Diagnostic triage is an essential guideline recommendation for low back pain (LBP), which is the most frequent musculoskeletal condition that general practitioners encounter in Australia. Clinical diagnosis of LBP — informed by a focused history and clinical examination — is the key initial step for GPs, and determines subsequent diagnostic workup and allied health and medical specialist referral. The goal of diagnostic triage of LBP is to exclude non-spinal causes and to allocate patients to one of three broad categories: specific spinal pathology (< 1% of cases), radicular syndrome (~5-10% of cases) or non-specific LBP (NSLBP), which represents 90-95% of cases and is diagnosed by exclusion of the first two categories. For specific spinal pathologies (eg, vertebral fracture, malignancy, infection, axial spondyloarthritis or cauda equina syndrome), a clinical assessment may reveal the key alerting features. For radicular syndrome, clinical features distinguish three subsets of nerve root involvement: radicular pain, radiculopathy and spinal stenosis. Differential diagnosis of back-related leg pain is complex and clinical manifestations are highly variable. However, distinctive clusters of characteristic history cues and positive clinical examination signs, particularly from neurological examination, guide differential diagnosis within this triage category. A diagnosis of NSLBP presumes exclusion of specific pathologies and nerve root involvement. A biopsychosocial model of care underpins NSLBP; this includes managing pain intensity and considering risk for disability, which directs matched pathways of care. Back pain is a symptom and not a diagnosis. Careful diagnostic differentiation is required and, in primary care, diagnostic triage of LBP is the anchor for a diagnosis.