What are the possible causes of left flank pain during urination?

Left Lower Back Pain During Urination: Differential Diagnosis

The most likely cause of left lower back (flank) pain during urination is urolithiasis (kidney stone), but you must also consider urinary tract infection with pyelonephritis, and less commonly, other urinary tract pathology. 1

Primary Differential Diagnoses

Urolithiasis (Kidney Stone) - Most Common

• This is the most frequent cause of acute flank pain in patients presenting with urinary symptoms 1, 2, 3
• Stones can occur anywhere from the kidney to the bladder, with ureteral stones being particularly symptomatic 1
• The pain typically occurs when stones cause obstruction and is often severe, colicky in nature 1
• Hematuria (blood in urine) is commonly present but not always visible 1, 3

Acute Pyelonephritis (Kidney Infection)

• Presents with flank pain, fever, and urinary symptoms including dysuria (painful urination), urgency, and frequency 1
• Costovertebral angle (CVA) tenderness is characteristic on physical examination 1
• Often accompanied by systemic symptoms: fever, chills, nausea, vomiting 1
• More common in women but can occur in men, particularly with anatomical abnormalities 4

Complicated Urinary Tract Infection

• Any infection beyond the bladder that involves anatomical or functional abnormalities of the urinary tract 1
• Risk factors include: obstruction, incomplete bladder emptying, diabetes, immunosuppression, recent urological procedures 1
• May present with similar symptoms to pyelonephritis but with additional complicating factors 1

Critical Diagnostic Approach

Immediate Evaluation Required

You need urgent evaluation if you have any of the following 1:

• Fever (suggests infection requiring immediate treatment)
• Solitary kidney (only one functioning kidney)
• Severe, unrelenting pain
• Inability to urinate (suggests complete obstruction)

Recommended Diagnostic Pathway

First-line imaging: Ultrasound 1

• Should be performed as the primary diagnostic tool 1
• Sensitivity of 45% for detecting stones but 94% specificity 1
• Can identify hydronephrosis (kidney swelling from obstruction) 1
• Safe, no radiation exposure 1

Second-line imaging: Non-contrast CT scan 1, 3

• This is the gold standard for acute flank pain after ultrasound 1
• Detects ureteral stones with 98-100% sensitivity and specificity regardless of size, location, or composition 3
• Identifies alternative diagnoses in approximately one-third of patients presenting with acute flank pain 3
• Low-dose CT protocols maintain 93.1% sensitivity and 96.6% specificity while reducing radiation exposure 1

Laboratory testing required 1:

• Urinalysis (dipstick): check for blood, white blood cells, nitrites 1
• Blood tests: creatinine (kidney function), complete blood count, C-reactive protein (inflammation marker) 1
• Note: Pyuria (white blood cells in urine) alone has very low positive predictive value and can indicate non-infectious inflammation 1

Urine culture indications 1:

• Suspected pyelonephritis (kidney infection)
• Complicated UTI
• Recurrent infections
• NOT routinely needed for simple uncomplicated cystitis in healthy patients 1

Important Clinical Pitfalls

Don't Anchor on Kidney Stones Alone

Flank pain and hematuria is not always a kidney stone 5

• Rare but serious alternative diagnoses exist, including Page kidney (subcapsular hematoma causing hypertension), eosinophilic ureteritis, and vascular abnormalities 5, 6
• Always consider imaging to confirm diagnosis rather than assuming stone disease 5

Distinguishing Infection from Stone Disease

• The presence or absence of flank pain alone does not reliably differentiate pyelonephritis from prostatitis in men 4
• Fever, systemic symptoms, and elevated inflammatory markers (CRP, white blood cell count) suggest infection over uncomplicated stone disease 1, 4
• Both conditions can coexist—stones can cause obstruction leading to infection 1

When Conservative Management Is Insufficient

Immediate intervention is indicated for 1:

• Infected obstructed kidney (fever + obstruction = urological emergency)
• Solitary kidney with obstruction
• Bilateral obstruction
• Acute kidney injury from obstruction
• Uncontrolled pain despite appropriate analgesia

Pain Management Considerations

NSAIDs (diclofenac, ibuprofen) are first-line treatment for renal colic 1

• More effective than opioids and reduce need for additional analgesia 1
• Use lowest effective dose due to cardiovascular and gastrointestinal risks 1