Background: MGRS is a relatively new concept for patients with renal paraprotein deposition (RPD) (except monoclonal cast nephropathy) with no bone marrow confirmed diagnosis of malignancy or premalignancy (< 10% monoclonal plasma cells). This concept has been used as a reason for a nephrology consult for a bone marrow biopsy (BMB), therefore it is important to evaluate the outcome of MGRS tentatively diagnosed via renal biopsy and confirmed by BMB. This study’s purpose was to identify what percentage of various subtypes of MGRS tentatively diagnosed via renal biopsy can be confirmed as MGRS by BMB and followup. Design: In total, 124 renal biopsies with variants of RPD were identified at our center out of 3811 renal biopsies (3.3% of overall cases) over past 10 years. Biopsy cases with known Myeloma, B Cell Lymphoma or Monoclonal Cast Nephropathy were separated as a heavy burden group. The remaining biopsies with RPD were considered as tentative MGRS diagnoses. Their BMB and clinical indices were followed up to determine the percent that resulted in a confirmed MGRS diagnosis. Results: Among the 124 renal paraprotein deposition cases, 43 cases (34.7%) were categorized to the heavy burden group (Figure). The remaining 81 cases with other variants of RPD were further divided into four categories based on the follow-up. Myeloma or Lymphoma were found in 34 cases (27.4%). Twelve outside consultation cases (10%) did not show follow-up biopsies as we were unable to obtain record access. BMB’s diagnosed as nonmalignant (6 cases, 4.8%) or premalignant (29 cases, 23.4%) were confirmed to be MGRS for a total of 35 cases (28.2%). Among the different categories of tentative MGRS, Monoclonal Light/Heavy Chain Deposition Diseases (L/HCDD) had the highest positivity rate for myeloma on the subsequent BMB. Proliferative Glomerulonephritis with Monoclonal Immunoglobulin Deposition (PGNMID) had a high rate of nonmalignant or premalignant diagnosis by BMB. Monoclonal Amyloidosis had some nonmalignant and pre-malignant diagnoses after BMB although myeloma was associated with this renal entity. Conclusions: The data indicate that BMB is crucial for confirming a diagnosis of MGRS. The study also provides pathologists and clinicians with general expectations regarding the possible BMB outcome for the many variants of MGRS identified tentatively via renal biopsy.
Metcalf B, Huang JZ, Li W, Kanaan H, Zhang P. Final counts of monoclonal gammopathy of renal significance (MGRS) after bone marrow biopsies (BMB) and follow-up. Mod Pathol. 2022 Mar;102(Suppl 2):990-991.