Early
Within the first 1–2 days of starting heparin
Seen in patients with prior exposure to heparin (usually in the preceding 3 months) and hence prior antibodies
Usual
Within 5–10 days of starting heparin therapy
Presumed to be due to the formation of new antibodies
Late
After discontinuation of heparin therapy. May be >2 weeks or more from last exposure to heparin
Can occur after the patient’s discharge from the hospital. Suspect in a patient returning to the hospital with a new thrombotic complication, particularly after an orthopedic or other surgery where heparin prophylaxis was used
Evaluation
HIT is a clinical diagnosis, but certain lab tests are useful in supporting the diagnosis. HIT is caused by antibodies against the heparin/platelet factor 4 complex, and multiple tests are available to assess for the presence of these antibodies.
The heparin antibody enzyme-linked immunosorbent assay (ELISA) is the most common test used; it is extremely sensitive but not specific.
A negative ELISA can be useful in ruling out the diagnosis, but a positive test does not confirm it without further supporting features.
We recommend first checking heparin antibody ELISA [3].
If the ELISA is positive and there is a high clinical suspicion, then treat as HIT positive.
If there is high clinical suspicion of false-positive, a serotonin release assay (SRA) can be checked [3].
There are multiple possible causes of postoperative thrombocytopenia. A pretest clinical score called “the 4T’s” has been developed and prospectively validated to help determine who may benefit most from laboratory testing for HIT (see Table 21.2) [3–5]. This score may also be useful in interpreting tests based on pretest probability, especially if there is suspicion for a false-positive.
Table 21.2
The 4T’s probability score
Thrombocytopenia | Platelet count fall >50 % and nadir ≥20,000/μL | 2 points |
Platelet count fall 30–50 % and nadir 10–19,000/μL | 1 point | |
Platelet count fall <30 % and nadir ≤10,000/μL | 0 point | |
Timing of platelet fall | Clear onset between days 5 and 10 or platelet count fall at ≤1 day if with prior heparin exposure within the last 30 days
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