||Electrodiagnostic testing, consisting of needle electromyography and nerve conduction studies, is the primary method used to objectively measure and document pathological changes or injury to the neuromuscular system, including proximally located spinal nerve roots. Clinicians employ electrodiagnostic testing to evaluate patients with low back pain (LBP). One specific cause of LBP is lumbosacral radicular syndrome (LRS), which is commonly known as sciatica or lumbar radiculopathy. The presence of radiating leg symptoms is common to all patients with LRS but radiculopathy is distinguished by the presence of measurable nerve root injury. Little is known about prognostic factors in these patients; however, recent evidence suggests the presence of radiculopathy found on needle electromyography may predict better functional outcomes. The primary purpose of this dissertation work was to investigate the prognostic value of electrodiagnostic testing in patients with LRS receiving physical therapy. Electrodiagnostic testing was performed on 38 patients with LRS participating in a randomized trial comparing different physical therapy treatment programs. Patients were grouped and analyzed according to the presence or absence of radiculopathy. The primary outcome measure was changes in LBPrelated disability assessed using the Roland and Morris disability questionnaire (RMDQ). Patients with radiculopathy (n=19) had statistically significant and iv clinically meaningful improvements in RMDQ scores at every posttreatment follow-up occasion regardless of physical therapy treatment received. The final multilevel growth model revealed improvements in RMDQ scores in patients with radiculopathy at the 6-week (-8.1, 95% CI, -12.6 to -2.6; P=.006) and 6-month (- 4.1, 95% CI, -7.4 to -0.7; P=.020) follow-up occasions compared to patients without radiculopathy. Physical therapy treatment group was not a significant predictive factor at any follow-up occasion. An interaction between electrodiagnostic status and time revealed faster weekly improvements in RMDQ scores in patients with radiculopathy at the 6-week (-0.72, 95% CI, -1.4 to -0.04; P=.040) through the 16-week (-0.30, 95% CI, -0.57 to -0.04; P=.028) follow-up occasions compared to patients without radiculopathy. The presence of lumbosacral radiculopathy identified with electrodiagnostic testing is a favorable prognostic factor for recovery in LBPrelated disability regardless of physical therapy treatment received.