Doctors diagnose sciatica primarily through a focused physical exam and detailed questions about your symptoms, not imaging. In most cases, an MRI or CT scan isn’t ordered unless your pain has persisted for at least six weeks without improvement or you have symptoms suggesting something more serious. Here’s what the diagnostic process actually looks like, step by step.
What Your Doctor Asks First
The diagnosis starts with your story. Two specific details carry the most weight, according to clinical guidelines from the UK’s National Institute for Health and Care Excellence: whether your pain radiates below the knee, and whether coughing, sneezing, or straining makes it worse. Both of these are strongly associated with true sciatica rather than general back pain. Your doctor will also ask which leg is affected, when the pain started, whether it’s constant or comes and goes, and what positions make it better or worse.
This matters because “sciatica” describes a specific pattern: pain that follows the path of the sciatic nerve from the lower back through the buttock and down the leg. Plenty of conditions cause back or leg pain, so your doctor is listening for that characteristic radiating quality. They’ll also ask about numbness, tingling, or weakness in your leg or foot, which can help pinpoint exactly which nerve root is involved.
The Physical Exam
The most common test is the straight leg raise, sometimes called Lasègue’s test. You lie on your back while the doctor slowly lifts your affected leg. The test is considered positive if it triggers radiating pain along the sciatic nerve path (not just tightness in the hamstring) when your leg is raised between 30 and 70 degrees. This test has high sensitivity, meaning it’s good at catching true cases of disc-related sciatica, but low specificity, meaning it can also be positive in people whose pain has a different cause.
If the straight leg raise is positive, your doctor may add the Bragard test: they lower your leg slightly until the pain eases, then flex your foot upward. If this brings the pain back, it helps confirm the issue is nerve-related rather than muscular. This variation can also reveal nerve irritation at a lower angle than the straight leg raise alone.
Checking Nerve Function
Your doctor will test specific muscles and skin areas to figure out which nerve root is compressed. The three most commonly affected levels each produce a distinct pattern:
- L4 nerve root: affects sensation along the inner leg and the top of the foot near the big toe
- L5 nerve root: affects the outer leg, the top of the foot, and the first three toes
- S1 nerve root: affects the back of the thigh, the calf, and the outer edge of the foot
The doctor tests these by lightly touching different areas of skin to check for numbness, then by asking you to walk on your heels, walk on your toes, or resist pressure against your foot or leg. Weakness in specific muscles points directly to a particular nerve root level, which helps guide treatment if imaging is eventually needed.
Reflexes are also checked. A diminished knee reflex suggests L4 involvement, while a reduced ankle reflex points to S1. These findings, combined with where you feel pain and numbness, give the doctor a fairly precise picture of which disc or nerve root is the problem.
When Imaging Is Ordered
Most people with sciatica improve within a few weeks with conservative care, so imaging right away is usually unnecessary. The American College of Radiology recommends imaging for patients who have completed up to six weeks of treatment (medication, physical therapy, or both) with little or no improvement.
MRI is the preferred imaging study because it shows soft tissues like discs and nerves in detail, not just bone. A bulging or herniated disc pressing on a nerve root is the most common finding. CT scans are an alternative when MRI isn’t available or when a patient can’t undergo MRI (for example, because of a pacemaker).
There are exceptions to the six-week waiting period. Your doctor will order imaging immediately if you have red flag symptoms, which include:
- Bladder or bowel changes: difficulty urinating, urinary retention, or loss of bowel control
- Saddle numbness: loss of sensation in the inner thighs, buttocks, or groin
- Progressive or bilateral leg weakness: weakness that’s getting worse or affecting both legs
- History of cancer, unexplained weight loss, or recent significant trauma
These red flags can signal cauda equina syndrome, a rare but serious condition where the bundle of nerves at the base of the spine is severely compressed. This is a medical emergency that requires urgent MRI and, in many cases, surgery within hours to prevent permanent damage.
Nerve Testing for Unclear Cases
When the diagnosis isn’t straightforward, or when symptoms have persisted for months, your doctor may order electrodiagnostic tests. These include nerve conduction studies, which measure how well electrical signals travel through your nerves, and electromyography (EMG), which measures the electrical activity in your muscles. Both are typically done in the same visit.
These tests are particularly useful when imaging looks normal but symptoms persist, or when the doctor needs to distinguish between a nerve problem and a muscle disorder. They can confirm that a specific nerve is damaged, pinpoint where the damage is occurring, and gauge how severe it is. The tests involve small electrical impulses and thin needle electrodes. They’re uncomfortable but not typically painful, and results are available quickly.
Ruling Out Other Conditions
Several conditions mimic sciatica, and part of the diagnostic process is ruling them out. The most common lookalike is piriformis syndrome, where a small muscle deep in the buttock spasms or tightens and irritates the sciatic nerve. Unlike disc-related sciatica, piriformis syndrome tends to cause pain centered in the buttock that worsens with sitting. Doctors test for it using the FAIR test: they flex your hip, rotate it inward, and push your knee toward the opposite shoulder. Research from Siriraj Hospital found that 87% of patients with confirmed piriformis syndrome had a positive FAIR test, and nearly 95% had tenderness when the doctor pressed directly over the piriformis muscle.
The key difference is that piriformis syndrome doesn’t involve a problem in the spine, so spinal MRI looks normal. If piriformis syndrome is suspected, an ultrasound-guided injection into the piriformis muscle can serve as both a diagnostic test and a treatment. Pain relief after the injection confirms the diagnosis.
Other conditions your doctor considers include spinal stenosis (narrowing of the spinal canal, more common in older adults), hip joint problems, and sacroiliac joint dysfunction. Each has its own pattern of symptoms and physical exam findings that an experienced clinician can distinguish from classic sciatica.
What the Diagnosis Means for You
In most cases, sciatica is diagnosed clinically, meaning your doctor reaches the diagnosis based on your symptoms and physical exam without needing advanced testing. If your pain follows the sciatic nerve distribution, worsens with coughing or straining, and produces a positive straight leg raise, the diagnosis is fairly clear. Imaging and nerve testing are reserved for cases that don’t improve, that present with alarming symptoms, or that need surgical planning. Understanding this can save you unnecessary worry about why your doctor didn’t immediately order an MRI. For the majority of sciatica cases, the clinical exam provides all the information needed to start effective treatment.