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Abnormalities of platelet number

From www.haematologyetc.co.uk



Summary


  • In normal health platelet counts lie between 150-400 x109/l.
  • There is approximately 1 platelet to 10-20 red cells*
  • A ratio of 1:15 platelets to red cells can be helpful in manually assessing platelet number, but take care since the actual ratio may vary according to the area of the blood film examined and the number of red cells present, and whether platelets are clumped
  • Elevated or reduced numbers of platelets can occur in a wide range of conditions, and altered number is not specific to single disease states. This section focuses mainly on morphological aspects, but the more frequent or important causes are outlined in the tables.


Myeloproliferation: platelet number exceeds red cell number (essential thrombocythaemia)




Causes of a raised platelet count

HAEMATOLOGICAL NEOPLASMS
Platelet counts may be very high (>1000x109/l). Look for marked variability and enlargement of platelets; if morphological abnormalities are more significant then this may suggest transformation. Where part of an MDS/MPD or acute leukaemia, the features affecting other cell types will be more conspicuous
Consider:
  • essential thrombocythaemia
  • prefibrotic stage of primary myelofibrosis
  • polycythaemia vera (there may also be iron deficiency)
  • chronic myeloid leukaemia (platelet count is rarely affected alone)
  • myelodysplastic/myeloproliferative neoplasm: refractory anaemia with ring sideroblasts and thrombocytosis
  • rare cases of acute myeloid leukaemia (t(3;3) or inv(3))
REACTIVE OR OTHER CAUSES

The platelet count is usually <1,000 x 109/l with relatively normal platelet appearances. The cause may be obvious to the clinical team, or through typical blood film appearances e.g. iron deficiency or splenic hypofunction

Consider:
  • Chronic inflammation: including rheumatoid arthritis, inflammatory bowel disease, tuberculosis
  • Acute blood loss: haemolysis or haemorrhage
  • Iron deficiency
  • Cancer
  • Splenectomy or hyposplenism
ALWAYS CONSIDER ARTEFACT
Although a relatively uncommon artefact, occasionally cryoglobulin or other particles in blood can be counted as platelets by automated analysers



Causes of a reduced platelet count


DECREASED PLATELET PRODUCTION
Platelets may have abnormal appearance, and there may be additional morphological clues within other cell lineages
The most commonly encountered are::
  • Bone marrow infiltration: as part of haematological (or less frequently non-haematological)
  • Bone marrow failure: aplastic anaemia or chemotherapy (rarely poisons)
  • Ineffective production: MDS or nutritional deficiency
  • Inherited states: Bernard‒Soulier syndrome, Glanzmann's thromaethaemia, MYH9-related disorders or grey platelet syndrome
INCREASED PLATELET DESTRUCTION

A compensatory increase in platelet number may cause the platelets to be larger; it is important to look for features of cause (although these may be absent in immune causes).

Consider:
  • Viral illness including mononucleosis, hepatitis and HIV
  • Malarial infection
  • MAHA including DIC and sepsis, HUS, TTP or physical destruction
  • Pregnancy gestational 100-150 but consider ITP, pre-eclampsia or HELLP
  • Immune ITP or autoimmune, drug induced
ALWAYS CONSIDER ARTEFACT
Clumping of platelets must be excluded as a cause to prevent uneccessaryconcern and investigation; but also consider that large platelets may not be counted accurately