Defined as <150 × 109/L: as platelet count decreases risk of bleeding and bruising increases. Risk of bleeding is moderately high <20 × 109/L and likely if <10 × 109/L.
· Selective megakaryocyte depression: viral (HIV, parvovirus, EBV) or more substantial bacterial infection, drugs, and poisons.
· Marrow failure: aplastic anaemia, Fanconi’s syndrome, severe IUGR, severe maternal pre-eclampsia, neonatal sepsis.
· Marrow infiltration: leukaemia, neuroblastoma, osteopetrosis.
· Marrow depression: radiotherapy, cytotoxic drugs, drug reaction.
· Hereditary: Wiskott–Aldrich syndrome (X-linked recessive: boys present with early thrombocytopenia, eczema, and immunocompromise due to immunoglobulin abnormalities), BSS, TAR syndrome.
· Nutritional deficiency: vitamin B12 or folate deficiency.
· Immune: ITP (most commonly in child, rarely in mother), neonatal alloimmune thrombocytopenia (NAIT), SLE, drug-induced (penicillin or heparin-induced thrombocytopenia (HIT)), infection (e.g. malaria or HIV).
· Non-immune: DIC, giant haemangioma (Kasabach–Merritt syndrome), HUS, cardiac disease (prosthetic valves or cardiopulmonary bypass).
· Hypersplenism: platelets pool in enlarged spleen from whatever cause—effect is dilutional, rather than destructive.
· History: drug history, family history, preceding viral illness.
· Examination: signs of bleeding, lymphadenopathy, hepatosplenomegaly, concurrent infection.
· FBC and blood film.
· Serology: anti-platelet antibodies (e.g. anti-HPA1) if NAIT suspected, autoimmune antibodies in those with chronic ITP, viral serology (CMV, EBV along with monospot if infectious mononucelosis suspected, or HIV if unusual unexplained thrombocytopenia).
· Bone marrow aspirate and trephine: very rarely required in cases of unexplained thrombocytopenia.
· Cranial CT scan: if any evidence of possible intracerebral haemorrhage.
· Treat underlying cause if possible.
· Platelet transfusion if very low platelet count (prophylactically, and guided by haematologists, except for ITP) or life-threatening bleeding.
· Splenectomy, e.g chronic ITP, hypersplenism.
Bone marrow transplant may be helpful in some inherited bone marrow failure syndromes.