Evaluation of flow cytometric HIT assays in relation to an IgG-Specific immunoassay and clinical outcome

Flow cytometry vs conventional methods for the evaluation of anti-PF4/heparin antibodies: a single center study

Aim: Heparin-induced thrombocytopenia (HIT) is a rare, life-threatening, immune-mediated adverse effect of heparin administration. This study compares frequently used laboratory assays in terms of their effectiveness in HIT diagnosis. Materials & methods: Fifty patients with suspected HIT were tested by gel immunoassay and solid phase PF4/heparin antibody ELISA. On positive results, platelet activation markers P-selectin and Annexin V were assayed using flow cytometry. Results: Thirty/50 patients were negative for both immunoassays. Flow cytometry was performed in the 20 immunoassay positive patients. Platelet activation was observed in 7/20 in the presence of low heparin concentration (0.2 IU/ml). Conclusion: The results are in accordance with the currently available literature and flow cytometry seems a promising alternative in HIT laboratory investigation.

An Optimized and Standardized Rapid Flow Cytometry Functional Method for Heparin-Induced Thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a thrombocytopenia caused by heparin and mediated by an atypical immune mechanism leading to a paradoxical high thrombotic risk, associated with severe morbidity or death. The diagnosis of HIT combines a clinical scoring of pretest probability and laboratory testing. First-line routine tests are antigen binding assays detecting specific antibodies. The most sensitive of these tests have a high HIT-negative predictive value enabling HIT diagnosis to be ruled out when negative. However, HIT-positive predictive value is low, and a functional assay evaluating the pathogenicity of the antibodies should be performed to exclude false-positive results. In contrast to screening assays, functional assays are highly specific but technically challenging, and are thus performed in referral laboratories, where platelet activation is detected using radioactive serotonin (serotonin release assay, SRA) or visually (heparin-induced platelet activation, HIPA). Flow cytometry is a possible alternative. It is, however, currently not widely used, mostly because of the lack of standardization of the published assays. This article describes and discusses the standardization of a HIT flow cytometry assay (HIT-FCA) method, which subsequently led to the development and commercialization of a CE-marked assay (HIT Confirm^®, Emosis, France) as a suitable rapid HIT functional test.

Heparin-induced thrombocytopenia: ELISA optical density value and 4T score in correlation with panel donor platelets activation in functional flow cytometric assay

BACKGROUND

Serological assays for the diagnosis of heparin-induced thrombocytopenia (HIT) detect both platelet-activating and platelet non-activating anti-heparin/platelet factor 4 (PF4) antibodies and have therefore a limited positive predictive value. Functional assays confirm the presence of platelet-activating antibodies but require platelets from healthy donors, whose response to patient serum can differ. Our aim was to investigate the correlation between the level of anti-heparin/PF4 antibodies, 4T score, and the extent of panel donor platelet activation in the functional assay.

MATERIALS AND METHODS

In total, 38 sera from enzyme immunoassays (ELISA) positive patients were tested against panel platelets obtained from 10 healthy, randomly selected donors, using our routine flow cytometry functional test for CD62P expression. Levels of anti-heparin/PF4 antibodies from medical and surgical patients and 4T pretest probability scores (where available) were correlated with the number of activated panel platelets.

RESULTS

Sera with low ELISA optical density (OD) values (0.4-1) activated on average 5.6, sera with intermediate ELISA OD values (>1-2.5) activated on average 7.3, and sera with high ELISA OD values (>2.5) activated on average 8.6 out of 10 panel platelets. One serum with low 4T score did not activate donor platelets, 12 sera with intermediate 4T score activated on average 6.3 donors, 8 sera with high 4T score activated on average 8.5 panel platelets.

DISCUSSION

Sera with higher ELISA OD values activated platelets from a higher number of platelet donors, independently of patient type (medical or surgical). The average number of activated panel platelets increased with rising 4T score. Results indicate that both donor platelet reactivity and quantity of anti-heparin/PF4 antibodies affect the result of the functional assay, meaning special attention is needed in platelet donor selection when testing sera with low levels of antibodies.

The use of flow cytometry in the diagnosis of heparin-induced thrombocytopenia (HIT)

Heparin-induced thrombocytopenia (HIT) affects some of the patients exposed to heparin. It is mediated by antibodies that recognize neoepitopes on platelet factor 4 (PF4)/heparin complexes. A HIT diagnosis requires both clinical and laboratory evaluation and remains a challenge. Since many patients develop antibodies in response to heparin, but only a few of them generate anti-PF4/heparin antibodies capable of activating platelets which consequently cause clinical complications, the performance of serologic assays is not enough to diagnose HIT. Functional assays can identify pathogenic antibodies capable of platelet activation, but they are more demanding and their limited availability contributes to the problem of diagnosing HIT. Restricted laboratories usually collect sera of multiple patients to perform functional assays only once or twice a week; hence, a HIT diagnosis can take several days. The use of flow cytometry appears to be a promising alternative in the confirmation of pathogenic anti-PF4/heparin antibodies. Flow cytometric assays detect either activation markers on a healthy donor's platelet surfaces or platelet derived microparticles formed after platelet incubation with a patient's serum. Flow cytometers are readily available in many clinical laboratories, so this technology introduces the possibility of an earlier HIT diagnosis. The objective of this review was to collect findings on flow cytometric HIT confirmations to the present date, and to review the currently available flow cytometric assays used in the diagnosis of HIT.

