The real strength of the 4T score for heparin-induced thrombocytopenia (HIT) is its negative predictive value, according to hematologist Adam Cuker, MD, of the department of medicine at the University of Pennsylvania, Philadelphia.
The score assigns patients points based on degree of thrombocytopenia, timing of platelet count fall in relation to heparin exposure, presence of thrombosis and other sequelae, and the likelihood of other causes of thrombocytopenia.
A low score – 3 points or less – has a negative predictive value of 99.8%, “so HIT is basically ruled out; you do not need to order lab testing for HIT or manage the patient empirically for HIT,” and should look for other causes of thrombocytopenia, said Dr. Cuker, lead author of the American Society of Hematology’s most recent HIT guidelines.
Intermediate scores of 4 or 5 points, and high scores of 6-8 points, are a different story. The positive predictive value of an intermediate score is only 14%, and of a high score, 64%, so although they don’t confirm the diagnosis, “you have to take the possibility of HIT seriously.” Discontinue heparin, start a nonheparin anticoagulant, and order a HIT immunoassay. If it’s positive, order a functional assay to confirm the diagnosis, he said.
Suspicion of HIT “is perhaps the most common consult that we get on the hematology service. These are tough consults because it is a high-stakes decision.” There is about a 6% risk of thromboembolism, amputation, and death for every day treatment is delayed. “On the other hand, the nonheparin anticoagulants are expensive, and they carry about a 1% daily risk of major bleeding,” Dr. Cuker explained during his presentation at the 2020 Update in Nonneoplastic Hematology virtual conference.
ELISA immunoassay detects antiplatelet factor 4 heparin antibodies but doesn’t tell whether or not they are able to activate platelets and cause HIT. Functional tests such as the serotonin-release assay detect only those antibodies able to do so, but the assays are difficult to perform, and often require samples to be sent out to a reference lab.
ASH did not specify a particular nonheparin anticoagulant in its 2018 guidelines because “the best choice for your patient” depends on which drugs you have available, your familiarity with them, and patient factors, Dr. Cuker said at the conference sponsored by MedscapeLive.
It makes sense, for instance, to use a short-acting agent such as argatroban or bivalirudin in patients who are critically ill, at high risk of bleeding, or likely to need an urgent unplanned procedure. Fondaparinux or direct oral anticoagulants (DOACs) make sense if patients are clinically stable with good organ function and no more than average bleeding risk, because they are easier to administer and facilitate transition to the outpatient setting.
DOACs are newcomers to ASH’s guidelines. Just 81 patients had been reported in the literature when they were being drafted, but only 2 patients had recurrence or progression of thromboembolic events, and there were no major bleeds. The results compared favorably with other options.
The studies were subject to selection and reporting biases, “but, nonetheless, the panel felt the results were positive enough that DOACs ought to be listed as an option,” Dr. Cuker said.
The guidelines note that parenteral options may be the best choice for life- or limb-threatening thrombosis “because few such patients have been treated with a DOAC.” Anticoagulation must continue until platelet counts recover.
Dr. Cuker is a consultant for Synergy and has institutional research support from Alexion, Bayer, Sanofi, and other companies. MedscapeLive and this news organization are owned by the same parent company.