Heparin-Induced Thrombocytopenia Testing

Heparin induced thrombocytopenia is an immune reaction to heparin that causes thrombocytopenia and paradoxically a high risk of thrombosis. Antibodies, mostly IgG, form against complexes of heparin and platelet factor 4. They activate platelets through the Fc gamma IIa receptor, release procoagulant microparticles and activate monocytes and endothelium, which generates thrombin. Testing combines a clinical probability score with an immunoassay, and a functional assay when needed.

It occurs in about 0.2 to 3% of patients exposed to unfractionated heparin, most often after orthopaedic and cardiac surgery, and in under 1% on low molecular weight heparin. It is rare with fondaparinux. Thrombosis develops in about a third to a half of untreated patients, and mostly affects veins, with arterial events, skin necrosis at injection sites and adrenal haemorrhage less often.

Typical timing. Platelet count falls 5 to 10 days after heparin starts. Onset within 24 hours occurs with heparin exposure in the previous 30 days (and up to 100 days). Delayed onset up to about 3 weeks after stopping heparin also occurs. The usual fall is over 30 to 50% from baseline, and the nadir is usually 20 to 150 x10⁹/L and rarely under 15 x10⁹/L. Bleeding is uncommon.

Specimen. Serum or citrated plasma, drawn before any platelet transfusion or intravenous immunoglobulin where possible, and stable for several days at 4 °C.

4Ts score. Each of 4 items scores 0, 1 or 2 points.

• Thrombocytopenia: 2 for a fall over 50% with a nadir of at least 20 x10⁹/L, 1 for a fall of 30 to 50% or a nadir of 10 to 19 x10⁹/L, 0 otherwise.
• Timing of fall: 2 for onset on day 5 to 10, or within 1 day with heparin exposure in the previous 30 days. 1 for a consistent but unclear timing, onset after day 10, or onset within 1 day with exposure 30 to 100 days earlier. 0 for a fall within 4 days without recent exposure.
• Thrombosis or other sequelae: 2 for new thrombosis, skin necrosis, or an acute systemic reaction after a heparin bolus. 1 for progressive or recurrent thrombosis, non-necrotising skin lesions, or suspected thrombosis. 0 for none.
• Other causes of thrombocytopenia: 2 if none apparent, 1 if possible, 0 if definite.

A score of 0 to 3 is low probability (under 5%), 4 to 5 intermediate (about 14%) and 6 to 8 high (about 64%). The score is used to decide whether to test. A low score has a negative predictive value over 99% and testing is usually unnecessary. An alternative score (the HIT expert probability score) adds weight to platelet fall timing and nadir, thrombosis and other causes.

Test sequence.

• Calculate the pre-test probability.
• If intermediate or high, send an immunoassay and, in a high probability patient, send the functional assay in parallel.
• Stop all heparin exposure in intermediate and high probability while waiting, and do not delay for results.

Immunoassay (anti-platelet factor 4/heparin). Enzyme linked, chemiluminescent, lateral flow and particle gel immunoassays detect antibodies. The IgG-specific assay is preferred. Sensitivity is over 95% and specificity is low, since many antibodies do not activate platelets. Optical density is reported, and the likelihood of a positive functional assay rises with the value.

A typical positive cut off is about 0.4 to 0.5, an optical density over about 2.0 gives a high likelihood of a positive functional test, and one under about 1.0 gives a low likelihood. Values are assay specific. A negative result with intermediate or low probability makes HIT very unlikely.

Functional assays. The serotonin release assay (the reference method) and heparin induced platelet activation assay measure donor platelet activation by patient serum with low dose heparin (about 0.1 to 0.3 IU/mL) and suppression at high dose (100 IU/mL). Sensitivity and specificity are both about 95 to 100%. They are available only in reference laboratories, and results take days.

Combined interpretation.

• Low probability and negative immunoassay: HIT excluded.
• Intermediate or high probability and positive immunoassay with a high optical density: HIT likely, and confirmed by a positive functional assay.
• High probability and positive immunoassay with a negative functional assay: HIT unlikely, though a false negative functional test can occur and clinical judgement applies.
• High probability and negative immunoassay: HIT very unlikely, and other causes of thrombocytopenia need review.

Other causes of thrombocytopenia with thrombosis or heparin exposure. Sepsis, DIC, post-cardiopulmonary bypass thrombocytopenia (platelets fall and then recover, and HIT shows a second fall after day 5), drug induced thrombocytopenia, antiphospholipid syndrome, thrombotic microangiopathy, post-transfusion purpura, and vaccine induced immune thrombotic thrombocytopenia (positive immunoassay in most cases, but heparin dependent functional assays are usually negative).

Critical findings needing immediate action.

• Intermediate or high 4Ts score with new thrombosis: stop all heparin, including flushes and heparin coated lines, and start a non-heparin anticoagulant under specialist direction.
• Skin necrosis at injection sites, or an acute systemic reaction within 30 minutes of an intravenous heparin bolus.
• Warfarin is avoided until the platelet count recovers, and platelet transfusion is avoided unless there is bleeding.

Interferences and pitfalls.

• Positive immunoassays are common without HIT, and are found in up to about half of patients after cardiac surgery and in many dialysis and critical care patients.
• A positive result in a patient with a low probability score is usually a false positive.
• IgG-specific assays are more specific than assays detecting IgG, IgA and IgM.
• Antibodies are transient and disappear after about 50 to 85 days, so late testing after heparin has stopped can be negative.
• Haemolysed, lipaemic and heavily clotted samples interfere with immunoassays.
• Platelet count may stay normal in early reactions, and thrombosis can precede the fall.
• Concurrent critical illness makes the 4Ts score less specific.

Special groups.

• Surgical patients carry a higher risk than medical patients, and the risk is higher with unfractionated than low molecular weight heparin.
• Obstetric and paediatric patients: HIT is rare.
• Patients with a history of HIT who need cardiac surgery: re-exposure to heparin is considered only when functional and immunoassays are negative, and with specialist input.
• Patients with spontaneous or autoimmune HIT can present without heparin exposure, or with a persistent fall after stopping heparin.

Follow up tests. Platelet count monitoring until recovery, repeat immunoassay, serotonin release assay, lower limb duplex ultrasound, imaging of suspected thrombosis, DIC screen (clotting screen, fibrinogen, D-dimer), blood film, antiphospholipid tests, anti-Xa level on non-heparin anticoagulant, adrenal imaging if abdominal pain or hypotension.