One-liner#

Flank pain requires differentiation between nephrolithiasis, pyelonephritis, and musculoskeletal causes—with urgent evaluation if infection plus obstruction is suspected.

Quick nav#

• Red flags / send to ED
• Key history
• Focused exam
• Differential (quick pattern recognition)
• Workup
• Initial management
• Management by diagnosis
• Follow-up
• Patient instructions
• Smartphrase snippets

Red flags / send to ED#

• Fever + flank pain + unable to urinate → infected obstructing stone (pyonephrosis); urologic emergency
• Fever + flank pain + sepsis signs (hypotension, tachycardia, altered mental status) → urosepsis; ED immediately
• Flank pain + pulsatile abdominal mass → ruptured or expanding AAA; call 911
• Severe pain uncontrolled with oral medications → may need IV pain control
• Solitary kidney with suspected obstruction → urgent imaging and urology
• Pregnant with flank pain + fever → pyelonephritis in pregnancy requires hospitalization

Key history#

Characterize the pain:

• Location: unilateral vs bilateral; radiation to groin, testicle, labia
• Quality: colicky/crampy (stone) vs constant/dull (infection, MSK)
• Severity: renal colic is classically severe (10/10)
• Timing: sudden onset (stone) vs gradual (infection, MSK)
• Position: worse with movement (MSK) vs no positional component (renal)

Associated symptoms:

• Nausea/vomiting → common with renal colic
• Dysuria, frequency, urgency → UTI, pyelonephritis
• Hematuria → stone, tumor, infection
• Fever/chills → pyelonephritis, infected stone
• Vaginal/urethral discharge → consider STI with ascending infection

Stone risk factors:

• Prior kidney stones (50% recurrence within 5 years)
• Family history of stones
• Dehydration, low fluid intake
• Diet: high sodium, high protein, low calcium (paradoxically)
• Medications: topiramate, indinavir, high-dose vitamin C
• Medical conditions: gout, hyperparathyroidism, obesity, diabetes
• GI conditions: Crohn’s, gastric bypass (oxalate stones)

Infection risk factors:

• Recent UTI
• Urinary catheter or instrumentation
• Diabetes, immunosuppression
• Anatomic abnormalities
• Pregnancy

MSK considerations:

• Recent lifting, twisting, exercise
• History of back problems
• Reproducible with movement or palpation

Focused exam#

Vital signs:

• Fever → infection (pyelonephritis, infected stone)
• Tachycardia → pain, dehydration, sepsis
• Hypotension → sepsis, dehydration

Abdominal exam:

• CVA tenderness → pyelonephritis, stone, renal mass
• Abdominal tenderness → consider other intra-abdominal pathology
• Pulsatile mass → AAA (especially if >50, smoker, hypertensive)
• Suprapubic tenderness → cystitis, retention

Back/MSK exam:

• Paraspinal tenderness → MSK cause
• Pain with range of motion → MSK
• Straight leg raise → radiculopathy

GU exam:

• Testicular exam in men (referred pain from stone can mimic testicular pathology)
• CVA tenderness is the key finding for renal pathology

General:

• Patient appearance: writhing/unable to get comfortable (stone) vs lying still (peritonitis)
• Signs of dehydration

Differential (quick pattern recognition)#

Common/likely (outpatient)#

Can’t-miss / urgent#

Workup#

Suspected nephrolithiasis:

• CT non-contrast (low-dose stone protocol): gold standard; sensitivity >95%
• Renal US: first-line in pregnancy, children, or if avoiding radiation; less sensitive for ureteral stones
• UA: hematuria supports diagnosis but absence doesn’t rule out stone
• BMP: assess renal function, especially if obstruction suspected
• Urine culture: if any concern for concurrent infection

Suspected pyelonephritis:

• UA and urine culture (always culture for pyelo)
• BMP: assess renal function
• CBC: if diagnostic uncertainty or assessing severity
• Blood cultures: if septic-appearing or immunocompromised
• Imaging: NOT routine; consider CT if: not improving in 48-72 hours, suspected abscess, anatomic abnormality, recurrent infections

When to image urgently:

• Fever + obstructing stone suspected
• Solitary kidney
• Bilateral obstruction
• Severe uncontrolled pain
• Pregnancy with concern for obstruction

When NOT to order:

• Do NOT order CT for obvious MSK pain with no red flags
• Do NOT delay antibiotics for imaging in clear pyelonephritis
• Do NOT order KUB alone—misses most stones and provides little useful information
• Do NOT repeat CT for known stone unless clinical change or intervention planned

Initial management#

Nephrolithiasis (uncomplicated):

• Pain control: NSAIDs first-line (ketorolac or ibuprofen)
• Hydration: encourage oral fluids (IV not proven to speed passage)
• Alpha-blocker: tamsulosin 0.4 mg daily for distal ureteral stones <10mm
• Strain urine: provide strainer; send stone for analysis if caught
• Antiemetic: ondansetron or promethazine PRN

Pyelonephritis (outpatient criteria met):

• Oral fluoroquinolone x 7 days (or TMP-SMX x 14 days if susceptible)
• Ensure patient can tolerate PO and has reliable follow-up
• Phone check in 24-48 hours

