Monday, 23 February 2009

Thrombocytopenia: an approach

In medicine, you need approaches to common presentations, so that you can systematically work out the cause for the problem without missing any obvious or dangerous things. Thrombocytopenia is one of those things.  What I've done here is to provide my own approach to the problem for reference.  If you have your own system that works for you, don't even bother reading what follows, and even if you do, feel free to debate or moderate anything.  OK, here goes.

There are 5 common or dangerous causes of thrombocytopenia that need to be excluded provisionally, before you go much further with the patient.  I find them easy to remember by the mnemonic DiSHMaT:
  • DIC (disseminated intravascular coagulation) - exclude by doing a DIC screen.  Platelets are consumed in the multiple small clots that are formed.
  • Sepsis - clinically, the patient will usually be very pyrexial, with a high white cell count and CRP.  Blood cultures may later become positive.
  • Hypersplenism - a very large spleen sequesters all 3 components of the peripheral blood, lowering the counts of them all.
  • Microangiopathic haemolytic anaemia - the peripheral blood smear will show marked red cell fragmentation.
  • Transfusion - if a patient has recently received a massive transfusion (a few units won't affect the platelet count significantly), they are sometimes thrombocytopenic because the transfused units contain no platelets. 
As you can see, it is possible to exclude or confirm any of these serious conditions within a few hours at most.  If the above are all negative, then congratulations, you have almost certainly excluded all the urgent causes of thrombocytopenia.  The next step is a bone marrow aspirate and biopsy.  There are two basic pictures that will emerge.

If there are adequate or increased megakaryocytes, this indicates peripheral consumption of platelets is the cause for your thrombocytopenia.  The commonest cause in this category is Immune (Idiopathic) Thrombocytopenia (ITP).  This condition is associated with certain drugs, viruses (HIV, EBV), connective tissue disorders (e.g. SLE) and lymphoproliferative disorders. Fundamentally, you need to consult a haematologist at this stage.

On the other hand, you bone marrow may show decreased megakaryocytes, which indicates that the bone marrow isn't producing enough platelets.  Once again, you need to consult a haematologist.  Options include amegakaryocytic thrombocytopenia, drug toxicity, bone marrow infiltration, myelofibrosis, and ineffective haemopoeisis.

The nice thing about this schema is that you can quickly find the cause in a large proportion of cases.  If you don't, you can safely and legitimately hand the problem on to a specialist.

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