ITP is a little-understood (Idiopathic1) disease of the blood involving a severely reduced platelet count (Thrombocytopenia). It affects roughly one in 10,000 people at some time in their life.
It should be of particular interest to anyone who has come into contact with recent (at the time of writing) British government meningitis information, since the initial indications can include bruises (Purpura) or a septicemia-like rash that fails the glass tumbler test2, and in children it is most common amongst a similar 4-6 age group.
Although a lack of platelets is not directly life-threatening, the risk of spontaneous internal haemorrhage is, so your urge to rush your child to the doctor upon finding such a rash should be undiminished3.
Diagnosis is made by a simple blood test, typically showing no abnormalities other than the low platelet count (less than 100,000 per mm3 is low). A bone marrow sample rules out other causes of similar symptoms, such as certain forms of leukaemia.
Patients who suffer or have suffered from ITP are divided into two categories: acute (recovered within six months) and chronic (didn't, and likely never will). In the 90% of children who get the acute form, the disease will often follow a viral illness of some kind, and spontaneously heal with no intervention. If the platelet count is very low at the onset of the disease (less than 40,000 per mm3), injections of globulin or a course of steroids may be given to block the action of the inferred4 platelet-killing antibodies5.
Treatment for the chronic form, more common in adults, may include a splenectomy (removal of the spleen) but even this drastic action has only a 70% chance of curing the problem. The theory is that the spleen makes most of the antibodies that destroy the blood platelets. It also destroys old or damaged blood cells.
Further (less jaded) Reading
Concise description of ITP from DrKoop.com's Medical Encyclopedia, with cross-referencing of technical terms and descriptions of procedures.
Description of ITP at NIDDK, one of the US National Institutes of Health.