What is the likely cause of chronic left-sided mid-back pain, worsened by movement, with slight leukocyturia (leukocytes in the urine) and no other symptoms?

Left-Sided Mid-Back Pain with Leukocyturia: Diagnostic Approach

This presentation most likely represents musculoskeletal back pain, but the leukocyturia warrants exclusion of urinary tract pathology before settling on a benign diagnosis. 1, 2

Initial Assessment Priority

The absence of red flags is reassuring but does not eliminate the need for targeted evaluation given the leukocyturia:

• No red flags present: Patient lacks fever, weight loss, dysuria, neurologic deficits, or systemic illness, making serious pathology less likely 2, 3
• Leukocyturia significance: Even slight leukocytes in urine without bacteriuria requires further diagnostic evaluation to exclude urinary tract pathology 4
• Movement-related pain: Pain worsening with movement strongly suggests musculoskeletal origin rather than visceral pathology 1

Recommended Diagnostic Algorithm

Step 1: Complete Urinalysis with Culture

• Obtain formal urinalysis with microscopy and urine culture to definitively rule out urinary tract infection, even without dysuria 4
• Leukocyturia without significant bacteriuria mandates further evaluation and cannot be dismissed as contamination 4
• Sterile pyuria (leukocytes without bacteria) can indicate renal pathology including papillary necrosis, interstitial nephritis, or early infection 5

Step 2: Imaging Decision Based on Clinical Context

Imaging is NOT routinely indicated for nonspecific back pain without red flags 1, 3. However, given the leukocyturia and localized unilateral pain, consider:

• Renal ultrasound as initial imaging if urinalysis suggests renal pathology (most cost-effective, no radiation) 6
• Plain radiography may be reasonable if symptoms persist beyond 1-2 months without improvement 1
• Avoid routine CT or MRI in the absence of red flags, as this does not improve outcomes and identifies incidental findings that lead to unnecessary interventions 1

Step 3: Reassessment Timeline

• Reevaluate in 1 month if symptoms persist without improvement, as most acute back pain improves substantially within the first month 1, 2
• Earlier reevaluation warranted if new symptoms develop (fever, neurologic changes, worsening pain) 1

Differential Diagnosis to Consider

Musculoskeletal causes (most likely):

• Paraspinal muscle strain or myofascial pain syndrome 1
• Facet joint arthropathy or rib dysfunction 1
• Thoracic spine degenerative changes (though poorly correlated with symptoms) 3

Urologic causes (must exclude given leukocyturia):

• Occult urinary tract infection without typical symptoms 4
• Renal papillary necrosis (especially if diabetic or frequent UTI history) 5
• Urogenital irritation causing referred back pain 6
• Early pyelonephritis or perinephric process 5

Critical Pitfalls to Avoid

• Do not dismiss leukocyturia as contamination without proper urine culture confirmation 4
• Do not order imaging prematurely for nonspecific back pain, as this leads to overdiagnosis of incidental findings and unnecessary interventions 1
• Do not assume normal-appearing urine excludes pathology—microscopic findings are more sensitive than gross appearance 4
• Do not delay investigation if symptoms worsen or new red flags emerge (fever, neurologic symptoms, progressive pain) 2

Management Approach

If urine culture is negative and renal imaging (if obtained) is normal:

• Treat as nonspecific musculoskeletal back pain 1
• Reassure patient that most back pain improves within 4-6 weeks 1
• Recommend continued activity as tolerated rather than bed rest 1
• Follow up in 1 month if no improvement 1

If urine culture is positive or imaging reveals pathology:

• Treat identified urinary tract infection appropriately 4
• Refer to urology if structural renal abnormality identified 5