[on page 507] . A diagnostic and treatment approach to suspected heparin-induced thrombocytopenia (HIT).

Physicians usually must make initial treatment decisions based upon the probability—judged clinically—that a patient has HIT (an 8-point scoring system, the 4 T’s, for judging pretest probability of HIT is shown). Treatment decisions range from stopping heparin and substituting an alternative anticoagulant (high probability of HIT) to continuing heparin (low probability of HIT). Results of diagnostic testing for HIT antibodies can influence whether an alternative anticoagulant is continued until platelet count recovery, or (low molecular weight) heparin is continued or resumed. Assessment for thrombosis should include routine imaging for lower-limb DVT, as subclinical lower-limb DVT occurs frequently in HIT and influences treatment duration. The transition from alternative anticoagulation to coumarin (eg, warfarin, phenprocoumon) should be managed cautiously, as HIT is a risk factor for coumarin-induced necrosis, including venous limb gangrene associated with DVT.

Abbreviations: aPTT, activated partial thromboplastin time; ASR, acute systemic reaction; DVT, deep vein thrombosis; INR, international normalized ratio; LMW, low molecular weight; PF4, platelet factor 4.