What does a positive straight leg raise (SLR) test indicate?

What Does a Positive Straight Leg Raise Test Indicate?

A positive straight leg raise (SLR) test indicates nerve root tension or compression, most commonly from lumbar disc herniation causing sciatica, though its high sensitivity (91%) but low specificity (26%) means it detects most cases of nerve root compromise but also produces many false positives. 1

Primary Clinical Significance

• The SLR test primarily detects lumbar disc herniation with nerve root compression, particularly when the test reproduces the patient's radiating leg pain (sciatica) at leg elevation between 30-70 degrees with the knee extended. 2

• The test works by passively stretching the sciatic nerve, inducing both linear motion (0.5-5 mm) and strain (2-4%) in spinal nerves L4, L5, and S1, which elicits symptoms when nerve root tension is present. 3

• Nerve roots actually move laterally toward the pedicle during SLR, meaning they move into a posterolaterally herniated disc, which explains why the test provokes symptoms in disc herniation cases. 3

Diagnostic Performance Characteristics

Standard SLR Test

• Sensitivity: 91% (95% CI, 82% to 94%) - meaning it catches most true cases of lumbar disc herniation. 1
• Specificity: 26% (95% CI, 16% to 38%) - meaning many positive tests are false positives and the test cannot rule in disc herniation definitively. 1

Crossed SLR Test (More Specific Variant)

• When raising the unaffected leg reproduces pain in the symptomatic leg, this has much higher specificity (88%) but lower sensitivity (29%), making it a stronger indicator of disc herniation when positive. 1, 2

Critical Age-Related Limitation

• The sensitivity of SLR decreases dramatically with age, dropping below 50% in patients over 60 years old, meaning negative tests in older adults do not rule out nerve root pathology. 1, 4

• The chance of a positive SLR becomes 0.27 times less for each additional year of age, and patients under 60 have 5.4-fold higher odds of a positive test compared to those over 60. 4

• Male patients have 1.3 times higher odds of a positive SLR compared to females. 4

What Constitutes a Positive Test

• The test is positive only when it reproduces the patient's sciatica (radiating leg pain below the knee in sciatic nerve distribution), not just back pain or hamstring tightness. 2

• Pain must be reproduced between 30-70 degrees of leg elevation - pain outside this range is less specific for nerve root pathology. 2

Clinical Context and Interpretation

• Sciatica can be diagnosed based on radicular pain pattern, dermatomal sensory changes, and motor weakness in a nerve root distribution even without a positive SLR, since the test's modest specificity means negative results don't exclude pathology. 1

• The SLR should be interpreted alongside a focused neurological examination including:

  • Knee strength and reflexes (L4 nerve root)
  • Great toe and foot dorsiflexion strength (L5 nerve root)
  • Foot plantarflexion and ankle reflexes (S1 nerve root)
  • Sensory distribution assessment 1, 2

Common Pitfalls to Avoid

• Do not rely on SLR alone to diagnose or exclude lumbar disc herniation - the combination of clinical findings (radicular pain pattern, motor/sensory deficits, reflex changes) is more reliable than any single test. 1

• Do not interpret hamstring tightness or non-specific back pain as a positive SLR - only reproduction of the patient's characteristic radiating leg pain counts as positive. 2

• In patients over 60 with high clinical suspicion but negative SLR, do not be falsely reassured - consider the Slump test (sensitivity 84% vs SLR 52%) or proceed with imaging based on clinical presentation. 5

• Remember that psychosocial factors are stronger predictors of low back pain outcomes than physical examination findings alone, so assess these alongside physical findings. 2