Monoclonal Gammopathy of Undetermined Significance (MGUS): This condition is characterized by the presence of a monoclonal protein (M-protein) in the blood, which can be detected as an elevated level of immunoglobulin kappa free light chain. The normal SPEP (Serum Protein Electrophoresis) result suggests that the monoclonal protein is not causing a significant spike in the serum protein electrophoresis, which is consistent with MGUS. Mild hypercalcemia and anemia can also be seen in MGUS.

Smoldering Multiple Myeloma: This is a precursor state to multiple myeloma, characterized by higher levels of monoclonal protein and/or more pronounced bone marrow involvement than MGUS, but without the end-organ damage that defines active multiple myeloma. Elevated kappa free light chains, mild hypercalcemia, and anemia could be indicative of this condition, especially if there's a significant monoclonal protein burden.
Primary Amyloidosis (AL Amyloidosis): This condition involves the deposition of amyloid fibrils composed of immunoglobulin light chains in various tissues, leading to organ dysfunction. While SPEP may be normal, the presence of elevated free light chains and systemic symptoms like anemia and hypercalcemia could suggest this diagnosis, particularly if other signs of organ involvement are present.

Multiple Myeloma: Although the SPEP is normal, which might suggest against this diagnosis, multiple myeloma is a "do not miss" diagnosis due to its potential for rapid progression and the availability of effective treatments. The presence of hypercalcemia and anemia, along with elevated kappa free light chains, necessitates consideration of this diagnosis, especially if other myeloma-defining events or CRAB criteria (Calcium elevation, Renal failure, Anemia, Bone lesions) are present.
Light Chain Amyloidosis with Renal Involvement: Similar to primary amyloidosis, but with a focus on renal involvement, which could be subtle and not immediately apparent. This diagnosis is critical to consider due to the potential for rapid renal deterioration.

Light Chain Deposition Disease (LCDD): A rare condition characterized by the deposition of immunoglobulin light chains in tissues, leading to organ dysfunction. It's less common than amyloidosis but could present with similar laboratory findings and systemic symptoms.
Heavy Chain Disease: A group of disorders characterized by the production of abnormal heavy chains of immunoglobulins. While less likely, these conditions can present with a variety of systemic symptoms and abnormal protein findings, including elevated free light chains in some cases.