Bence Jones protein urine test: what it detects and why it is done

Cancer Research UK explains that blood tests for myeloma look for abnormal types of antibodies (immunoglobulins) called paraproteins, and that myeloma makes paraproteins. Healthy antibodies are a mixture of many types, whereas a paraprotein is a single abnormal type produced in excess, which is what makes it detectable.

Cancer Research UK explains that myeloma makes paraproteins and that one part of the paraprotein is called the light chain, also known as the Bence Jones protein. The body gets rid of the light chain in the urine. That is the origin of the old urine test: if free light chains are being produced in excess, they spill into the urine where they can be detected.

The older terminology persists on request forms and in reports, so you may see it written as Bence Jones protein, urine paraprotein or urine protein electrophoresis. They broadly refer to the same question: are abnormal free light chains passing into the urine?

The test is requested when a doctor is looking for or monitoring a plasma cell disorder, most commonly myeloma. The NHS describes myeloma as a type of blood cancer that affects your bones and different parts of your body. Symptoms can be vague and varied, so GPs typically start with a panel of blood and urine tests rather than a single test.

Cancer Research UK says that a GP can arrange blood tests to check blood cell levels (a full blood count), how well the liver and kidneys are working, calcium, and how thick the blood is (plasma viscosity), and that these tests also look for paraproteins. A urine sample may be asked for to check for kidney problems (urinalysis) and for Bence Jones protein.

Once myeloma is diagnosed, Cancer Research UK says urine tests are also a way of monitoring the disease, and the choice between urine and blood tests for light chains is a decision for the haematology team.

Cancer Research UK describes two ways of giving the urine sample: you might give a sample of your urine in a pot to your doctor or nurse, or you might collect your urine over 24 hours, taking a container home to collect all of the urine you pass in the 24 hours.

A 24-hour collection is more demanding than it sounds. The usual approach is to pass urine into the toilet at the start of the collection, then collect every subsequent sample in the container provided for the next 24 hours, finishing with the first sample the following morning. The laboratory or ward will give you precise instructions, including whether the container holds a preservative and how to store it, and those instructions take precedence over any general description.

Cancer Research UK also notes that it is now more common to have a blood test instead of a 24-hour urine collection. The blood test is called a serum free light chain (sFLC) assay. This is a pragmatic change: a blood test is easier for the person and gives a result that is easier to compare over time.

A positive Bence Jones protein result means that free light chains were found in the urine. It is not by itself a diagnosis of myeloma. The same finding may be seen in related conditions, which is why paraprotein results are interpreted by haematologists together with the blood count, kidney function, calcium, imaging and bone marrow examination.

Other conditions are on the differential for a paraprotein. MGUS (monoclonal gammopathy of undetermined significance) is a condition in which a paraprotein is present without myeloma. Cancer Research UK also mentions a condition similar to myeloma called plasmacytoma, which a PET-CT scan can help diagnose.

A negative result is less informative than people expect. The NHS lists the main tests for myeloma as a CT or MRI scan and a bone marrow biopsy, in which a small piece of bone marrow is removed for testing. If myeloma is still suspected, a negative urine light chain test would not end the work-up.

If a GP thinks your symptoms could be due to myeloma, Cancer Research UK says you will have more tests at the hospital, which might include further blood tests, a bone marrow test, an x-ray, an MRI scan, a PET-CT scan or a CT scan. Further blood tests can include beta 2 microglobulin, immunofixation electrophoresis, albumin and LDH, which help to understand how myeloma is affecting you and how it might develop.

The NHS adds that if you are told you have myeloma you will be helped through diagnosis and treatment by a team of specialists including a clinical nurse specialist who will be your main point of contact. If you have been told you have myeloma, you may need more tests, such as further blood tests and scans, to help your specialist team decide on treatment.

If you are waiting for results and worried, the NHS says you can call the hospital or your GP and they may be able to update you if your results are ready. A paraprotein result that is only mildly abnormal is often followed up with a repeat rather than urgent action, but that decision belongs to the haematology team.