Recommendations For Platelet Count Monitoring For

1. Monitoring for typical-onset HIT: stratifying the intensity of platelet count monitoring for HIT based upon its risk

A. Patients at highest risk for HIT (1-5%) (e.g., postoperative patients receiving prophylactic-dose UFH after major surgery, patients receiving therapeutic-dose UFH): monitoring during heparin therapy, at least every second day from day 4 to day 14a,b

B. Patients at intermediate risk for HIT (0.1-1%) (e.g., medical/obstetrical patients receiving prophylactic-dose UFH, or postoperative patients receiving prophylactic-dose LMWH, or postoperative patients receiving intravascular catheter "flushes" with UFH): monitoring during heparin therapy, at least every 2 or 3 days from day 4 to day 14a, when practical

C. Patients at low risk for HIT (<0.1%) (e.g., medical/obstetrical patients receiving prophylactic- or therapeutic-dose LMWH, or medical patients receiving only intravascular catheter "flushes" with UFH): routine platelet count monitoring is not recommendedd

2. Monitoring for rapid-onset HIT: for a patient recently exposed to heparin (within the past 100 days), a repeat platelet count within 24 h following reinitiation of heparin

3. When to suspect HIT

A relative (proportional) platelet count fall of 50% or greater that is otherwise clinically unexplained should be considered suspicious for HIT, even if the platelet count nadir remains above 150x109/L. For any patient who develops thrombosis during (day 5 to 14) or within several days after stopping heparin therapy, or who develops an unusual clinical event in association with heparin therapy (e.g., inflammatory or necrotic skin lesions at heparin injection sites, acute systemic reaction post-intravenous heparin therapy), a repeat platelet count should be measured promptly and compared with recent values.

Note: These recommendations parallel those of the Seventh American College of Chest Physicians (ACCP) Concersus Conference on Antithrombotic and Thrombolytic Therapy (Warkentin and Greinacher, 2004). aThe crucial time period for monitoring "typical-onset" HIT is between days 4 to 14 (first day of heparin = day 0), where the highest platelet count from day 4 (inclusive) onwards represents the "baseline." Platelet count monitoring can cease before day 14 when heparin is stopped.

bOnce-daily platelet count monitoring is reasonable in patients receiving therapeutic-dose UFH given that daily blood draws required for aPTT monitoring are usually required.

cFrequent platelet count monitoring may not be practical when UFH or LMWH is given to outpatients. dMonitoring as per "intermediate" risk is appropriate if UFH was given before initiating LMWH. Abbreviations: HIT, heparin-induced thrombocytopenia; LMWH, low molecular weight heparin; UFH, unfractio-nated heparin.

Source: Adapted from Warkentin and Greinacher, 2004.

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