Pathogenic heparin-induced thrombocytopenia and thrombosis (HIT) antibodies determined by rapid functional flow cytometry

OBJECTIVES

Reliable diagnosis of heparin-induced thrombocytopenia and thrombosis (HIT) is mandatory for patient management, yet prompt determination of pathogenic antibodies remains an unmet clinical challenge. Common immunoassays carry inherent limitations and functional assays which detect antibody-mediated platelet activation are not usually readily available to routine laboratories, especially the serotonin release assay (SRA), being technically demanding, time consuming, and requires high level expertise. To overcome some of these limitations, we have developed a practical functional flow cytometric assay (FCA) for routine clinical use.

METHODS

A simple FCA is described which avoids platelet manipulation, is highly specific and sensitive compared with SRA, and provides rapid results.

RESULTS

Of the 650 consecutive samples, from HIT-suspected patients, 99 (15.3%) were positive by the PaGIA Heparin/PF4 immunoassay and 31 (4.8%) by FCA. Average platelet activation was 11-fold higher in PaGIA+/FCA+ vs PaGIA-/FCA- samples. Of 21 SRA-positive samples, 19 were FCA-positive (relative sensitivity 90.5%), and of 42 SRA-negative samples, 40 were FCA-negative (relative specificity 95.2%). The FCA showed significantly higher correlation with the clinical presentation of HIT (4Ts score) performed on 182 patients, compared with PaGIA Heparin/PF4 (ROC-plot analysis, AUC 0.93 vs 0.63, P < 0.001). At a 92% sensitivity, the assay specificity was 96%.

CONCLUSIONS

The present FCA is practical for routine testing, providing prompt reliable results for initial diagnosis and confirmation, to effectively assist in HIT patient management.

Functional Assays in the Diagnosis of Heparin-Induced Thrombocytopenia: A Review

A rapid and accurate diagnosis in patients with suspected heparin-induced thrombocytopenia (HIT) is essential for patient management but remains challenging. Current HIT diagnosis ideally relies on a combination of clinical information, immunoassay and functional assay results. Platelet activation assays or functional assays detect HIT antibodies that are more clinically significant. Several functional assays have been developed and evaluated in the literature. They differ in the activation endpoint studied; the technique or technology used; the platelet donor selection; the platelet suspension (washed platelets, platelet rich plasma or whole blood); the patient sample (serum or plasma); and the heparin used (type and concentrations). Inconsistencies in controls performed and associated results interpretation are common. Thresholds and performances are determined differently among papers. Functional assays suffer from interlaboratory variability. This lack of standardization limits the evaluation and the accessibility of functional assays in laboratories. In the present article, we review all the current activation endpoints, techniques and methodologies of functional assays developed for HIT diagnosis.

Activated Platelet-Derived and Leukocyte-Derived Circulating Microparticles and the Risk of Thrombosis in Heparin-Induced Thrombocytopenia: A Role for PF4-Bearing Microparticles?

BACKGROUND

Though the presence of platelets-derived microparticles (MPs) have previously been described in heparin-induced thrombocytopenia (HIT), the mechanism of thrombosis in HIT remains poorly understood. We aimed to assess the presence and origin of MPs in patients with HIT and their possible contribution to HIT with thrombosis (HITT).

METHODS

Forty-five patients with HIT and 45 matched hospitalized patients with not confirmed HIT (HIT-negative) were enrolled. Twelve HIT patients (27%) developed HITT. MPs expressing phosphatidylserine (Annexin V-MP), activated platelet-derived (P-Selectin+), activated leukocyte-derived (L-Selectin+), PF4-bearing and tissue factor-bearing (TF+) MPs were measured by flow-cytometry.

RESULTS

HIT patients showed significantly higher median levels of P-Selectin+, L-Selectin+, PF4-bearing, L-Selectin+/TF + MPs than HIT-negative; PF4-bearing MP showed the highest statistical difference. As compared to HIT patients, HITT patients showed a trend of higher median levels of all MP subtypes considered but the differences were not statistically significant. Only levels of activated-leukocyte/TF + MPs (L-Selectin + CD142+) were significantly higher (P = 0.015). Sensitive analyses showed that HIT patients with activated-leukocyte/TF + MPs above the cut-off (52 MP/µL) had an odds ratio (OR) for thrombosis of 3.78 (95%CI, 0.98-14.5, P = 0.045). The combination of activated-leukocyte/TF + MPs and PF4-bearing-MPs above the cut-off (1416 MP/uL) resulted in a higher risk of HITT (OR 4.49 (95% CI, 1.17-8.05, P = 0.014).

CONCLUSIONS

We showed for the first time the presence of circulating PF4-bearing MPs derived from activated platelets in patients with HIT; activated leukocyte/TF + MPs are associated with an increased thrombotic risk. Our findings confirm that HIT antibodies complexes may determine a TF-driven prothrombotic state through the activation of platelets and leukocytes. © 2017 International Clinical Cytometry Society.