From the Guidelines
The straight leg test has a likelihood ratio of approximately 3.5 for a positive test, as it has a relatively high sensitivity of 91% but modest specificity of 26% for diagnosing herniated disc. This means a positive test roughly triples the probability of disc herniation. The test is performed by having the patient lie supine while the examiner passively lifts the patient's straightened leg, with pain radiating down the leg at 30-70 degrees of elevation considered positive, suggesting nerve root irritation 1. The straight leg test works by stretching the sciatic nerve and its nerve roots, which causes pain when there is compression from a herniated disc.
- Key points to consider when interpreting the results of the straight leg test include:
- A positive result on the straight-leg-raise test has a relatively high sensitivity (91% [95% CI, 82% to 94%]) but modest specificity (26% [CI, 16% to 38%]) for diagnosing herniated disc 1.
- The crossed straight-leg-raise test is more specific (88% [CI, 86% to 90%]) but less sensitive (29% [CI, 24% to 34%]) 1.
- The test should be combined with other clinical findings and imaging studies when appropriate to confirm the diagnosis of lumbar disc herniation. While the straight leg raise test is useful as a screening tool due to its moderate to high sensitivity, its lower specificity means it's better at ruling out disc herniation than confirming it, and should be used in conjunction with a thorough clinical evaluation, including a focused examination and neurologic examination 1.
From the Research
Likelihood Ratio of Straight Leg Test
- The likelihood ratio of the straight leg test is reported in several studies, including 2, which found a positive likelihood ratio of 1.38 and a negative likelihood ratio of 0.87.
- Another study, 3, reported a sensitivity of 0.875 and a specificity of 0.429 for the straight leg raise test in detecting lumbar disc herniation, but did not provide a likelihood ratio.
- The study 4 reported a pooled estimate of sensitivity of 0.92 and specificity of 0.28 for the straight leg raising test in surgical populations, but did not provide a likelihood ratio.
- The likelihood ratio can be calculated from the sensitivity and specificity values, but it is not directly reported in all studies.
Factors Affecting the Likelihood Ratio
- Age is reported to affect the likelihood ratio of the straight leg test, with older patients having a lower likelihood ratio 2.
- The study 5 found that the rate of positive straight leg raising test results declined with increasing age, and that the chance of a positive test result was 5.4 times higher in patients under 60 than in those over 60.
- Sex is also reported to affect the likelihood ratio, with men having a higher likelihood ratio than women 5.
Comparison with Other Tests
- The study 3 compared the accuracy of the straight leg raise test with the slump test, and found that the straight leg raise test was more accurate in detecting lumbar disc herniation.
- The study 4 reported that the crossed straight leg raising test had a higher specificity than the straight leg raising test, but a lower sensitivity.