Basic understanding of Monoclonal gamapathy
Normal plasma cells secrete antibodies directed against specific antigens. It is capable of producing 5 different heavy chains (IgA, IgG, IgM,IgD,IgE)and 2 different light chains(Kappa and lambda). So, there is a possible 10 different combination of antibodies(IgG kappa, IgG lambda, IgM kappa, IgM lambda and such)For some reason there is always an excess of light chains than heavy chain which in physiologic state is excreted into the tubules but reabsorbed completely and broken down into amino acids (recycled by tubules). If there is an excess of light chains than what the tubules can handle(as in the case of monoclonal gamapathy), light chains are excreted in urine (Benze jones protein) .
In Monoclonal gamapathy(MM, Waldenstroms and such) there is clonal proliferation of plasma cell that produces the same antibodies/clonal(not directed against antigen). Some clonal plasma cells produce just light chains and not heavy chains. Rarely some clonal plasma cells produce heavy chains and not light chains.
Now let us see what information we could get from SPEP/Immunofixation, UPEP and free light chains.
Method : Serum proteins are electrophoretically separated in SPEP based on its electrical charge. Serum proteins generally categorize in 5 different zones( albumin, alpha 1 globulin, alpha 2 globulin, beta 1 globulin, beta 2 globulin and gamma) Gamma region has the polyclonal immunoglobulin. If there is a monoclonal protein , we see a spike in the gamma region(occasionally the M spike can be seen in the alpha or beta region). The peak of the M spike in electrophoresis in relation to the total protein will help quantify the M protein. say if the spike is 40% of the total protein , then if we know the total protein quantification , we should be able to calculate the amount of M protein ex / total protein in 10 gm/dl and spike is 40% , then M protein is 4 gram/dl . So, SPEP helps determine two information.
1) The presence of M protein
2) Quantification of M protein
This is used in conjunction with SPEP. By using antibodies against the 5 different known classes of heavy chain and 2 different classes of light chain, we can determine the type of M protein. So, the immunofixation helps
1) Identify the type of M protein such as IgG Kappa or IgM lambda and such..
Some light chains are freely filtered in the glomerulus and quickly cleared from blood. So, SPEP cannot identify these light chains.Under these circumstances, UPEP will increase the sensitivity of identifying the myeloma protein when combined with SPEP. SPEP alone has 80% sensitivity and combining UPEP increases the specificity to 95%. The UPEP is done the same way as SPEP and can quantify the M protein just as in SPEP.
Free light chains
Technically, if we can obtain serum free light chains , we do not need to order UPEP since SPEP and free light chains can together increase sensitivity of identifying M protein to >95%. It is just that it is expensive compared to SPEP and UPEP combined together.
SPEP — For knowing if there is M protein and if so quantification of M protein
Immunofixation – For identifying the M protein (using antibodies agains heavy and light chains)
UPEP – For light chains in urine
Free light chain — Highly sensitive for identifying even small increase in M protein-expensive though!