Lab Dept:


Test Name:


General Information

Lab Order Codes:



Heparin Induced Antibody; Heparin-Dependent Antibody; HIT; PF4

CPT Codes:

86022 – Antibody ID, platelet antibodies

Test Includes:

Heparin P4 Antibody level reported as Reactivity (%), Heparin Inhibition (%) and Interpretation.


Test Indications:

Useful for evaluation of possible immune-mediated HIT-II. In patients with prior suspected or documented HIT, with or without thrombosis, assay for presence of H/PF4 antibody may be useful in assessment of the risk of recurrence of HIT with additional exposure to heparin, however, prospective data is limited. Routine screening of all patients prior to, during, or following heparin use is currently not recommended. A positive H/PF4 ELISA result has relatively low and uncertain predictive value for the development of clinical HIT-II.

Lab Testing Sections:

Serology - Sendouts

Referred to:

Mayo Medical Laboratory (MML Test: HIT)

Phone Numbers:

MIN Lab: 612-813-6280

STP Lab: 651-220-6550

Test Availability:

Daily, 24 hours

Turnaround Time:

1 – 3 days

Special Instructions:

Serum gel tubes are not acceptable


Specimen Type:



Red top NO GEL tube

Draw Volume:

3 mL (Minimum: 1.5 mL) blood

Processed Volume:

1 mL (Minimum: 0.5 mL) serum


Routine venipuncture

Special Processing:

Lab Staff: Centrifuge specimen. Remove serum aliquot into a screw-capped plastic vial. Store and ship at frozen temperatures. Forward promptly.

Patient Preparation:


Sample Rejection:

Warm specimens, unlabeled or mislabeled specimens; gross hemolysis; gross lipemia; specimens collected in gel tubes


Reference Range:

Results are reported as: 1) Reactivity (%); 2) Heparin inhibition (%); 3) Interpretation. Typical patterns of results and interpretations are depicted in the following table. Interpretive comments will also accompany test reports when indicated.

Results Field

Reactivity (%)

Heparin Inhibition (%)


Normal Range


Not done







20 – 40




20 – 40







A negative result of testing for H/PF4 antibodies has about 90% negative predictive value for exclusion of clinical Type II (HIT-II).

Because up to 10% of patients with HIT may have a negative H/PF4 ELISA result, a negative H/PF4 antibody ELISA result does not exclude the diagnosis of HIT when clinical suspicion remains high. A functional assay for HIT antibodies (eg, HDPA or SRA) may be helpful in these circumstances.

A positive result is indicative of the presence of H/PF4 complex antibodies. However, this test’s specificity is as low as 20-50% for clinical diagnosis of HIT, depending on the patient population studies. For example, up to 50% of surgical patients and up to 20% of medical patients treated with heparin may develop H/PF4 antibodies as measured by ELISA, and only a small proportion (1-5%) develop clinical HIT. Accordingly, this test does not confirm the diagnosis of Type II HIT. The diagnosis must be made in conjunction with clinical findings, including evaluation for other potential causes of thrombocytopenia.

The presence of H/PF4 antibodes likely increases the risk of clinical HIT, with risk probably partly dependent on associated medical and surgical conditions, but currently there are few data about relative risk of HIT in various populations with positive tests for H/PF4 antibodies.

Critical Values:



HIT is a clinical diagnosis that is complimented by laboratory testing for HDPA antibodies and/or antibodies to H/PF4 complexes. Assay results provide information on the presence or absence of H/PF4 antibodies which are implicated in the pathogenesis of the HIT-II with or without thrombosis. However, results of the H/PF4 antibody assay must be interpreted in conjunction with clinical findings and other pertinent tests to evaluate other causes of thrombocytopenia (eg, sepsis, intravascular coagulation, and fibrinolysis, thrombotic thrombocytopenia purpura, post-transfusion purpura, malignancy, drug-induced thrombocytopenia, autoimmune thrombocytopenia) or to confirm the findings of this assay.

Some low titer, low avidity antibodies and some antibodies that recognize sites on H/PF4 complex may not be detected using this assay.

Some patients may have naturally occurring antibodies for PF4 (no evidence of heparin dependence) of no known significance with respect to pathogenesis of HIT-II.


Enzyme-Linked Immunosorbent Assay (ELISA)


Mayo Medical Laboratory Web Page December 2014