The straight leg raise test (SLR) has been proposed to detect increased nerve mechanosensitivity of the lower limbs in individuals with low back pain or leg-related low back pain. However, its validity in the diagnosis of lumbosacral radiculopathy show very variable results. The aim of this study was to analyse the diagnostic validity of the SLR including well defined diagnostic criteria in a sample of participants in phase III with suspicion of lumbar radiculopathy using the nerve conduction studies (NDS) as the reference standard. A phase III diagnostic accuracy study was designed. 142 individuals with suspected lumbosacral radiculopathy referred for NCS participated in the study. Each participant was tested with NCS and SLR. SLR was considered positive using three diagnostic criteria. The sensitivity of the SLR for the criterion 3 was 89.02% (CI 81.65-96.40), the specificity was 25.00% (CI 13.21-36.79), and the +LR and -LR were 1.19 (CI 1.01-1.40) and 0.44 (0.21-0.94), respectively. SLR showed limited validity in the diagnosis of lumbosacral radiculopathy. The incorporation of more objective diagnostic criteria (asymmetry in range of motion or localization of symptoms) improved diagnostic validity but the imprecision of the confidence intervals limited the interpretation of the results.