Challenges in Detecting Clinically Relevant Heparin-Induced Thrombocytopenia Antibodies

Abstract

Heparin-induced thrombocytopenia (HIT) is an antibody-mediated hypercoagulable state featuring high thrombosis risk and distinct pathogenesis involving immunoglobulin G-mediated platelet activation. The target of the immune response is a cationic “self” protein, platelet factor 4 (PF4), rendered antigenic by heparin. A key problem is that only a minority of anti-PF4/polyanion antibodies induced by heparin are pathogenic, i.e., capable of causing platelet activation and thereby clinical HIT. Since thrombocytopenia occurs frequently in hospitalized, heparin-treated patients, testing for “HIT antibodies” is common; thus, the problem of distinguishing between pathogenic and nonpathogenic antibodies is important. The central concept is that those antibodies that have platelet-activating properties demonstrable in vitro correlate well with pathogenicity, as shown by platelet activation tests such as the serotonin-release assay (SRA) and heparin-induced platelet activation assay. However, in most circumstances, immunoassays are used for first-line testing, and so it is important for clinicians to appreciate which immunoassay result profiles—in the appropriate clinical context—predict the presence of platelet-activating antibodies (Bayesian analysis). Clinicians with access to rapid, on-demand HIT immunoassays (e.g., particle gel immunoassay, latex immunoturbidimetric assay, chemiluminescent immunoassay) can look beyond simple dichotomous result interpretation (“negative”/“positive”) and incorporate semiquantitative interpretation, where, for example, a strong-positive immunoassay result (or even combination of two immunoassays) points to a greater probability of detecting platelet-activating antibodies, and hence supporting a diagnosis of HIT. Recent recognition of “SRA-negative HIT” has increased the importance of semiquantitative interpretation of immunoassays, given that strong immunoassay reactivity is a potential clue indicating possible HIT despite a (false) negative platelet activation assay.

Heparin-Induced Thrombocytopenia: A Review of New Concepts in Pathogenesis, Diagnosis, and Management

“…Ticagrelor can induce false negative HIPA among HIT patients, and other anti-platelet drugs might also cause false negative HIT functional assays [ 56 ]. Another mechanism is the novel concept of seroconversion preceding functional positivity in vitro [ 58 ]. To address this lack of sensitivity, several methods to avoid false-negative functional assays have been reported, but not validated by a dedicated study: (i) removing/inhibiting antiplatelet agents in the first case and (ii) performing a PF4-enhanced assay (PF4-SRA) [ 59 , 60 ] when suspecting the second case, although this approach could lead to false positive results [ 56 , 57 , 58 , 61 ].…”

“…Another mechanism is the novel concept of seroconversion preceding functional positivity in vitro [ 58 ]. To address this lack of sensitivity, several methods to avoid false-negative functional assays have been reported, but not validated by a dedicated study: (i) removing/inhibiting antiplatelet agents in the first case and (ii) performing a PF4-enhanced assay (PF4-SRA) [ 59 , 60 ] when suspecting the second case, although this approach could lead to false positive results [ 56 , 57 , 58 , 61 ]. Although such methods increase the performances of the gold standards functional assays, HIPA and SRA, false negative-negative results are still possible.…”

“…They are nowadays still broadly used, and the ASH guidelines recommend their use as first-line laboratory test [ 48 ]. They are recognized to be highly sensitive (i.e., allow to accurately rule out HIT), but unspecific (i.e., leading to false-positive results and unnecessary non-heparin anticoagulation while awaiting the definitive result of a functional assay or for long course) [ 46 , 58 ]. Recently, Warkentin et al highlighted that ELISA are not considered as rapid immunoassays anymore [ 58 ].…”

“…They are recognized to be highly sensitive (i.e., allow to accurately rule out HIT), but unspecific (i.e., leading to false-positive results and unnecessary non-heparin anticoagulation while awaiting the definitive result of a functional assay or for long course) [ 46 , 58 ]. Recently, Warkentin et al highlighted that ELISA are not considered as rapid immunoassays anymore [ 58 ]. In addition, they underscored the original observation of Lindhoff-Last [ 58 ], that IgG-specific ELISA are more specific without being less sensitive for HIT, echoing a communication of the scientific and standardization committee of the International Society on Thrombosis and Haemostasis (ISTH) [ 63 ] in favor of use of IgG-specific immunoassays [ 58 ].…”

“…Recently, Warkentin et al highlighted that ELISA are not considered as rapid immunoassays anymore [ 58 ]. In addition, they underscored the original observation of Lindhoff-Last [ 58 ], that IgG-specific ELISA are more specific without being less sensitive for HIT, echoing a communication of the scientific and standardization committee of the International Society on Thrombosis and Haemostasis (ISTH) [ 63 ] in favor of use of IgG-specific immunoassays [ 58 ]. In a review published in 2017, Arepally also underscored that anti-PF4/H IgG antibodies are the most relevant isotype for HIT pathogenesis, IgA and IgM contribution to HIT being subordinate [ 46 ].…”