GPAtlas🧭 When to suspect
The platelet count is one of the most common abnormalities a GP meets on a full blood count (FBC), and the great majority are incidental and benign. The skill is to separate a transient, reactive change from the minority that signal serious disease – occult cancer, a myeloproliferative neoplasm (MPN), or a bleeding risk that needs same-day action.
Thrombocytosis (platelets > 450 × 10⁹/L) is most often reactive – to infection, inflammation, tissue damage, iron deficiency, or malignancy. The key primary-care point is that a raised platelet count is a NICE-recognised marker of occult cancer, with a positive predictive value above the 3% threshold that mandates investigation. Persistent, unexplained thrombocytosis also raises the possibility of essential thrombocythaemia (ET) and other MPNs. Suspect on an incidental result, or with headache, visual disturbance, burning pain in the hands or feet (erythromelalgia), abnormal bleeding, or unprovoked thrombosis.
Thrombocytopenia (platelets < 150 × 10⁹/L) presents with bruising, a petechial (non-blanching) rash, mucosal bleeding such as epistaxis or menorrhagia, or – commonly – as an incidental finding. Frequent causes are viral infection, drugs, alcohol excess, liver disease, pregnancy (gestational thrombocytopenia), and immune thrombocytopenia (ITP). The job is to grade severity, find a reversible cause, and recognise the few that need urgent referral or admission.
| Thrombocytopenia – platelet count | Severity | Primary-care action |
|---|---|---|
| 100–149 | Mild | Often reactive; if isolated, stable and without atypical features, recheck FBC and monitor in primary care. |
| 50–99 | Moderate | Refer haematology if persistent/unexplained; urgent if age > 60, systemic symptoms, splenomegaly, abnormal FBC, pregnant, or pre-procedure. |
| 20–49 | Severe | Urgent haematology referral. |
| < 20 | Very severe | Same-day admission – risk of spontaneous bleeding. |
| Any count + active bleeding | – | Same-day assessment/admission regardless of the number. |
| Thrombocytosis – platelet count | Category | Primary-care action |
|---|---|---|
| 450–600 | Mild | Usually reactive; check ferritin and CRP, treat the cause, recheck. Pursue cancer pathways (LEGO-C) if symptoms fit. |
| 600–1000 | Marked | Refer haematology if persistent and unexplained, or symptomatic; suspected-cancer pathway if cancer features. |
| > 1000 | Extreme | Urgent haematology – risk of thrombosis and paradoxical bleeding (acquired von Willebrand syndrome). |
| Persistent > 450 (confirmed, no reactive cause) | – | Routine haematology referral to exclude ET / MPN. |
| Thrombocytosis + cancer features | – | Site-specific suspected-cancer pathway (NICE NG12). |
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🧠 Clinical pearl A raised platelet count can be the only clue to cancer. The cancers most associated with thrombocytosis are remembered as LEGO-C: Lung, Endometrial, Gastric, Oesophageal, Colorectal. In the landmark UK primary-care cohort, roughly a third of patients later diagnosed with lung or colorectal cancer had thrombocytosis with no other referable symptom – so do not wait for a second red flag before investigating. |
Source: NICE NG12 · British Society for Haematology
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