What does the straight leg raise (SLR) test measure?

The Straight Leg Raise Test: Diagnostic Purpose and Interpretation

The straight leg raise (SLR) test primarily measures nerve root tension and is designed to diagnose lumbar disc herniation with nerve root compression, demonstrating high sensitivity (91%) but modest specificity (26%) for this condition. 1, 2

Mechanism and Procedure

• The SLR test works by passively stretching the sciatic nerve to elicit symptoms suggesting nerve root tension when the leg is raised between 30-70 degrees with the knee extended 3, 1
• During the test, spinal nerve roots (particularly L4, L5, and S1) undergo both linear motion (0.5-5 mm) and strain (2-4%) 4
• The test involves hip flexion with knee extension, which places mechanical pressure on the sciatic nerve and can reproduce the patient's radiating leg pain (sciatica) 3, 2

Diagnostic Value

• A positive test is defined as reproduction of the patient's sciatica when the leg is raised between 30 and 70 degrees 3, 1
• The crossed SLR test (pain produced when raising the unaffected leg) is more specific (88%) but less sensitive (29%) for diagnosing disc herniation 1, 2
• The SLR test is most valuable for identifying nerve root compression from disc herniation that may require surgical intervention 2
• Research shows that nerve roots move laterally toward the pedicle during SLR, potentially moving into a posterolaterally herniated disc 4

Clinical Applications and Interpretation

• The SLR should be combined with a focused neurological examination including:
  • Evaluation of knee strength and reflexes (L4 nerve root) 1, 2
  • Assessment of great toe and foot dorsiflexion strength (L5 nerve root) 1, 2
  • Testing of foot plantarflexion and ankle reflexes (S1 nerve root) 1, 2
  • Evaluation of the distribution of sensory symptoms 1, 2

Limitations and Considerations

• The diagnostic accuracy of the SLR test decreases with increasing patient age, with the area under the ROC curve decreasing from 0.730 in younger patients (16-25 years) to 0.515 in older patients (76-85 years) 5
• The Slump test, a variant of SLR performed in the seated position, has been found to be more sensitive (0.84) than the traditional SLR (0.52) for lumbar disc herniation, though slightly less specific (0.83 vs 0.89) 6
• When both tests are interpreted as positive based on reproduction of presenting leg pain that intensifies with ankle dorsiflexion, they show substantial agreement (kappa = 0.69) and good correlation (r = 0.64) 7

Common Pitfalls

• Using the SLR test in isolation rather than as part of a comprehensive assessment can lead to misdiagnosis 2
• Relying solely on the SLR test without confirmatory imaging when serious pathology is suspected can delay appropriate treatment 1, 2
• Failing to recognize that psychosocial factors are stronger predictors of low back pain outcomes than physical examination findings or pain severity/duration 1, 2
• Not considering that spinal fusion does not decrease nerve root motion or strain during SLR, which has implications for post-surgical expectations 4