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Electrophoresis (EPG): The
separation of proteins (usually from serum or urine) on the basis of their charge and
molecular weight. This can be done in a gel (EPG) or more recently in a capillary (CZE)
Immunoglobulin (Ig) levels: Measurement of
the levels (in serum or other fluid) of IgG, IgA and IgM.
Paraprotein: The immunoglobulin product of
a single clone of immunoglobulin producing cells. Coming from a single cell clone, these
proteins have identical size and charge characteristics and thus migrate as a discrete
band on EPG or CZE.
Immunofixation: This is an immunological
technique whereby a presumed paraprotein is incubated in separate samples with antibodies
selective for each of the heavy (G, M, A) and light (kappa, lambda) chains. A monoclonal
paraprotein will only react with a single heavy chain and a single light chain type e.g.
IgG lambda. This indicates that the immunoglobulin comprising the paraprotein consists of
only a single heavy chain combined with a single light chain class and hence originates
from a single clone of cells.
Bence Jones protein: In haematopoetic
malignancies producing paraproteins (especially myeloma) there can be excess production of
free immunoglobulin light chains which can thus not assemble into whole immunoglobulin
molecules. Sometimes a malignant clone will produce only light chains and not synthesise
heavy chains. This free light chain (or Bence Jones Protein), because of its low molecular
weight, is not retained by the glomerular basement membrane and passes freely into urine.
It is not normally detected in serum unless renal impairment is present.
The incidence of paraproteins increases with age and in the
elderly, they can be quite a common finding. They are also found more frequently in
association with chronic inflammatory processes. In most individuals these paraproteins
are of no clinical significance. However these must be differentiated from paraproteins
that are associated with lymphoproliferative disorders such as multiple myeloma, lymphoma
or CLL. ‘Benign’ paraproteins are usually at low concentrations (usually less
than 9g/L) and the concentration does not increase in amount over a period of observation
of months to years. Additionally, they are not associated with suppression of normal
immunoglobulin levels, nor with the presence of free light chains (Bence Jones protein) in
the urine.
Malignant paraproteins on the other hand, are present at higher concentrations (usually
greater than 10g/litre). Even if detected very early when their initial concentration is
low, their concentration increases over time. They are often associated with other
clinical abnormalities, depression of normal immunoglobulin levels and sometimes, the
presence of a free light chains in urine. Paraproteins in this category with an IgM heavy
chain are associated with lymphomas or CLL, whilst IgG, IgA with myeloma. Bence Jones
protein is much more common in myeloma than lymphoma
Serum paraproteins
The primary method of detection of paraproteins is serum
EPG. However serum paraproteins, especially of IgM or IgA isotype, can sometimes be missed
on EPG examination. This occurs most frequently because of the co-migration of the
paraprotein with bands of major serum protein or because of a variety of other technical
factors. Therefore, to exclude most clinically significant serum paraprotein, it is
suggested that Ig levels and EPG should both be ordered. The possibility of a masked
paraprotein is suggested by marked elevation of an immunoglobulin of only a single heavy
chain isotype. For example, very high IgA, but normal IgG and IgM. Masked paraproteins can
only be detected with immunofixation of the serum sample.
Urine paraproteins
The second reason why paraproteins may not be noticed in
serum is that they are present only in urine. This occurs if only Bence Jones protein is
secreted by the malignant clone. If significant quantities of Bence Jones protein are
produced, then marked reduction of serum immunoglobulins usually occurs. Urine
paraproteins are detected with urine EPG
Bands on serum or urine EPG are designated probable
paraproteins, because bands can sometimes be seen with non-immunoglobulin proteins or may
be due to artefacts. Immunofixation must be carried out in order to confirm monoclonality
of the band and be able to label the band as a paraprotein.
An approach to initial assessment
of patients with paraproteins
Paraproteins >10g/L, or Bence Jones present. In this
situation, a lymphoproliferative disorder needs to be excluded and expert evaluation is
desirable.
Paraproteins <10g/L. Look for anaemia, lymphocytosis, elevated serum Ca2+, Bence Jones
Protein on urine EPG or depression of normal immunoglobulins. If none of these is present
and there are no other clinical features to suggest a lymphoproliferative disorder, it is
reasonable to presume that this represents a ’benign’ paraprotein. This can be
managed by periodic re-evaluation of the patients over a number of years (e.g. 3-4
times/year over 2-3 years, then annually thereafter). Progressive rise in the paraprotein
or development of the aforementioned features suggests the possible evolution of a
lymphoproliferative disorder. This should be excluded by evaluation of the patient by an
appropriately experienced physician.
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