Apport diagnostique du dosage des chaînes légères libres sériques d'immunoglobulines pour l'exploration des gammapathies monoconales

Mononoclonal gammopathies are not unfrequent in people over 50 years old (3,2 %). At presentation about half of them are classified as MGUS (monoclonal gammopathy of undetermined significance) whereas only 25 % have clinically documented plasma cell dyscrasias (myeloma, Waldenström macroglobulinemia or amyloidosis). MGUS has a 1 % per year risk of progression to such plasma cell malignant disorders. Establishing the diagnosis of monoclonal gammopathy requires immunofixation of serum and urine. Serum free light chain (fFLC) assay is a recently available test for diagnosis and monitoring patients with those plasma cell dyscrasias. The used antisera are specific for hidden épitopes on light chains bound to the heavy chain in the intact immunoglobulin, but exposed on the free light chains. Abnormal calculated serum κ/λ ratio is assumed to give information on large monoclonal production excess. In clinical context with absence or minimal monoclonal immunoglobulin marker, such as light chain multiple myeloma, non-secretory multiple myeloma, amyloidosis and light chain deposition disease, fFLC has proven to be a useful laboratory test. For the more frequent plasma cell dyscrasias (multiple myeloma with intact immunoglobulin, MGUS) the clinical utility of the fFLC for diagnosis and monitoring needs more extensive prospective studies to be established.