Background <p>Acquired dysfibrinogenemia is a rare and often underrecognized coagulation disorder characterized by impaired fibrinogen function rather than absolute deficiency. In plasma cell dyscrasias, including multiple myeloma, monoclonal gammopathy of undetermined significance, and monoclonal gammopathy of clinical significance, monoclonal immunoglobulins or light chains may interfere with fibrin polymerization, resulting in discordant fibrinogen assay results and diagnostic challenges.</p> Case presentation <p>A middle-aged woman exhibited persistently prolonged thrombin time (TT) and markedly reduced fibrinogen by the Clauss method, while PT-derived fibrinogen remained relatively preserved, yielding a stable elevated PT-derived/Clauss ratio (~ 4.0–4.7) suggestive of qualitative fibrinogen dysfunction. These abnormalities were detected more than three years before the diagnosis of λ light-chain multiple myeloma (MM), in the absence of CRAB features. Following anti-myeloma therapy, TT and fibrinogen parameters normalized in parallel with hematologic response.</p> Conclusions <p>This case illustrates that acquired dysfibrinogenemia with discordant fibrinogen assays may serve as an early laboratory clue to an underlying plasma cell dyscrasia and may precede overt multiple myeloma in some patients. Similar coagulation abnormalities may also occur in MGUS or MGCS and should not be interpreted as specific predictors of progression, but rather as potential manifestations of paraprotein-related hemostatic dysfunction.</p>