The Lasegue test: Defining its specificity and sensitivity based on geometry.

The Lasegue test -also known as the straight leg raise test-, used for diagnostic purposes in cases of lumbar disc herniation can be explained by geometrical rules. This study aimed to describe the relationship between the apical and basal size of the herniated disc and the Lasegue test, and to describe the specificity and sensitivity of the herniated disc as a function of its height, location, and size. The study evaluated cases diagnosed with lumbar disc herniation with available MRI scans and Lasegue test results. The radix length to the foramen was calculated by recording the apical length and the base length of the herniated disc. The level and apical direction of the herniated discs were also recorded to determine the specificity and sensitivity of the test. Data from a total of 865 cases were analyzed. The apical length of the herniated disc was the most critical criterion for determining the Lasegue test. The Lasegue test is most sensitive and specific in lateral herniated discs at the level of L4-5. The specificity and sensitivity are decreased in L3-4, migrated, and sequestered disc herniations. In these cases, false-negative results may occur. Sensitivity decreases in the median and paramedian herniated discs. Compression of the sciatic nerve reduces herniated disc straightness. The Lasegue test is a test that can be defined according to the rules of geometry. Knowing the factors that decrease specificity and sensitivity and cause false positive/negative results will facilitate correct diagnosis.