Heparin induced thrombocytopenia (HIT) is a transient autoimmune disorder paradoxically associated with venous or arterial thrombotic complications. The clinical diagnosis is supported by a laboratory test, either a functional assay or an enzyme-linked immunosorbent assay (ELISA). In the absence of heparin reexposure, HIT antibodies are usually undetectable within 100 days after their appearance.1 Anticoagulation with unfractionated heparin (UFH) is preferred for cardiopulmonary bypass (CPB) in patients with a remote history of HIT who are antibody negative. This report illustrates the clinical dilemma that can arise when a “screening only” HIT antibody test (without confirmatory procedure) is used for patients with remote HIT whose test results are moderately positive and who need urgent cardiac surgery.
CASE DESCRIPTION
A 62-yr-old, 93-kg, man with prosthetic valve endocarditis and occluded saphenous vein grafts was scheduled for a redo aortic valve replacement and single vessel coronary bypass graft (AVR/CABG). His comorbidities included biventricular dysfunction, diabetes, cerebrovascular disease with cognitive impairment, and acute renal insufficiency. He was diagnosed with HIT 2 years before this hospitalization after his initial AVR/CABG. Per his medical history, he had no further exposure to heparin. On admission, his laboratory results included a stable platelet count of 123 × 109/L, international normalized ratio of 4.1 on warfarin and a creatinine of 1.0 mg/dL (glomerular filtration rate of >60 mL/min/1.7). He subsequently developed acute renal insufficiency (creatinine 2.8 mg/dL, glomerular filtration rate 25/mL/min/1.7) that was attributed to aminoglycoside toxicity. Given his remote history of HIT with thrombosis (bilateral lacunar infarcts), a commercial anti-PF4/heparin ELISA (GTI PF4 Enhanced®) was performed during routine preoperative laboratory testing to screen for residual HIT antibodies. The optical density (OD) value was 0.976 and 0.955 after high-concentration heparin (100 IU/mL UFH) was added in vitro. This yielded a percent inhibition of 2.4%. Inhibition of a positive reaction must be at least 50% to be considered confirmatory for characteristic pathogenic HIT antibodies. The pathologist reported the result as “equivocal” because of the negative confirmatory procedure. The hematology consultant recommended using a direct thrombin inhibitor (DTI) for anticoagulation during CPB. The anesthesiologist of record questioned the recommendation because of the positive ELISA in conjunction with a negative confirmatory inhibition given the patient's low pretest probability of HIT (Warkentin clinical score = 0; stable platelet count, remote heparin exposure, no new thrombosis).2 A functional serotonin release assay (SRA) was sent to a reference laboratory (Blood Center of Wisconsin, Milwaukee, WI) to help resolve the uncertainty associated with the initial positive OD value of 0.976. The sample was sent on a Monday morning and available 3 days later on Wednesday afternoon. The patient's serum did not release serotonin after the addition of a therapeutic amount of heparin and was reported as negative for the presence of platelet-activating HIT antibodies. Thus, UFH was used for anticoagulation.
The patient underwent CABG ×1 and a redo AVR with a mechanical prosthesis. He also required debridement of a periannular abscess and placement of a pericardial patch. Porcine heparin was administered as an initial bolus of 400 U/kg and 10,000 U in the pump priming volume. The activated clotting time was >700 s throughout the CPB period. The patient received an initial loading dose, pump prime, and continuous infusion of aminocaproic acid during the procedure. The patient was on CPB for 230 min. No further UFH was administered. The patient received 9 U of packed red blood cells, 6 U of plasma, and 1500 mL of cell saver blood in the operating room. The mediastinal chest tube drainage was 670 mL in 24 h. The nadir postoperative platelet count was 75 × 109/L and increased to 160 × 109/L on postoperative day 5 before stabilizing. A repeat GTI PF4 Enhanced assay was performed which was negative (absorbance <0.2). Thus, warfarin was started without prior administration of a DTI.
