Additional biochemical tests
Beta 2-microglobulin
Serum beta 2-microglobulin β2M is the single best indicator of the extent of the myeloma and is therefore an important prognostic factor. A high β2M level indicates a poor prognosis.
β2M is a protein that is present on the surface of white blood cells. It is not diagnostic for any specific disease but serum levels correlate with the number of myeloma cells in the body. High levels of β2M also occur in the context of kidney damage.
It is a useful marker of tumour burden, an increasingly important prognostic marker, as well as an indicator of renal impairment.
The normal values for β2M are <2 mg/L in blood and <0.12 mg/24 hours in urine although this may vary between centres. A serum β2M <3.5mg/l alongside albumin >35g/L indicate stage I disease, while serum β2M >5.5mg/l indicates stage III disease, according to the most commonly used staging system for myeloma – the Revised International Staging System (R-ISS) (see Module 4 – Staging and Prognosis).
Lactate dehydrogenase (LDH)
Lactate dehydrogenase is an enzyme found in most cells of the body. Although it is normally involved in the production of cellular energy, it is often used as a marker of tissue damage. In myeloma, the LDH level provides a useful indicator of tumour burden, staging and prognosis as outlined also in the R-ISS.
When disease or injury affects cells, the enzyme is released into the blood stream and can be measured by a simple blood test. Five different subtypes of LDH exist but the blood test measures all types of LDH and provides a value for total LDH. LDH can be elevated in a number of conditions in addition to myeloma, including other cancers, infection and cardiac arrest.
The normal total LDH for adults is approx. 140 – 280 U/L although this may vary between centres.
Paraprotein evaluation
Approximately 80% of myeloma patients have detectable paraprotein levels in their blood. The detection of paraprotein in these patients is an important diagnostic feature as well as a useful marker of disease activity. For patients, their paraprotein figure is often the single most important test result they look for, so nurses should be very aware of this.
The screening test used for paraprotein is serum protein electrophoresis (SPEP). If paraprotein is found it is then typed to identify the heavy and light chain components so that the myeloma can be classified and quantified by immunofixation electrophoresis.
Another ~20% patients have ‘light chain’ myeloma and only produce abnormal light chains from the abnormal plasma cells that should be measured by serum free light chain analysis. A very small number – around 1% patients – produce no detectable paraprotein or light chains and they are diagnosed as having ‘non-secretory myeloma’. This is generally monitored using bone marrow biopsies and imaging.