Heparin-Induced Thrombotic Thrombocytopenia (HITT) and Vaccine-Induced Immune Thrombotic Thrombocytopenia (VITT): Similar but Different

Heparin-induced thrombocytopenia (HIT) represents an autoimmune process whereby antibodies are formed against heparin in complex with platelet factor 4 (PF4) after heparin administration. These antibodies can be detected by a variety of immunological assays, including ELISA (enzyme-linked immunosorbent assay) and by chemiluminescence on the AcuStar instrument. However, pathological HIT antibodies are those that activate platelets in a platelet activation assay and cause thrombosis in vivo. We would tend to call this condition heparin-induced thrombotic thrombocytopenia (HITT), although some workers instead use the truncated abbreviation HIT. Vaccine-induced (immune) thrombotic thrombocytopenia (VITT) instead reflects an autoimmune process whereby antibodies are formed against PF4 after administration of a vaccine, most notably adenovirus-based vaccines directed against COVID-19 (coronavirus disease 2019). Although both VITT and HITT reflect similar pathological processes, they have different origins and are detected in different ways. Most notable is that anti-PF4 antibodies in VITT can only be detected immunologically by ELISA assays, tending to be negative in rapid assays such as that using the AcuStar. Moreover, functional platelet activation assays otherwise used for HITT may need to be modified to detect platelet activation in VITT.

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.

Critical laboratory values in hemostasis: toward consensus

The term "critical values" can be defined to entail laboratory test results that significantly lie outside the normal (reference) range and necessitate immediate reporting to safeguard patient health, as well as those displaying a highly and clinically significant variation compared to previous data. The identification and effective communication of "highly pathological" values has engaged the minds of many clinicians, health care and laboratory professionals for decades, since these activities are vital to good laboratory practice. This is especially true in hemostasis, where a timely and efficient communication of critical values strongly impacts patient management. Due to the heterogeneity of available data, this paper is hence aimed to analyze the state of the art and provide an expert opinion about the parameters, measurement units and alert limits pertaining to critical values in hemostasis, thus providing a basic document for future consultation that assists laboratory professionals and clinicians alike. KEY MESSAGES Critical values are laboratory test results significantly lying outside the normal (reference) range and necessitating immediate reporting to safeguard patient health. A broad heterogeneity exists about critical values in hemostasis worldwide. We provide here an expert opinion about the parameters, measurement units and alert limits pertaining to critical values in hemostasis.