Diagnosis
HIT may be suspected if blood tests show a falling platelet count in someone receiving heparin, even if the heparin has already been discontinued. Professional guidelines recommend that people receiving heparin have a complete blood count (which includes a platelet count) on a regular basis while receiving heparin.
However, not all people with a falling platelet count while receiving heparin turn out to have HIT. The timing, severity of the thrombocytopenia, the occurrence of new thrombosis, and the presence of alternative explanations, all determine the likelihood that HIT is present. A commonly used score to predict the likelihood of HIT is the "4 Ts" score introduced in 2003. A score of 0–8 points is generated; if the score is 0-3, HIT is unlikely. A score of 4–5 indicates intermediate probability, while a score of 6–8 makes it highly likely. Those with a high score may need to be treated with an alternative drug while more sensitive and specific tests for HIT are performed, while those with a low score can safely continue receiving heparin as the likelihood that they have HIT is extremely low. In an analysis of the reliability of the 4T score, a low score had a negative predictive value of 0.998, while an intermediate score had a positive predictive value of 0.14 and a high score a positive predictive value of 0.64; intermediate and high scores therefore warrant further investigation.
| Element | The 4T score for heparin-induced thrombocytopenia |
|---|---|
| Thrombocytopenia | 2 points if the fall in platelet count is >50% of the previous value, or the lowest count (nadir) is 20–100 × 109/liter 1 point if the fall is 30–50% or the nadir is 10–19 × 109/liter No points if the fall is less than 30% or the nadir is <10 × 109/liter. |
| Timing | 2 points if the fall is between days 5–10 after commencement of treatment 1 point if the fall is after day 10. If someone has been exposed to heparin within the last 30 days and then has a drop in platelet count within a day of reexposure, 2 points are given. If the previous exposure was 30–100 days ago, 1 point If the fall is early but there has been no previous heparin exposure, no points. |
| Thrombosis | 2 points in new proven thrombosis, skin necrosis (see below), or systemic reaction 1 point if progressive or recurrent thrombosis, silent thrombosis or red skin lesions No points if there are no symptoms. |
| AlTernative cause possible | 2 points if no other cause 1 point if there is a possible alternative cause No points if there is a definite alternative cause. |
The first screening test in someone suspected of having HIT is aimed at detecting antibodies against heparin-PF4 complexes. This may be with a laboratory test of the ELISA (enzyme-linked immunosorbent assay) type. The ELISA test, however, detects all circulating antibodies that bind heparin-PF4 complexes, and may also falsely identify antibodies that do not cause HIT. Therefore, those with a positive ELISA are tested further with a functional assay. This test uses platelets and serum from the patient; the platelets are washed and mixed with serum and heparin. The sample is then tested for the release of serotonin, a marker of platelet activation. If this serotonin release assay (SRA) shows high serotonin release, the diagnosis of HIT is confirmed. The SRA test is difficult to perform and is usually only done in regional laboratories.
If someone has been diagnosed with HIT, some recommend routine Doppler sonography of the leg veins to identify deep vein thromboses, as this is very common in HIT.
Read more about this topic: Heparin-induced Thrombocytopenia