Drug Induced Thrombocytopenia

• The decrease of the platelet count typically occurs within 2-3 days (sometimes a few hours) of taking a drug that has been taken previously or 1-3 weeks after starting a new drug.
• Once stopped, the thrombocytopenia usually resolves in 5-10 days after drug withdrawal. Clinically, DITP can be easily confused with ITP and only an accurate history collection can help in the diagnostic process
• DITP should also be suspected when patients have recurrent episodes of thrombocytopenia with prompt recovery. 
• Sometimes the diagnosis is very challenging because the substance causing the thrombocytopenia is not a drug, but a food or beverage, such as walnuts, cow’s milk, cranberry juice, tonic water (which contains quinine), and certain herbal remedies
• The diagnosis of DITP is mostly empirical, being based on platelet recovery after discontinuation of the drug or after abstaining from certain foods or beverages
• The frequency of DITP in critically ill patients is approximately 20%. Antibiotics in this setting are a frequent etiology

Heparin Induced Thrombocytopenia

• HIT occurs in 0.5%-5% of heparin-treated subjects, depending on the patient population studied.
• It should be suspected in any patient who, while on heparin therapy, has a 50% decrease in platelet count compared with baseline or a total platelet count of 100 109/L with normal baseline counts. 
• usually moderate, with median platelet counts ranging from 50-80 109/L and nadir counts rarely below 20 109/L. 
• Clinical manifestations may include venous or arterial thrombosis, necrotic
  skin lesions at heparin injection sites, or acute systemic reactions
  subsequent to IV heparin bolus administration.
• not possible to predict which patients will develop these complications
• typically develops after exposure to heparin for 5-10 days.
  Earlier onset of HIT (sometimes a few hours after the first heparin administration) occurs in approximately 30% of cases. 
• These patients have a history of exposure to heparin in the prior 3 months,
  thereby developing Abs against the heparin-PF4 complex. 
• Less commonly (in 10%-15% of cases), HIT has a delayed onset, occurring several days after all heparin has been stopped.

Reference:

1. How to approach thrombocytopenia. Hematology 2012
2. http://www.aafp.org/afp/2012/0315/p612.html