eGFR to determine duration of kidney dysfunction 1
• Small echogenic kidneys on ultrasound suggest chronic disease, though normal-sized kidneys do not exclude CKD (diabetic nephropathy and infiltrative disorders initially preserve renal size) 2
• Cystatin C measurement should be considered as confirmatory testing when eGFR based on creatinine may be inaccurate (muscle wasting, extremes of body mass) 2, 4

Special Populations

Pregnant women: Use ultrasound as first-line, MRI as second-line, and reserve low-dose CT as last option 1

Children: Ultrasound is first-line imaging; reserve low-dose CT for cases where ultrasound is inadequate or high-energy trauma with hematuria ≥50 RBCs/HPF 1

Patients with solitary kidney: Require immediate comprehensive evaluation regardless of pain severity 1