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Investigations for low back and leg pain

Back pain is one of the most heavily investigated conditions.

However, the reasons for doing these procedures is often not well understood, even by the medical or paramedical professions.

To avoid unfulfilled expectations and disappointments, it is important to figure out what can and cannot be done with the many investigations used for low back and leg pains.

Magnetic Resonance Imaging (MRI) Scanning
CT scanning
Blood Tests
Bone Scan
Bone Densitometry


This is the commonest investigation used for low back pain and associated conditions.

In the UK The Royal College of Radiologists have issued guidelines for the use of x-rays. These points come from the guidelines and other sources.

It is not necessary to routinely x-ray people with “simple” low back pain.

X-rays of the lumbosacral spine involve a radiation dose which is 120 times that needed for a chest x-ray.

X-ray investigations cannot usually diagnose the conditions which may cause acute low back pain. Age-related changes which show on x-rays do not correlate well with back pain, so are likely not to be relevant but just incidental.

X-rays show up much less than often thought. Bone is well shown but soft tissues are poorly imaged and need specialised interpretation to make any conclusions. A person can have normal x-rays in the presence of early infection or tumours (which are rare) so a normal x-ray does not rule out serious illness. On the other hand, people without back pain have abnormalities on their x-rays, so such changes on x-ray are not always connected with back pain in many people. This makes the interpretation of “age-related degenerative changes” in people with back pain very difficult.

In a study of 2500 people with acute (recent) back pain, the x-rays were reviewed and an important finding noted in one out of the 2500 x-rays. In other words, 2499 x-rays showed nothing important and made no difference to the management of the back pain. In another study the incidence of useful x-rays was given at 1 in 100.

Magnetic Resonance Imaging (MRI) Scanning

MRI scanning has been an extremely important advance in understanding what is going on inside people when they have health problems. There seem to be no side effects as a very strong magnetic field is used, rather than radiation.

MRI scanning seems to be safe for almost everyone, although there are a few exceptions - pregnancy, people who have metal fragments in their eyes (shrapnel) and anyone who has had a metal clip inserted in their head (e.g. for aneurysm treatment) may be judged unsuitable.

There may be problems with other people but they are less serious: people who cannot keep still or are agitated will cause blurring of the pictures; people who are claustrophobic often cannot tolerate the enclosed space; people over a certain weight or size may be too heavy for the equipment.

Anyone who has had a joint replacement or lumbar fusion may have a large metal object inside their body, but this does not bar them from having this kind of scan. However, the pictures of the area around the metal are badly affected and of little use.

MRI scanning was not designed to help diagnose low back pain so is not well suited to making these kind of decisions.

Advantages and uses of MRI scanning

MRI scans provide wonderfully detailed information about the anatomy of the back, and were designed to show up serious pathology and nerve compression. This they do extremely well. For the routine investigation of ordinary back pain these scans are much less useful due to the commonness of incidental findings.

Main uses for MRI scanning:

  • If serious pathology is suspected - eg infections, tumours
  • If operation for nerve compression or “segmental failure” is considered

It is difficult to justify ordering an MRI scan in other circumstances, especially in the light of McDonald et al’s (1996) paper which showed that a normal test result does not reassure people. In fact, the ordering of the test in the first place may cause more anxiety than that relieved by a normal outcome.

CT scanning

CT stands for Computerised Tomography. CT scans preceded MRI and still have many uses in medicine. They deliver a relatively high dose of x-rays so their use is considered carefully.In back pain CT scanning is less useful since the advent of MRI scanning, but is particularly good at imaging bone. This means it is the investigation of choice when a bony injury is suspected, such as a spinal injury where operation may be necessary to stabilise the spine.

Blood Tests

Blood tests for someone with back pain are done for a few specific reasons, but the answers they give are not always clear.

  • Suspicion of serious problems such as infection or tumours. Various blood tests can give indications that there is a systemic (ie body wide) problem of some kind. These tests need careful interpretation as their message is seldom clear
  • Checking for an arthritic disease. These diseases, such as rheumatoid arthritis and ankylosing spondylitis, are quite different from osteoarthritis which is the an age-related joint condition very common in the population.

Typical blood tests for back pain are:

  • ESR - Erythrocyte Sedimentation Rate. This test measures the rate at which the red blood cells settle in a test tube. The normal rate is 15-25 millimetres in the first hour, but an increased result is rather non-specific in telling what is wrong. A high ESR can be due to many things, some of them not at all serious (colds, ‘flu) and so does not tell the doctor what is wrong, but rather that there is something going on which needs attention. In some cases there may be a serious disease when the ESR is normal
  • CRP - C-reactive protein is a test which is more specific for infection, an uncommon cause of back pain.
  • HLA B27 is a protein found on white blood cells. If positive, it raises the possibility that the person may have ankylosing spondylitis (AS). 90% of AS sufferers are HLA B27 positive. However, you can be HLA B27 positive without having AS.

Other tests may be ordered in specific circumstances.

Bone Scan

A bone scan is the investigation of choice if, once x-rays have been performed, there is some suspicion of a change in the rate of bone turnover. An injection of a radioactive labelled chemical is given then after a certain time delay an image is taken on x-ray film. Areas of the body where the bone is very active show up dark on the film, and it is these areas which may indicate a problem of some kind.

Bone Densitometry

Osteoporosis is an increasingly recognised cause of back pain in older people. Osteoporosis is managed in a different manner from simple back pain, and there are successful treatments now available which can reverse the bone loss. Dexa scanning, or bone densitometry, is the investigation which give an indication of bone density. Interpretation of the results is a matter for a specialist in this field.


This investigation is becoming more common performed by a specialist radiologist. It is performed to find out exactly which disc level is causing the pain. Other investigations may give some indication of where the problem is, but before embarking on major surgery the surgeon may need to be more sure of the most likely pain source. To give a positive result the investigation needs to reproduce the normal pain the person is complaining of. Again, the results need to be interpreted by a specialist and taken in context.


This investigation is rarely performed now, to check for significant disc prolapses but has been overtaken by MRI scanning which is non-invasive. In a myelogram, dye in injected into the spinal canal and a series of x-rays taken. Anything, such as a disc prolapse, which gets in the way of the dye shows up as a dark area on the film. The films need specialist reading and interpretation to make sense.

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