Laboratory testing in monoclonal gammopathy of renal significance (MGRS)

Recently, monoclonal gammopathy of renal significance (MGRS) reclassified all monoclonal (M) gammopathies that are associated with the development of a kidney disease but do not meet the definition of symptomatic multiple myeloma (MM) or malignant lymphoma. The purpose was to distinguish the M gammopathy as the nephrotoxic agent independent from the clonal mass. The diagnosis of MGRS obviously depends on the detection of the M-protein. More importantly, the success of treatment is correlated with the reduction of the M-protein. Therefore, familiarity with the M-protein tests is a must. Protein electrophoresis performed in serum or urine is inexpensive and rapid due to automation. However, poor sensitivity especially with the urine is an issue particularly with the low-level M gammopathy often encountered with MGRS. Immunofixation adds to the sensitivity and specificity but also the cost. Serum free light chain (sFLC) assays have significantly increased the sensitivity of M-protein detection and is relatively inexpensive. It is important to recognize that there is more than one assay on the market and their results are not interchangeable. In addition, in certain diseases, immunofixation is more sensitive than sFLC. Finally, novel techniques with promising results are adding to the ability to identify M-proteins. Using the time of flight method, the use of mass spectrometry of serum samples has been shown to dramatically increase the sensitivity of M-protein detection. In another technique, oligomeric LCs are identified on urinary exosomes amplifying the specificity for the nephrotoxic M-protein.