Heparin-induced thrombocytopenia laboratory findings

Heparin-induced thrombocytopenia
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor-In-Chief: Cafer Zorkun, M.D., Ph.D. [2], Aric C. Hall, M.D., [3] Shyam Patel [4]

Overview

A variety of laboratory tests can be used to aid in the diagnosis of HIT. A complete blood count is always necessary and will show low platelet count. A high value of the PF4 IgG optical density can suggest HIT. Confirmatory testing is done with the serotonin release assay (SRA), heparin-induced platelet aggregation (HIPA) assay, or other functional assay. These confirmatory tests have high specificity.

Lab tests

The most sensitive lab tests are:

• Complete blood count (CBC): This will show a low platelet count, (also known as thrombocytopenia). There is no specific threshold below which HIT is diagnosed, but the typical standard is a decrease in platelet count by approximately 50% or more.
• Anti-heparin-PF4 IgG: This is a test that employs enzyme-linked immunosorbent assay (ELISA) to detect antibodies against the heparin-PF4 complex. The higher the optical density (OD), the stronger the antibody titer and the higher the likelihood of HIT.

The most specific tests are:

• Serotonin release assay (SRA)
• Heparin induce platelet aggregation (HIPA) assays
• Solid-phase immunoassay (SPI) (H-PF4 enzyme-linked immunosorbent assay [ELISA])
• Particle gel immunoassay (PIFA)

These specific tests are used for confirmatory testing for HIT after the initial PF4-IgG test is found to be high. The sensitivity of these confirmatory tests, however, is 94% at best.

Serotonin release assay (SRA)

• The gold standard is the serotonin release assay (SRA) where antibodies from the patient’s serum result in release of carbon-14 radiolabeled serotonin attached to platelets from a normal patient.

Heparin-induced platelet aggregation (HIPA) assay

• The HIPA assay is done via a light transmission aggregometer. It is performed by mixing platelet-poor plasma from a patient with suspected HIT with platelet-rich plasma from donors.
• It has a >90% specificity but is limited by low sensitivity.

Solid-phase immunoassay (SPI)

• The SPI is an enzyme-linked immunosorbent assay (ELISA) that tests for the presence or absence of heparin-PF4 complexes.
• As it does not determine whether the antibodies are functionally significant, it is best used in conjunction with one of the two prior tests.

If HIT is suspected it may take hours to days to obtain the laboratory back. In the meantime it may simply be a safer approach to substitute another agent (eg agatroban) for heparin.