. 2021 Jun 22;195(1):76–84. doi: 10.1111/bjh.17613

Table III.

Assays used in the assessment of VITT.

Test name	Assay type	Mechanism	Notes
Anti‐PF4 ELISA	Immunological assay	ELISA based approach to detect the presence of anti‐PF4/heparin antibodies	Used in both HIT and VITT assessment
Heparin‐induced platelet activation (HIPA)	Functional assay	Investigates if patient antibodies activate platelets in the presence of heparin. Donor platelets are incubated with patient serum/plasma and heparin. HIPA positive implies formation of heparin‐antibody‐PF4 complex that binds to platelet receptors and induces donor platelet activation and aggregation.	In contrast to HIT, patients with VITT may show inhibition rather than enhancement of platelet aggregation in the presence of low dose heparin.
PF4‐induced platelet activation (PIPA)	Functional assay	Modified HIPA, first described by Greinacher et al. ⁴ Investigates if patient antibodies activate platelets in the presence of increased PF4. Donor platelets are incubated with patient serum/plasma but instead of heparin, additional PF4 is added. In PIPA‐positive cases, the presence of PF4 with patient serum/plasma enhances platelet activation, independent of heparin.	Unclear if positive PIPA is due to anti‐PF4 autoantibody production due to vaccine or vaccine‐related antibodies that cross‐react with PF4 and platelets.

Test name

Assay type

Mechanism

Notes

Anti‐PF4 ELISA

Immunological assay

ELISA based approach to detect the presence of anti‐PF4/heparin antibodies

Used in both HIT and VITT assessment

Heparin‐induced platelet activation (HIPA)

Functional assay

Investigates if patient antibodies activate platelets in the presence of heparin.

Donor platelets are incubated with patient serum/plasma and heparin.

HIPA positive implies formation of heparin‐antibody‐PF4 complex that binds to platelet receptors and induces donor platelet activation and aggregation.

In contrast to HIT, patients with VITT may show inhibition rather than enhancement of platelet aggregation in the presence of low dose heparin.

PF4‐induced platelet activation (PIPA)

Functional assay

Modified HIPA, first described by Greinacher et al. ⁴

Investigates if patient antibodies activate platelets in the presence of increased PF4.

Donor platelets are incubated with patient serum/plasma but instead of heparin, additional PF4 is added.

In PIPA‐positive cases, the presence of PF4 with patient serum/plasma enhances platelet activation, independent of heparin.

Unclear if positive PIPA is due to anti‐PF4 autoantibody production due to vaccine or vaccine‐related antibodies that cross‐react with PF4 and platelets.

ELISA, enzyme‐linked immunosorbent assay; HIT, heparin induced thrombocytopenia; PF4, platelet factor 4; VITT, vaccine‐induced immune thrombotic thrombocytopenia.