Clinical and Diagnostic Complexity in ELISA- and SRA-Negative Monoclonal Gammopathy of Thrombotic Significance
Authors: Kanack Adam , Leung Nelson , Mauch Emily , Splinter Noah , Kohlhagen Mindy , Murray David , Jones Curtis , Johns Gretchen Schaef , Pabmanabhan Anand 
Published: December 2024

Monoclonal gammopathy of thrombotic significance (MGTS) is a recently recognized thrombophilia characterized by the production of a monoclonal antibody that binds to platelet factor 4 (PF4) and stimulates platelet activation (Blood. 2022 Jul 7;140(1):73-77). Typical signs of MGTS include a history of recurrent thrombosis, persistent thrombocytopenia, and positive laboratory results in PF4/polyanion enzyme-linked immunosorbent assay (ELISA) and serotonin release assay (SRA) testing, as in the related thrombotic disorder heparin-induced thrombocytopenia (HIT). Here, we present a case of MGTS, presenting with transient thrombocytopenia coincident with thrombotic episodes and negative test results in SRA and PF4/polyanion-based testing platforms (ELISAs and automated assays).
A 56-year-old male patient initially presented with acute left lower extremity deep vein thrombosis (DVT) and a pulmonary embolism. Following the administration of warfarin, the patient experienced a second DVT four months later and was transitioned to a factor Xa inhibitor. Fifty-two months later, the patient developed occlusion of the splenic and superior mesenteric veins despite additional anticoagulation therapy. Further testing led to a diagnosis of an IgG2 kappa monoclonal gammopathy of undetermined significance (MGUS). Eleven months later, the patient again suffered breakthrough thrombosis, specifically radial artery thrombosis, and a thrombotic cerebrovascular accident, leading to the addition of aspirin to the patient's treatment regimen. Notably, where detailed blood counts were available in the patient's clinical history, the patient experienced only transient thrombocytopenia coinciding with his two most recent thrombotic episodes and has had platelet counts in the normal range in the four-year period since his last thrombotic event. After initiating dual anticoagulant/antiplatelet therapy, the patient has not developed recurrent thrombocytopenia or new thrombotic events.
Antigen-based and functional testing for HIT antibodies (ELISA, SRA, and automated immunoassays) were negative in this patient, unlike in the previously described index case of MGTS, where serological testing mimicked classical HIT serology (i.e., HIT ELISA and SRA positivity). The sample was also non-reactive in a vaccine-induced immune thrombotic thrombocytopenia (VITT)-specific ELISA employing uncomplexed PF4 targets, as previously described (Am J Hematol. 2022 May;97(5):519-526). Conversely, the patient's antibody consistently stimulated platelet activation in a PF4- and IgG-dependent manner in functional testing using PF4-treated platelets. This included PF4-treated platelets that were freshly isolated and used in the PF4-dependent P-selectin expression assay (PEA) and cryopreserved platelets used in the thrombospondin-1 release assay. Interestingly, consistently positive PEA results have been documented from four samples over the last four years, during which the patient had normal platelet counts.
To further characterize the patient's IgG2 MGUS antibody, the patient's monoclonal was immuno-enriched. Patient sera and the immuno-enriched MGUS were then compared by mass spectrometry, which generated identical mass profiles. After confirming the effective enrichment of the patient's MGUS by mass spectrometry, the antibody was de novo sequenced or “reverse-engineered” to obtain the antibody's amino acid sequence. The antibody was then expressed and tested in ELISA-based and functional platelet assays. Importantly, the recombinant antibody replicated the PF4-dependent platelet activation observed with patient serum and did not bind PF4/polyanion targets in ELISA testing. Together, these data strongly support the identification of a new class of ELISA- and SRA-negative anti-PF4 monoclonal antibodies that cause MGTS.
This case illustrates the diagnostic challenges in MGTS. MGTS patients may not exhibit persistent thrombocytopenia, and diagnostic tests for detecting HIT antibodies may be ineffective in accurately diagnosing this new thrombophilia. This case emphasizes the importance of reviewing platelet counts in the peri-thrombotic period to recognize transient thrombocytopenia and establishes the utility of PF4-enhanced functional assays to detect pathogenic anti-PF4 antibodies, even in settings where classical HIT testing (i.e., ELISA/SRA) is negative.