DISCUSSION
HIT develops 5-15 days after heparin exposure and is characterized by a >50% decrease in platelet count with or without thrombosis. HIT antibodies (IgG) recognize a complex of heparin and PF4 and activate platelets by binding to FcγRIIa receptors on their surface.3 In the presence of platelet-activiating HIT antibodies, heparin can cause rapid platelet activation and aggregation with clinically significant thrombosis. HIT antibodies are usually undetectable 100 days after their appearance and anamnestic responses are rare in seronegative patients upon heparin reexposure.1 The current recommendation for managing patients with previous HIT undergoing CPB is to use UFH provided HIT antibodies are no longer detectable.4 Some authors believe that routine testing for HIT antibodies should be performed on all patients undergoing cardiac surgery,5,6 whereas others recommend that it should only be performed in the presence of thrombocytopenia or increased clinical risk of HIT.7 The proponents of routine testing argue that positive anti-PF4/heparin ELISA results can also be useful in identifying patients at increased risk of postoperative complications.5,6 Other experts believe that routine testing will result in the over-diagnosis of HIT and unnecessary use of less familiar anticoagulants.7
The SRA and the anti-PF4/heparin ELISA are among the sensitive assays used to detect the presence of HIT antibodies. The SRA is a functional assay and has the highest specificity.8 Unfortunately, it is performed in only a few reference laboratories. Furthermore, it takes 3-4 working days to obtain the results, requires use of radioactive material, and is both time-consuming and expensive. By comparison, the anti-PF4/heparin ELISA has a higher sensitivity, but a lower specificity. It is widely available in most major medical centers with results available within 4 h. The high sensitivity of the anti-PF4/heparin ELISA makes its clinical utility greatest when the result is negative (OD <0.4). This virtually assures that platelet-activating HIT antibodies are not present.9
Studies have demonstrated that the specificity of the anti-PF4/heparin ELISA for detecting platelet-activating HIT antibodies is greatest when the OD value is >1.0.10,11 The interpretation of a positive OD value between 0.4 and 1.0 is much less certain and is heavily dependent on clinical factors to determine if platelet-activating HIT antibodies are truly present. The incidence of “false-positive” ELISA results (OD >0.4 in the absence of platelet-activating HIT antibodies) is highest in the 0.4-1.0 range and is due to nonpathogenic IgM and IgA antibodies or nonplatelet-activating IgG antibodies.12 This gives rise to a clinical dilemma when an anti-PF4/heparin ELISA is performed as a routine preoperative screening test to exclude residual HIT antibodies and the test result is in the 0.4-1.0 range. Anesthesiologists (and hematologists) feel compelled to use unfamiliar anticoagulation strategies based solely on positive ELISA results, despite their patients’ increased risk of excessive bleeding, blood transfusion, and mediastinal reexploration.13 In urgent cases, the surgical team does not have the option of waiting for an outside reference laboratory to perform a functional SRA to confirm the diagnosis, since the results of this test are usually not available for at least 3 working days.
More recently, an anti-PF4/heparin ELISA (Genetics Testing Institute GTI PF4 Enhanced) is commercially available that includes a confirmatory procedure by adding a high concentration of heparin to measure the inhibition of the positive reaction seen in the screening step. The addition of a high concentration of heparin as a confirmatory step is analogous to its use in the SRA where it increases specificity for pathogenic HIT antibodies.14 Thus, the ELISA is considered positive only when the OD without heparin is >0.4 and inhibition with heparin is >50%.
Our patient had an OD value of 0.976 without heparin and 0.955 with heparin (2.4% inhibition). According to the manufacturer's instructions, inhibition <50% shifts the interpretation of the ELISA from positive to equivocal. The equivocal laboratory result must be interpreted in light of the patient's clinical history. Any screening methodology must integrate pretest probability of HIT with the magnitude of the laboratory result. In our case, we had a patient with a very low pretest clinical probability of HIT antibodies who required urgent complex cardiac surgery. We suspected the anti-PF4/heparin antibodies detected were not pathogenic for HIT but were a marker for inflammation given the patient's active endocarditis. The negative ELISA confirmatory step using a high concentration of heparin was used as an adjunct to the low pretest probability to give us more confidence in the validity of our diagnostic hypothesis. This clinicopathologic approach suggested this patient's probability of HIT was low. Had the patient's condition decompensated while waiting for the SRA results, we were prepared to proceed with surgery using UFH. The negative SRA confirmed the absence of platelet-activating HIT antibodies in our patient.
For patients with a history of HIT who are HIT antibody-negative and require cardiac surgery, current guidelines recommend the use of UFH. Our case report is consistent with these recommendations, yet tries to illustrate that a weak to moderately positive ELISA (OD 0.4-1.0) can lead to over-diagnosis of HIT. Because HIT is a clinicopathologic diagnosis, assessment of clinical risk is essential. In our case, the negative (large-dose heparin) confirmatory test in conjunction with a low clinical (pretest) probability led us to believe that the moderately positive screening ELISA test did not represent the presence of pathogenic heparin/PF-4 antibodies in our patient. Thus, we conclude that the risk of using UFH appeared lower than that of excessive bleeding from the use a DTI for anticoagulation in a patient undergoing complex cardiac surgery.
In summary, our report illustrates the use of a clinicopathologic approach to the diagnosis of HIT that integrates the assessment of pretest probability (4 T's clinical score) and the laboratory test likelihood score (OD >0.4 and inhibition >50%) to determine the probability of HIT. There is growing evidence that HIT may be overdiagnosed when routinely using the anti-PF4/heparin ELISA in patients who have a low clinical probability of HIT. In our experience, the addition of a high concentration heparin confirmatory step (akin to SRA) has been a useful adjunct in our attempt to improve the diagnostic specificity of this laboratory test. Further studies are needed to confirm the validity of this approach.
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