Platelets - abnormal counts (Thrombocytosis and Thrombocytopenia)

🔍 When to Suspect

Thrombocytosis (platelets >450) with unexplained bleeding, headache, or clotting. Thrombocytopenia (platelets <150) with bleeding, bruising, petechiae, or found incidentally on FBC

From the full topic in The Ocean Library: Platelets - abnormal counts (Thrombocytosis and Thrombocytopenia)

🧭 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).

• 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: 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 myeloproliferative neoplasms (MPNs).

Suspect on an incidental result, or with:

• Headache

• Visual disturbance

• Burning pain in the hands or feet (erythromelalgia)

• Abnormal bleeding

• Unprovoked thrombosis

Thrombocytopenia (platelets < 150 × 10⁹/L) presents with:

• Bruising

• A petechial (non-blanching) rash

• Mucosal bleeding such as epistaxis or menorrhagia

• 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).

🧠 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. • Do not wait for a second red flag before investigating.

Source: NICE NG12 · British Society for Haematology

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