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.

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.

• 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.

• The HIPA assay is done via a light transmission aggregometer.^[1] 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.

• 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. ^{[2] [3] [4] [5]}

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.