Pitfalls in the diagnosis of heparin‐Induced thrombocytopenia: A 6‐year experience from a reference laboratory

Abstract

Heparin-induced thrombocytopenia (HIT) is caused by platelet-activating antibodies against complexes of platelet factor 4 (PF4) and heparin. The diagnosis of HIT is contingent on accurate and timely laboratory testing. Recently, alternative anticoagulants for the treatment of HIT have been introduced along with algorithms for better HIT diagnosis. However, the increased reliance on immunoassays for the diagnosis of HIT may have harmful consequences due to the high rate of false positive results. To compare trends and implications of current HIT testing approaches, we analyzed results over a six-year period from the McMaster University Platelet Immunology Reference Laboratory. From 2008 to 2013, 8,546 samples were investigated for HIT using both an in-house IgG-specific anti-PF4/heparin enzyme immunoassay (EIA) and the serotoninrelease assay (SRA). Of 8,546 samples tested, 13.4% were true-positives (positive in both assays); 65.6% were true-negatives (negative in both assays); 20.9% were presumed false positive for HIT (EIA-positive/SRAnegative); and 0.2% were EIA-negative/SRA-positive. The frequency of EIA-positive/SRA-negative results increased over time (from 12.9% in 2008 to 22.9% in 2013). We found that the number of SRA-negative samples was reduced from referring centers that used an immunoassay as an initial screen; however, 41% of those samples tested negative in the immunoassay and in the SRA at the reference laboratory. The suspicion of HIT continues at a high rate and the agreement between the EIA and SRA test results remains problematic.

Characterization of platelet factor 4 amino acids that bind pathogenic antibodies in heparin‐induced thrombocytopenia

“…The disparities between anti‐PF4/heparin antibodies that activate platelets (pathogenic HIT antibodies) and those that do not (non‐pathogenic anti‐PF4/heparin antibodies) present a significant challenge in diagnosing HIT patients . This is because of the absence of a rapid and widely available laboratory diagnostic test with high specificity that can distinguish pathogenic from non‐pathogenic antibodies .…”

Distinguishing between anti–platelet factor 4/heparin antibodies that can and cannot cause heparin‐induced thrombocytopenia

“…The 14 samples that were positive in the PF4‐SRA had a significantly higher OD value in the EIA compared with the 25 samples that remained negative in the PF4‐SRA. In addition, we recently reported that although high OD values in the EIA can predict SRA reactivity (93% of samples with OD 405 >2.0 are SRA‐positive), there remains a subset of samples (7%) that will have a negative SRA result even in the presence of high OD 405 values . Similarly, in this study, three of the EIA‐positive/SRA‐negative samples that were negative in the PF4‐SRA actually had the highest OD 405 values in the EIA (Fig.…”

“…Evidence to support the need for a critical titer of HIT antibodies are that (i) high titers of anti‐PF4/heparin antibodies strongly correlate with positive results in HIT functional assays ; (ii) high titers of anti‐PF4/heparin antibodies precede the onset of thrombocytopenia and thrombosis in patients with HIT by a median of 2 and 5.5 days, respectively ; and (iii) ~88% of patients who are positive in the SRA have sufficient levels of platelet‐activating antibodies to cause > 50% release in the SRA . Therefore, a threshold level of platelet‐activating anti‐PF4/heparin antibodies might be required to initiate in vivo platelet activation and subsequently trigger HIT.…”

Adverse Drug Reactions in the ICU

“…For moderate or high scores, further testing is recommended, and the patient started on alternative anticoagulation until the diagnosis can be conclusively excluded. The absence of PF4 IgG antibodies has a high negative predictive value and rules out HIT; specificity is poor so positive tests require additional analysis [142][143][144][145]. Serotonin release assay has high specificity (95-100 %) and positive predictive value for HIT; however, the availability is limited.…”