Thrombotic thrombocytopenic purpura (TTP) is a rare blood condition characterised by the formation of small clots (thrombi) within the circulation, which results in the consumption of platelets and thus a low platelet count (thrombocytopenia).

It is a rare blood clotting disorder where small clots form throughout blood vessels causing damage to red blood cells which encourages more clotting. The initial clotting is caused by the inhibition of an enzyme that is critical to breaking up blood clots together with the formation of antibodies which attack the enzyme.

As a result, instead of breaking up quickly after any initial damage is repaired, clots continue to grow unabated, further damaging red blood cells.

Treatment is plasmapheresis to re-introduce the critical enzyme to the system while removing the antibodies that attack it. Although many cases of TTP can be treated by removing the underlying cause, some patients require a continuing regimen of immunosuppressants or steroids. Where plasmapheresis is contra-indicated (such as when the patient suffers from anemia), the usual treatment is a splenectomy, although this procedure is riskier.

TTP is very rare, affecting about 4 to 6 out of every million people every year. However, African-Americans are slightly more likely to get it, and in pregnant women it occurs about once in every 25,000 pregnancies.