Overview of SPEP/UPEP and Immunofixation for Monoclonal Proteins
1) SPEP – screening test to look for M-protein, but sometimes get false positives where M-protein is actually a polyclonal increase in Ig. Benefit is cheap, easy, and also gives a quantitative estimate of the concentration of the M-protein.
2) Serum Immunofixation – always done with SPEP, uses monospecific Abs for heavy and light chains – increases sensitivity to pick up M-protein and determines clonality (monoclonal vs polyclonal). Unlike SPEP, does not give quantitative estimate.
3) UPEP and Urine Immunofixation – 24 hour urine collecton – similar concepts as above. Main reason to do this is that it picks up many patients who have light chain secretion only, as those are rapidly excreted in the urine and thus are often not picked up on SPEP.
4) Serum Free Light Chains Assay – even more sensitive for detection of low levels of monoclonal free light chains in the serum than is the UPEP, and is also important in following response to therapy.
Basically, SPEP has a sensitivity of ~80% for multiple myeloma, and sensitivity is increase to >95% with addition of Serum immunofixation and UPEP/Urine Immunofixation, as ~20% of multiple myeloma patients secrete ONLY light chains which are better picked up by the UPEP.
(Chanu Rhee MD, 5/10/11)