What is the appropriate evaluation and management for a patient presenting with left flank pain radiating to the left abdomen and decreased urine output?

Evaluation and Management of Left Flank Pain with Decreased Urine Output

Begin with ultrasound as the primary diagnostic tool, followed by non-contrast CT if ultrasound is inconclusive or shows moderate-to-severe hydronephrosis, while simultaneously obtaining urinalysis, serum creatinine, and initiating pain control with NSAIDs. 1

Immediate Diagnostic Workup

Initial Laboratory Assessment

Obtain the following tests immediately:

• Urinalysis (dipstick) - check for hematuria, infection, crystals
• Serum creatinine - assess for acute kidney injury (especially critical with decreased urine output)
• Complete blood count and CRP - evaluate for infection/inflammation
• Serum calcium, uric acid, sodium, potassium - metabolic evaluation 1

The decreased urine output is particularly concerning and may indicate:

• Obstructive uropathy (unilateral obstruction with contralateral compensation, or bilateral disease)
• Acute kidney injury from obstruction
• Volume depletion from pain/nausea

Imaging Algorithm

Step 1: Ultrasound (US) - First-Line Imaging

• Sensitivity: 45% for ureteral stones, 88% for renal stones
• Specificity: 94% for ureteral stones 1
• US can detect hydronephrosis with high accuracy (sensitivity 88%, specificity 85% for predicting obstructing calculi) 2
• If moderate-to-severe hydronephrosis is present on US in a patient with high clinical suspicion for stone disease, this provides strong evidence for ureteral calculus without requiring immediate CT 2

Step 2: Non-Contrast CT - Follow-Up Imaging Non-contrast CT abdomen/pelvis is indicated when:

• US is inconclusive or technically limited
• Moderate-to-severe hydronephrosis is detected on US (to determine stone size/location for treatment planning) 2
• Alternative diagnosis is suspected
• Sensitivity/specificity: 98-100% for detecting ureteral stones regardless of size, location, or composition 1
• Use low-dose CT protocol (sensitivity 93.1%, specificity 96.6%) to minimize radiation exposure 1

Important Caveat: The presence of hydronephrosis on US reduces the likelihood of alternative diagnoses (OR 0.31), making stone disease highly probable 2. However, 15% of patients with flank pain and negative urinalysis without stone history may have alternative diagnoses better characterized by CT 2.

Pain Management

Initiate NSAIDs immediately as first-line treatment:

• Diclofenac, ibuprofen, or metamizole
• NSAIDs are superior to opioids for renal colic and reduce need for additional analgesia 1
• Use lowest effective dose due to cardiovascular and gastrointestinal risks 1

Critical Decision Points

Indications for Urgent/Emergency Intervention

Immediate evaluation and potential intervention required if:

• Solitary kidney (risk of complete renal failure)
• Fever (suggests infected obstructed system - pyonephrosis - requires urgent decompression)
• Bilateral obstruction (suggested by elevated creatinine with decreased urine output)
• Acute kidney injury (elevated creatinine beyond baseline) 1

Risk Stratification

High-risk features requiring more aggressive workup:

• Moderate-to-severe hydronephrosis (higher risk of stone passage failure) 2
• Elevated creatinine
• Fever/signs of infection
• Persistent symptoms despite adequate analgesia

Alternative Diagnoses to Consider

While urolithiasis is most common, CT may identify alternative causes in approximately one-third of patients with acute flank pain 3:

• Pyelonephritis (especially if fever present)
• Renal infarction (consider in patients with cardiovascular disease/atrial fibrillation) 4
• Abdominal aortic aneurysm (especially in elderly patients) 5
• Adrenal pathology (rare but can present with flank pain) 6

Treatment Planning Based on Stone Characteristics

If stone disease confirmed:

• Stone <5mm: High likelihood of spontaneous passage, conservative management appropriate
• Stone 5-10mm: Intermediate likelihood of passage, consider medical expulsive therapy
• Stone >10mm or >20mm: Low likelihood of spontaneous passage, likely requires intervention 1

Common Pitfall: Do not delay imaging or intervention in patients with decreased urine output and elevated creatinine - this suggests significant obstruction requiring urgent evaluation and possible decompression to prevent permanent nephron loss 2.