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Investigation to write in lowerback pain radiating to thigh and knwe

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"low back pain"[MeSH Terms] AND radiculopathy

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NICE low back pain and sciatica investigations imaging recommendations adults

For low-back pain radiating to the thigh and knee, the cause may be lumbar nerve-root irritation, hip disease, sacroiliac pathology, or less commonly infection/inflammation. Investigations should be selected after examination, not all done routinely.

Common investigations to write

If pain persists, there is neurological deficit, or a clinician suspects radiculopathy:
  1. MRI lumbosacral spine without contrast
    • Best test for disc prolapse, nerve-root compression, spinal stenosis.
    • Add contrast only if prior spinal surgery, suspected infection, tumour, or inflammatory lesion.
  2. X-ray lumbosacral spine - AP and lateral views
    • Useful after trauma or when fracture, deformity, spondylolisthesis, or significant degeneration is suspected.
    • It does not show discs or nerves well.
  3. X-ray pelvis with both hips - AP view
    • Important when pain is mainly in the groin, front/side of thigh, or referred to the knee, because hip arthritis can present as knee pain.
  4. Nerve conduction study and EMG
    • Consider if the diagnosis is unclear, weakness/numbness persists, or to distinguish lumbar radiculopathy from peripheral neuropathy.

Blood tests only if clinically indicated

  • CBC, ESR, CRP: fever, unexplained severe pain, suspected infection/inflammatory disease.
  • Blood sugar/HbA1c, vitamin B12, TSH: if neuropathy is suspected.
  • Calcium, vitamin D, renal function: only when relevant to bone health, medications, or systemic symptoms.

A practical prescription-style line

MRI lumbosacral spine (non-contrast) + X-ray pelvis with both hips AP view.
CBC, ESR, CRP if fever, weight loss, night pain, or inflammatory symptoms are present.
Routine scans are not usually needed for a recent uncomplicated episode of back pain/sciatica. Current NICE guidance recommends imaging in specialist care only when it is likely to change management. Textbook guidance also notes that many acute cases of low-back pain with sciatica can initially be assessed clinically without spinal imaging.
Urgent assessment or emergency MRI is needed now if there is new leg weakness, numbness around the genitals/anus, inability to pass urine or loss of bladder/bowel control, fever, cancer history, major trauma, or rapidly worsening pain.
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