Distinguishing between anti-platelet factor 4/heparin antibodies that can and cannot cause heparin-induced thrombocytopenia.
BACKGROUND
Many patients exposed to heparin develop antibodies against platelet factor 4 (PF4) and heparin, yet only those antibodies that activate platelets cause heparin-induced thrombocytopenia (HIT). Patients who produce anti-PF4/heparin antibodies without developing HIT either have antibodies that do not cause platelet activation or produce pathogenic antibodies at levels that are insufficient to cause HIT. Understanding the differences between anti-PF4/heparin antibodies with and without HIT will improve test methods and reduce overdiagnosis.


OBJECTIVE
To investigate the presence of low levels of platelet-activating antibodies in patients investigated for HIT who had anti-PF4/heparin antibodies but failed to cause platelet activation in the (14) C-serotonin release assay (SRA).


METHODS
We developed a platelet activation assay similar to the SRA using exogenous PF4 without added heparin (PF4-SRA). This assay was able to detect low levels of platelet-activating antibodies. We used this PF4-SRA to test for platelet-activating antibodies in patients investigated for HIT.


RESULTS
The PF4-SRA detected platelet-activating antibodies in seven (100%) of seven SRA-positive sera even after the samples were diluted until they were no longer positive in the standard SRA. Platelet-activating antibodies were detected in 14 (36%) of 39 patients who had anti-PF4/heparin antibodies but tested negative in the SRA and did not have clinical HIT. The clinical diagnosis of HIT was confirmed by chart review and concordant with the SRA results.


CONCLUSIONS
A subset of heparin-treated patients produce subthreshold levels of platelet-activating anti-PF4/heparin antibodies that do not cause HIT. An increase in the titer of these pathogenic antibodies, along with permissive clinical conditions, could lead to HIT.
Department of Medicine, Michael G. DeGroote School of Medicine, McMaster University, Hamilton, ON, Canada.
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I
Nazi
Department of Medicine, Michael G. DeGroote School of Medicine, McMaster University, Hamilton, ON, Canada.|Canadian Blood Services, Hamilton, ON, Canada.
D M
DM
Arnold
Department of Medicine, Michael G. DeGroote School of Medicine, McMaster University, Hamilton, ON, Canada.|Department of Pathology and Molecular Medicine, McMaster University, Hamilton, ON, Canada.
T E
TE
Warkentin
Department of Medicine, Michael G. DeGroote School of Medicine, McMaster University, Hamilton, ON, Canada.
J W
JW
Smith
Department of Medicine, Michael G. DeGroote School of Medicine, McMaster University, Hamilton, ON, Canada.
P
P
Staibano
Department of Medicine, Michael G. DeGroote School of Medicine, McMaster University, Hamilton, ON, Canada.
J G
JG
Kelton
D000906:Antibodies / Q000097:blood*; D000925:Anticoagulants / Q000009:adverse effects / Q000276:immunology*; D015415:Biomarkers / Q000097:blood; D001792:Blood Platelets / Q000276:immunology* / Q000378:metabolism; D016022:Case-Control Studies; D006493:Heparin / Q000009:adverse effects / Q000276:immunology*; D006801:Humans; D015539:Platelet Activation; D010978:Platelet Factor 4 / Q000276:immunology*; D010979:Platelet Function Tests; D011237:Predictive Value of Tests; D012698:Serologic Tests; D013921:Thrombocytopenia / Q000097:blood / Q000139:chemically induced / Q000175:diagnosis / Q000276:immunology*
D000906:Antibodies; D000925:Anticoagulants; D015415:Biomarkers; D010978:Platelet Factor 4; D006493:Heparin
heparin; platelet activation; platelet factor 4; thrombocytopenia; thrombosis