Platelet Factor 4/Heparin Antibodies in Blood Bank Donors
Platelet Factor 4/Heparin Antibodies in Blood Bank Donors
Platelet factor 4 (PF4)/heparin antibody, typically associated with heparin therapy, is reported in some heparin-naive people. Seroprevalence in the general population, however, remains unclear. We prospectively evaluated PF4/heparin antibody in approximately 4,000 blood bank donors using a commercial enzyme-linked immunosorbent assay for initial and then repeated (confirmatory) testing. Antibody was detected initially in 249 (6.6%; 95% confidence interval [CI], 5.8%–7.4%) of 3,795 donors and repeatedly in 163 (4.3%; 95% CI, 3.7%–5.0%) of 3,789 evaluable donors. "Unconfirmed" positives were mostly (93%) low positives (optical density [OD] = 0.40–0.59). Of 163 repeatedly positive samples, 116 (71.2%) were low positives, and 124 (76.1%) exhibited heparin-dependent binding. Predominant isotypes of intermediate to high seropositive samples (OD >0.6) were IgG (20/39 [51%]), IgM (9/39 [23%]), and indeterminate (10/39 [26%]). The marked background seroprevalence of PF4/heparin antibody (4.3%–6.6%) with the preponderance of low (and frequently nonreproducible) positives in blood donors suggests the need for further assay calibration, categorization of antibody level, and studies evaluating clinical relevance of "naturally occurring" PF4/heparin antibodies.
Heparin-induced thrombocytopenia (HIT) is an immunemediated disorder caused by antibodies that recognize a complex of 2 naturally occurring antigens: platelet factor 4 (PF4), which is a platelet-derived protein, and heparin, which is a member of the glycosaminoglycan family. HIT occurs in approximately 1% to 5% of patients treated with unfractionated heparin (UFH) and 0.2% to 0.8% of patients treated with low-molecular-weight heparin (LMWH). The immune response to PF4/heparin characteristically begins 4 days after initial exposure to UFH or LMWH. With few exceptions, all cases of HIT are acquired in the wake of heparin exposure.
Diagnosis of HIT is based on clinical criteria and laboratory demonstration of PF4/heparin antibodies by immunologic or functional assays. At most medical centers, immunoassays are preferred over functional assays because of their technical simplicity, high sensitivity (>99%), and rapid turnaround time. The specificity of immunoassays, however, is compromised by the high rates of asymptomatic PF4/heparin seroconversions that occur in various clinical settings. Asymptomatic seroconversions are seen in approximately 8% to 17% of patients receiving UFH, 2% to 8% receiving LMWH, and 1% to 2% receiving fondaparinux. For unknown reasons, exceptionally high rates of seroconversion (approximately 27%–61%) are documented in patients undergoing cardiac surgery. The clinical significance of asymptomatic seroconversions is presently unknown.
Until recently, it was believed that the specificity of immunoassays was principally affected by the occurrence of "false-positive" antibodies during heparin treatment and that PF4/heparin antibodies do not occur in healthy subjects. This perception was based on studies of healthy subjects performed by the manufacturers of commercial immunoassays (eg, PF4 Enhanced, Genetics Technology Institute [GTI], Waukesha, WI; and Asserachrom HPIA, Diagnostica Stago, Asnieres, France) and smaller seroprevalence studies comparing healthy donors with various clinical populations. Recent studies, however, suggest that PF4/heparin antibodies may occur spontaneously in the absence of exogenous heparin. Case reports of "spontaneous" HIT have documented seropositivity and clinical symptoms resembling HIT in several patients with recent infection or surgery. Another recent study describes the occurrence of false-positive PF4/heparin antibodies among patients diagnosed with antiphospholipid antibodies (APLAs) or systemic lupus erythematosus. Last, in our retrospective analysis of 11 studies containing serologic data on healthy subjects (n = 860), we noted PF4/heparin seropositivity in 17 (2.2%; 95% confidence interval [CI], 1.1%–3.2%) of 790 subjects tested by Stago enzyme-linked immunosorbent assay (ELISA), 1 (1.0%; 95% CI, 0%–3.0%) of 100 subjects tested by GTI ELISA, and 3 (4%; 95% CI, 0%–9.0%) of 70 subjects by particle gel immunoassay. The seroprevalence of PF4/heparin antibody in the general population, however, remains unclear.
In this study of prospectively collected samples, we determined the seroprevalence of PF4/heparin antibodies in approximately 4,000 blood bank donors. We also characterized the serologic features of PF4/heparin antibodies detected, including relative titer (ie, optical density [OD] result from ELISA), heparin dependency, and isotype.
