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.

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Heparin-Induced Thrombocytopenia (HIT) is classically viewed as a drug-induced immune disorder triggered by exposure to the anticoagulant drug, heparin. However, HIT may also be considered a dysregulated autoimmune response to two naturally occurring substances, Platelet Factor 4 (PF4), a platelet protein, and heparin, a tissuederived glycosaminoglycan. Until recently, studies of HIT and PF4/heparin antibody seroconversion have been exclusively focused on patients exposed to heparin or heparinlike drugs, including low-molecular weight heparin, the heparinoids and the synthetic agent, fondaparinux. Recent reports, however, indicate that PF4/heparin antibodies may arise spontaneously in some individuals in the absence of heparin (Warkentin Am J Med 2008; Hursting J Thromb Thrombolysis, 2008). We undertook this study to document the seroprevalance of “naturally-occurring” PF4/heparin antibodies in healthy subjects without prior heparin exposure. To study “healthy” subjects, we used blood bank donors as a surrogate population. Samples from blood bank units were obtained through Duke Transfusion services/American Red Cross (ARC). Donors met eligibility criteria for blood donation to the ARC and generally, represent a healthy population (http://www.redcross.org/services/biomed/0,1082,0_557_,00.html#hem). A Duke IRB waiver was issued due to use of anonymous samples. Plasma from donor units/segments were assayed in duplicate for the presence of PF4/heparin antibodies using a commercially available PF4/heparin ELISA (PF4 enhanced IgG,A,M kits, Genetics Technology Institute, Waukesha, WY). Positivity was defined by the manufacturer as A405nm ≥0.4. Positive samples were stratified into low, (A405nm=0.4–0.6), intermediate (A405nm=0.61–0.99) or high-positive values (A405nm≥1.0). High and intermediate positive samples were further isotyped using IgG and IgM specific antibodies (Sigma, St. Louis, MO). Of the 970 blood bank donors tested to date, 31 samples (3.2%) were positive using the manufacturer’s cut-off value. Repeat testing confirmed positivity in all 31 samples. Of those testing positive, 26 were low-positive (2.3% of overall cohort; 84% of all positive samples), 1 sample was intermediate positive (0.1% of overall cohort; 3.2% of positive samples) and 5 samples were high-positive (0.5% overall cohort; 16% of positive samples). Isotyping of 6 high/intermediate positive samples revealed IgG in 3 patients, IgM in the remainder. There was no correlation between antibody positivity and ABO blood type. In summary, our data reveals a high prevalence of PF4/heparin antibodies, the majority of which are low-positive values (~84%). Many of these low-positive samples likely reflect background “noise” of the assay and could be eliminated using a higher cut-off for positivity. We also found a seroprevalance of 0.5% of high positive antibodies, of IgG and IgM isotype, supporting the notion that some individuals may have “naturally-occuring”