How is a herniated lumbar disc diagnosed?

The physician will use a combination of history (patient interview), physical examination, and imaging (Xray, MRI, CT scan, etc.) to make a diagnosis of herniated disc.

The history is the information the patient gives the physician about the injury. The doctor will want to know:

how the pain began, how long it has been going on,and how severe it is.
if an accident or fall preceded the pain.
The the state of the patient's general health.
which medications the patient is taking (both prescription and over the counter).
other treatments that have been tried and whether any of them have been helpful.

The doctor will check several functions in the physical examination:

Gait: The doctor may ask the patient to walk normally, then walk on heels and finally on toes to check balance, as well as perform other maneuvers.
Range of motion: The patient may be asked to bend over and lean backwards and sideways to examine the mobility of the back.
Back examination: The back will be checked for curvatures, muscle spasm, and tenderness.
Examination of the legs: Other conditions such as hip problems, leg length differences, circulation problems and injuries to the legs must be checked.
Nerve root tension signs: The doctor may stretch the legs in various ways to see if stretching a nerve will reproduce the pain. This helps determine if a nerve is compressed. The most common of these tests is the "straight leg raise".
Neurologic examination: The doctor will evaluate as many nerve functions as possible in the patient's legs. This will include checking the sense of touch using a pin prick and evaluating the strength in several leg muscles. The reflexes at the knee and ankle will also be tested.

It is often necessary to obtain an X-ray, MRI, bone scan, or myelogram. Since many people recover completely in a short time, most doctors suggest waiting until the condition has failed to respond to several weeks of conservative (non-operative) treatment before getting imaging studies.

X-rays: The most common imaging study is the X-ray, which is best suited for looking at the bones. Fractures, disc degeneration, arthritis and spinal deformities are also visible on regular X-rays. Abnormal movements of the vertebrae can be identified if X-rays are made while the patient is bending forward or leaning backward.
Computed Tomography (CT scan): This is a special type of X-ray that shows soft tissues (disc, nerve, muscle and ligament) more clearly than a regular X-ray. CT scans are about 83% accurate at diagnosing a herniated disc.
MRI (Magnetic Resonance Imaging): The MRI is ordered only after a period of unsuccessful conservative treatment. The MRI is extremely good at showing the discs, nerves, spinal cord, spinal fluid, muscles and ligaments. It is about 93% accurate at finding herniated discs.
Myelogram and Post Myelogram CT Scan: During a myelogram the radiologist injects dye into the spinal fluid. A series of X-rays is followed by a CT scan. The dye outlines the nerves and makes them easier to see. This method is also about 93% accurate at identifying herniated discs, but most doctors prefer an MRI because it does not require an injection. A myelogram and post myelograin CT scan is often done when an earlier MRI was not conclusive.