Abstract and Introduction
Abstract
Platelet factor 4 (PF4)/heparin antibody, typically associated with heparin therapy, is reported in some heparin-naive people. Seroprevalence in the general population, however, remains unclear. We prospectively evaluated PF4/heparin antibody in approximately 4,000 blood bank donors using a commercial enzyme-linked immunosorbent assay for initial and then repeated (confirmatory) testing. Antibody was detected initially in 249 (6.6%; 95% confidence interval [CI], 5.8%–7.4%) of 3,795 donors and repeatedly in 163 (4.3%; 95% CI, 3.7%–5.0%) of 3,789 evaluable donors. "Unconfirmed" positives were mostly (93%) low positives (optical density [OD] = 0.40–0.59). Of 163 repeatedly positive samples, 116 (71.2%) were low positives, and 124 (76.1%) exhibited heparin-dependent binding. Predominant isotypes of intermediate to high seropositive samples (OD >0.6) were IgG (20/39 [51%]), IgM (9/39 [23%]), and indeterminate (10/39 [26%]). The marked background seroprevalence of PF4/heparin antibody (4.3%–6.6%) with the preponderance of low (and frequently nonreproducible) positives in blood donors suggests the need for further assay calibration, categorization of antibody level, and studies evaluating clinical relevance of "naturally occurring" PF4/heparin antibodies.
Introduction
Heparin-induced thrombocytopenia (HIT) is an immunemediated disorder caused by antibodies that recognize a complex of 2 naturally occurring antigens: platelet factor 4 (PF4), which is a platelet-derived protein, and heparin, which is a member of the glycosaminoglycan family. HIT occurs in approximately 1% to 5% of patients treated with unfractionated heparin (UFH) and 0.2% to 0.8% of patients treated with low-molecular-weight heparin (LMWH). The immune response to PF4/heparin characteristically begins 4 days after initial exposure to UFH or LMWH. With few exceptions, all cases of HIT are acquired in the wake of heparin exposure.
Diagnosis of HIT is based on clinical criteria and laboratory demonstration of PF4/heparin antibodies by immunologic or functional assays. At most medical centers, immunoassays are preferred over functional assays because of their technical simplicity, high sensitivity (>99%), and rapid turnaround time. The specificity of immunoassays, however, is compromised by the high rates of asymptomatic PF4/heparin seroconversions that occur in various clinical settings. Asymptomatic seroconversions are seen in approximately 8% to 17% of patients receiving UFH, 2% to 8% receiving LMWH, and 1% to 2% receiving fondaparinux. For unknown reasons, exceptionally high rates of seroconversion (approximately 27%–61%) are documented in patients undergoing cardiac surgery. The clinical significance of asymptomatic seroconversions is presently unknown.
Until recently, it was believed that the specificity of immunoassays was principally affected by the occurrence of "false-positive" antibodies during heparin treatment and that PF4/heparin antibodies do not occur in healthy subjects. This perception was based on studies of healthy subjects performed by the manufacturers of commercial immunoassays (eg, PF4 Enhanced, Genetics Technology Institute [GTI], Waukesha, WI; and Asserachrom HPIA, Diagnostica Stago, Asnieres, France) and smaller seroprevalence studies comparing healthy donors with various clinical populations. Recent studies, however, suggest that PF4/heparin antibodies may occur spontaneously in the absence of exogenous heparin. Case reports of "spontaneous" HIT have documented seropositivity and clinical symptoms resembling HIT in several patients with recent infection or surgery. Another recent study describes the occurrence of false-positive PF4/heparin antibodies among patients diagnosed with antiphospholipid antibodies (APLAs) or systemic lupus erythematosus. Last, in our retrospective analysis of 11 studies containing serologic data on healthy subjects (n = 860), we noted PF4/heparin seropositivity in 17 (2.2%; 95% confidence interval [CI], 1.1%–3.2%) of 790 subjects tested by Stago enzyme-linked immunosorbent assay (ELISA), 1 (1.0%; 95% CI, 0%–3.0%) of 100 subjects tested by GTI ELISA, and 3 (4%; 95% CI, 0%–9.0%) of 70 subjects by particle gel immunoassay. The seroprevalence of PF4/heparin antibody in the general population, however, remains unclear.
In this study of prospectively collected samples, we determined the seroprevalence of PF4/heparin antibodies in approximately 4,000 blood bank donors. We also characterized the serologic features of PF4/heparin antibodies detected, including relative titer (ie, optical density [OD] result from ELISA), heparin dependency, and isotype.