Platelet Count
PLT· also: Platelets, Thrombocytes
Clinical Overview
Platelets (thrombocytes) are small cell fragments produced by megakaryocytes in the bone marrow. They are essential for primary hemostasis — forming the initial platelet plug at sites of vascular injury to stop bleeding. A platelet count is routinely measured in every CBC.
Why This Test Matters
Platelet count guides the diagnosis and management of bleeding disorders and clotting abnormalities. Critically low platelets (thrombocytopenia) increase bleeding risk, while elevated platelets (thrombocytosis) may promote clot formation. The count alone does not capture platelet function — additional tests (PFA-100, aggregation studies) are needed when platelet dysfunction is suspected despite a normal count.
Reference RangesWHO/IFCC standards
| Reference Range | Unit | Notes |
|---|---|---|
| 150 – 400 | ×10³/µL | — |
Also reported in: ×10⁹/L.
Critical (Panic) Values
Critical Low: < 50 ×10³/µL. Critical High: > 1000 ×10³/µL. Values outside these limits require immediate clinical attention.
What Causes Abnormal Results?
High PLT Causes
- Reactive thrombocytosis — after infection, inflammation, or surgery (most common)
- Iron deficiency anemia
- Post-splenectomy
- Essential thrombocythemia (bone marrow disorder)
- Polycythemia vera
- Malignancy (paraneoplastic)
Low PLT Causes
- Immune thrombocytopenic purpura (ITP) — autoimmune destruction
- Dengue fever (platelet destruction is characteristic)
- Bone marrow failure (aplastic anemia, leukemia)
- Chemotherapy or radiation
- Heparin-induced thrombocytopenia (HIT)
- Disseminated intravascular coagulation (DIC)
- Hypersplenism (spleen trapping platelets)
- Vitamin B12 or folate deficiency
- Sepsis
Signs & Symptoms to Watch For
How to Prepare for This Test
No special preparation needed. Tell your doctor if you take aspirin, NSAIDs, or anticoagulants, as these affect platelet function (but not the count itself).
Factors That Can Affect Results
- EDTA-dependent pseudothrombocytopenia (platelet clumping in the collection tube — artifact)
- Giant platelets counted as white cells (instrument error)
- Heparin therapy can lower platelet count (HIT)
- Aspirin and NSAIDs affect function but not count
Related Topics
Frequently Asked Questions
At what platelet count does spontaneous bleeding occur?
Spontaneous bleeding risk increases significantly when platelets fall below 20,000/µL. Counts between 20,000–50,000/µL carry risk during procedures or trauma. Platelet transfusions are typically given prophylactically when the count is below 10,000–20,000/µL, or below 50,000/µL before surgery.
Why are platelets low in dengue fever?
In dengue fever, the dengue virus infects and destroys platelets directly and triggers immune-mediated platelet destruction. Infected bone marrow megakaryocytes are also damaged, reducing platelet production. A platelet count below 100,000/µL is common in dengue, and counts below 20,000/µL can cause life-threatening bleeding.
Is a high platelet count dangerous?
Mildly elevated platelets from reactive thrombocytosis (infection or iron deficiency) rarely cause problems and resolve when the underlying cause is treated. However, in essential thrombocythemia, very high counts (over 1,000,000/µL) can paradoxically cause both clotting and bleeding complications.
What is the difference between thrombocytopenia and ITP?
Thrombocytopenia is the general term for a low platelet count, with many possible causes. Immune thrombocytopenic purpura (ITP) is a specific autoimmune cause where antibodies attack platelets. ITP is diagnosed after ruling out other causes and is treated with steroids, IVIG, or TPO receptor agonists.