Detection of paraproteins
Agarose electrophoresis is the most common method currently in use in clinical diagnostic for the detection of paraproteins in serum and urine.
Paraprotein bands may be 'missed' if they are at low serum concentration (<5.0 g/l) and or where their mobility coincides with other bands such as beta globulins. It is also possible to miss paraproteins where there is no suppression of normal immunoglobulin concentrations
Immunofixation should be done on samples when no obvious paraprotein band is detected but where there is raised IgA or IgM without the increased staining of the beta-gamma region that is associated with a polyclonal increase in IgA or IgM.
IgD paraproteins and free heavy chains are susceptible to post-synthetic degradation, which results in diffuse paraprotein bands on electrophoresis. These may be missed if present at low concentrations or if there is an expectation of seeing a clearly defined band.
There are a number of situations where a band is seen in a serum electrophoretic separation that is not monoclonal immunoglobulin; these include:
- additional bands in the alpha-1 region due to allotypic variation in α-1 antitrypsin
- split alpha-2 zone due to the different mobility of the haptoglobin-haemoglobin complex after intravascular haemolysis
- an additional band in the beta-gamma region due to high concentrations of C-reactive protein
- additional bands in the fast gamma region due to the presence of fibrinogen.
It is also worth noting that some paraproteins precipitate at temperatures below 37oC - so called cryoproteins [Cryoglobulins]. Samples where cryoprotein is being considered must be collected, transported and separated at 37oC. Failure to do this may result in the precipitation of the cryoprotein which will be subsequently discarded with the call pellet.