College Answer

a)

• Cause of thrombocytopenia in most ICU patients is multi-factorial, due to some combination of following four mechanisms:
  • Increased destruction or consumption
  • Decreased production 
  • Dilution
  • Sequestration

• Increased destruction 
  • Non-immune mediated – DIC, TTP, HELLP
  • Immune-mediated (drug) – eg Type 2 HITS, Glycoprotein IIb/IIIa inhibitors, NSAIDS, anti-epileptic drugs
  • Immune-mediated (non-drug) – ITP Mechanical – extracorporeal circuits
• Decreased production
  • Bone marrow suppression – drugs (eg linezolid), toxins, infections, nutritional deficiencies, metastases
• Dilutional
  • After massive transfusion and fluid resuscitation
• Sequestration 
  • Splenomegaly
  • Portal hypertension
  • May also be spurious due to clumping of platelets in collection tubes

Management consists of establishing the diagnosis and specific and supportive treatment.

The most likely causes in this patient are:

• DIC
• HITS 
• Other anti-platelet agents
• Dilutional from massive transfusion

Other causes to be considered if indicated from history or examination

• Investigations 
  • FBC and Blood film
  • Coagulation screen including DIC screen 
  • HITS screen
  • LFTs 
  • Sepsis screen
  • Consider ADAMTS-13 (for TTP)

Treatment

• Stop heparin and any other possible causative agents
• Stop ongoing bleeding
• Alternative agent for heparin in CRRT and alternative strategy for thromboprophylaxis
• Platelet transfusion if bleeding, high risk or interventions scheduled
• Look for and treat sepsis

Discussion

The differential diagnosis of thrombocytopenia in general is presented in a table in the discussion section of Question 11.3 from the first paper of 2012.

At risk of damaging SEO, I will reproduce it here:

No sensible or specific additions can be made to the non-specific manageemnt suggested by the college in the second part of this question. This patient is probably having his platelets eaten by the dialysis filter, or is developing DIC from sepsis, or he has developed HITS. The bottom line is, you would stop anticoagulating him.

The following list of generic steps applies to thrombocytopenia of any cause:

Minimise platelet destruction

• Withhold heparin and rationalise the indications for heparin, eg.:
  • Use alternative anticoagulants for the extracorporeal circuit (citrate comes to mind but there are numerous others) 
  • Use mechanical thromboprophylaxis or LMWH
  • Rationalise the use of dialysis
• Manage the sepsis with appropriate antibiotics and resuscitation (as sepsis improves, DIC will resolve)
• Address specific destructive aetiologies with appropriately targeted therapies, eg.:
  • Plasmapheresis for TTP
  • High dose methylprednisone for MAHA
  • Delivery for HELLP

Maximise platelet production

• Ensure supply of haematinics is uninterrupted
• Optimise nutrition, focusing on vitamins and trace elements
• Withhold or rationalise any drugs which are bone marrow toxins
• Correct the correctable causes of bone marrow failure and liver disease
• Think about thrombopoietin receptor agonists (eg. eltrombopag) - some promising results have come from the RAISE trial (Cheng et al, 2010)

Protect the patient from complications of thrombocytopenia

• Cancel or postpone all nonessential invasive procedures
• Cover unavoidable procedures with transfusion of pooled platelets (up to a level of 50)
• For neurosurgical procedures (or lumbar puncture, etc) aim for a level above 100
• Otherwise, keep the level above 20
  (the above numbers derived from recommendations made by Van der Linden et al, 2012)