Differential Diagnosis for Lower Back Pain
The differential diagnosis for lower back pain should be organized into three broad categories: nonspecific low back pain (>85% of cases), back pain with radiculopathy or spinal stenosis, and back pain from specific spinal causes requiring urgent evaluation. 1
Primary Diagnostic Framework
The American College of Physicians/American Pain Society guideline provides a structured triage system that prioritizes identifying serious conditions while recognizing that most low back pain is nonspecific 1.
Category 1: Nonspecific Low Back Pain (>85% of cases)
- Musculoskeletal strain/sprain - most common
- Degenerative disc disease - poorly correlates with symptoms
- Facet joint arthropathy
- Myofascial pain
Critical point: Attempting to identify specific anatomical sources in this group has not been validated and does not improve outcomes 1.
Category 2: Back Pain with Neurologic Involvement
Radiculopathy/Sciatica (4% of cases):
- Herniated disc - 90% occur at L4/L5 or L5/S1 levels 1
- Look for: leg pain below knee in dermatomal distribution, positive straight-leg raise (30-70 degrees), specific motor/sensory deficits
Spinal Stenosis (3% of cases):
- Look for: neurogenic claudication (leg pain with walking/standing, relieved by sitting), age >65 years (positive likelihood ratio 2.5) 1
Category 3: Specific Spinal Causes Requiring Urgent Evaluation
RED FLAGS - Screen Every Patient:
Cauda Equina Syndrome (0.04% prevalence):
- Urinary retention (90% sensitivity) - most critical finding 1
- Fecal incontinence
- Saddle anesthesia
- Bilateral lower extremity weakness
- Action required: Immediate imaging and surgical consultation
Malignancy (0.7% prevalence): Screen for these risk factors with specific likelihood ratios 1:
- History of cancer - increases probability from 0.7% to 9% (LR+ 14.7)
- Age >50 years (LR+ 2.7)
- Unexplained weight loss (LR+ 2.7)
- Failure to improve after 1 month (LR+ 3.0)
Vertebral Compression Fracture (4% prevalence):
- Older age
- Osteoporosis history
- Corticosteroid use
- Trauma history
Spinal Infection (0.01% prevalence):
- Fever
- IV drug use
- Recent infection
- Immunosuppression 1
Ankylosing Spondylitis (0.3-5% prevalence):
- Age <40 years at onset
- Morning stiffness >30 minutes
- Improvement with exercise (not rest)
- Alternating buttock pain
- Awakening in second half of night due to back pain 1
Non-Spinal Causes to Consider
Always evaluate for referred pain from: 1
- Vascular: Abdominal aortic aneurysm (pulsatile mass, age >60)
- Renal: Nephrolithiasis (colicky pain, hematuria)
- Gastrointestinal: Pancreatitis (epigastric radiation, elevated lipase)
- Infectious: Endocarditis (fever, new murmur)
- Gynecologic: Pelvic pathology in women
Critical Physical Examination Maneuvers
For Radiculopathy:
- Straight-leg raise: Sensitivity 91%, specificity 26% for herniated disc 1
- Crossed straight-leg raise: Sensitivity 29%, specificity 88% - more specific 1
- Neurologic exam by level:
- L4: Knee extension strength, patellar reflex
- L5: Great toe/foot dorsiflexion
- S1: Foot plantarflexion, ankle reflex
For Cauda Equina:
- Post-void residual (urinary retention is key finding)
- Rectal tone
- Perianal sensation
Common Pitfalls to Avoid
Don't over-rely on imaging for nonspecific pain - degenerative changes correlate poorly with symptoms 1
Don't miss cauda equina - if no urinary retention, probability drops to 1 in 10,000, but still check 1
Don't dismiss cancer risk in patients >50 - even without prior cancer history, age alone increases risk 1
Don't forget psychosocial factors - stronger predictors of chronicity than physical findings 1
Don't ignore inflammatory back pain features - ankylosing spondylitis criteria are evolving and can present before radiographic changes 1
The key is systematic triage using this three-category framework, with aggressive screening for red flags that signal serious pathology requiring immediate intervention, while recognizing that the vast majority of cases are nonspecific and self-limited.