Infected stone (fever + obstruction):

• ED immediately
• Needs IV antibiotics AND urologic decompression (stent or nephrostomy)
• Do NOT attempt outpatient management

MSK flank pain:

• NSAIDs, heat/ice
• Activity modification
• Physical therapy if not improving

Management by diagnosis#

Nephrolithiasis#

Education:

• Kidney stones are common; most pass on their own
• Small stones (<5mm) usually pass within 1-2 weeks
• Larger stones (5-10mm) may take longer; some need intervention
• Drink plenty of fluids; strain urine to catch the stone
• Pain can be severe but is manageable

Treatment:

Stone passage expectations:

• <5mm: 90% pass spontaneously
• 5-10mm: 50% pass; consider tamsulosin

• 10mm: unlikely to pass; urology referral

Urology referral indications:

• Stone >10mm
• Not passing after 4-6 weeks
• Infection with obstruction (emergency)
• Intractable pain or vomiting
• Solitary kidney
• Bilateral stones
• Renal insufficiency

Follow-up: 2-4 weeks to confirm passage. If stone caught, send for analysis. Consider 24-hour urine metabolic evaluation for recurrent stone formers.

Pyelonephritis (Outpatient)#

Education:

• Kidney infection; more serious than bladder infection
• Must complete full antibiotic course
• Should feel better in 24-48 hours; if not, need reassessment
• Stay hydrated; rest

Treatment:

Outpatient criteria (all must be met):

• Non-pregnant
• Tolerating oral intake
• No sepsis criteria (fever alone is OK)
• Reliable follow-up
• No urinary obstruction
• Not immunocompromised

Follow-up: Phone check in 24-48 hours. If not improving, reassess (imaging, possible hospitalization).

Recurrent Nephrolithiasis#

Education:

• 50% of stone formers will have another stone within 5 years
• Dietary and lifestyle changes can reduce recurrence
• 24-hour urine testing helps identify specific risk factors

Prevention strategies (general):

• Increase fluid intake: goal urine output >2.5 L/day
• Dietary sodium restriction: <2300 mg/day
• Moderate protein intake
• Normal calcium intake (low calcium paradoxically increases stone risk)
• Limit oxalate-rich foods if calcium oxalate stones (spinach, nuts, chocolate)

Medications (based on stone type and 24-hour urine):

Follow-up: 24-hour urine collection 6 weeks after starting prevention. Repeat annually or if recurrence.

Musculoskeletal Flank Pain#

Education:

• Muscle strain or spasm causing pain
• Usually improves with rest and anti-inflammatory medication
• No kidney problem identified

Treatment:

• NSAIDs: ibuprofen 400-600 mg TID or naproxen 500 mg BID
• Heat or ice for comfort
• Gentle stretching once acute pain improves
• Activity modification; avoid aggravating movements

Follow-up: Return if not improving in 1-2 weeks or if new symptoms develop (fever, urinary symptoms, hematuria).

Follow-up#

Nephrolithiasis:

• 2-4 weeks to confirm stone passage
• If passed: consider metabolic workup for recurrent formers
• If not passed: repeat imaging, urology referral

Pyelonephritis:

• Phone check 24-48 hours
• If not improving: office visit, imaging, possible hospitalization
• No routine follow-up UA needed if symptoms resolve

Return precautions (all patients):

• Fever or chills (new or worsening)
• Unable to keep fluids or medications down
• Pain not controlled with prescribed medications
• Unable to urinate
• Blood in urine (if not already present)
• Feeling much worse

Patient instructions#

For kidney stones:

• Drink plenty of water—aim for light yellow urine
• Take pain medication as prescribed; NSAIDs work best for this type of pain
• Use the strainer every time you urinate; save any stone you catch
• Most stones pass on their own, but it can take days to weeks
• Call or return if: fever, vomiting and can’t keep fluids down, pain not controlled, or unable to urinate

For kidney infection:

• Take all your antibiotics, even if you feel better
• Drink plenty of fluids
• Rest; you should start feeling better in 1-2 days
• Call or return if: not improving after 2 days, fever getting higher, vomiting, or feeling much worse

Smartphrase snippets#

Nephrolithiasis (outpatient management): “CT confirms [X]mm stone in [location]. No hydronephrosis. Afebrile. Plan: NSAIDs for pain, tamsulosin for medical expulsive therapy, strain urine. Urology referral if not passed in 4-6 weeks or sooner if worsening. Return precautions for fever, intractable pain/vomiting, or inability to urinate.”

Pyelonephritis (outpatient): “Clinical presentation consistent with acute pyelonephritis—fever, CVA tenderness, pyuria. Meets outpatient criteria: tolerating PO, non-pregnant, no sepsis, reliable follow-up. Treated with [fluoroquinolone] x 7 days. Urine culture pending. Phone follow-up in 24-48 hours. ED precautions given.”

Flank pain, MSK etiology: “Flank pain with MSK features: reproducible with palpation/movement, no CVA tenderness, afebrile, UA negative. No imaging indicated. Treated with NSAIDs and activity modification. Return if fever, urinary symptoms, or not improving in 1-2 weeks.”