Platelet CountPLT
Anucleate fragments central to hemostasis; a sentinel for inflammation, marrow function, and consumptive disease.
What it measures
Produced by megakaryocytes in marrow under TPO control. Lifespan ~7–10 days. Aggregation is platelet-driven primary hemostasis.
Why we order it
Bleeding/thrombosis risk assessment, monitor chemo and immunosuppression, detect ITP/TTP/HIT and reactive thrombocytosis.
Interpretation by lens
Spontaneous bleeding risk rises sharply <20 ×10⁹/L; surgical risk rises <50.
Persistent platelets >350 with high ferritin and CRP often reflect smoldering inflammation.
Mean platelet volume (MPV), immature platelet fraction (IPF), and platelet RNA are emerging cardiovascular and oncology biomarkers.
Causes of abnormal values
- ↑Reactive (infection, inflammation, iron deficiency, post-splenectomy, malignancy)
- ↑Essential thrombocythemia (with JAK2/CALR/MPL mutations)
- ↑Polycythemia vera, primary myelofibrosis
- ↓Pseudothrombocytopenia (EDTA-induced clumping — re-draw in citrate)
- ↓ITP, TTP, HIT, DIC
- ↓Drug-induced (heparin, vancomycin, quinine, valproate, linezolid)
- ↓Liver disease/cirrhosis (sequestration + decreased TPO)
- ↓Marrow failure or infiltration
- ↓Sepsis
Pattern recognition
TTP/HUS — emergency. ADAMTS13 activity, plasma exchange.
HIT until proven otherwise. 4Ts score, anti-PF4, stop all heparin including flushes.
Pre-analytic & interpretation traps
- !EDTA pseudothrombocytopenia is common; always confirm a low platelet count with a citrate tube before workup.
- !Giant platelets (e.g., MYH9 disorders) are miscounted by some analyzers as RBCs.