Using HitAlert Flow Cytometry to Detect Heparin-Induced Thrombocytopenia Antibodies in a Tertiary Care Hospital

We aimed to assess the utility of HitAlert flow cytometry as a
diagnostic functional heparin-induced thrombocytopenia (HIT) assay in a
tertiary care hospital. The 4Ts score was used to assess pretest
probability of HIT in 37 patients. Serum was analysed for HIT antibodies
by the flow cytometry HitAlert assay. Results were compared with an
antigenic assay, the particle gel immunoassay, PaGIA ID PF4/Hep Ab assay; and two functional assays, the Multiplate whole blood
impedance aggregometry assay (WBIA), and the serotonin release assay
(SRA). Flow cytometry was positive in 14 out of 37 patients, including
zero out of eight, five out of 19 and nine out of 10 in the low,
intermediate and high-risk groups by 4Ts score, respectively. Using the
SRA as a ‘gold standard’, flow cytometry has a sensitivity of 81% and a
specificity of 100% for the diagnosis of HIT. The other functional assay
(WBIA) had similar sensitivity (81%) and specificity (90%) to flow
cytometry. In contrast, the PaGIA maintained a high sensitivity of 100%
but a specificity of only 20%. The improved specificity of flow
cytometry over the antigenic assay was most marked in the 4T
intermediate-risk group in which similar results were obtained between
all three functional assays. We demonstrate that compared with an
immunological assay (PaGIA), flow cytometry can improve the specificity
of laboratory diagnosis of HIT without loss of sensitivity using SRA as a
standard. Flow cytometry may have a role in the first-line laboratory
diagnosis of HIT, especially when combined with an immunological assay
such as PaGIA.