Reasoning GP · Algorithm · 1-page summary
Thrombocytosis — raised platelet count
For: adults in primary care with platelets >450 × 10⁹/L. Not for: a raised Hb or haematocrit (use the polycythaemia algorithm); known ET or PV on treatment follows the haematology plan and local shared-care protocol.
Core rule: most thrombocytosis is reactive, but a persistent, unexplained raised count is an NG12 cancer prompt and may be a myeloproliferative neoplasm (MPN). Treat the cause and recheck; never start aspirin or cytoreduction for the count in primary care.
0Stop — call 999 now — whatever the platelet count
- Stroke or ongoing neurological deficit, confusion, seizure, sudden vision loss or suspected intracranial bleed → 999
- Acute coronary syndrome, suspected PE, splanchnic or cerebral venous thrombosis (severe abdominal pain + ascites: Budd–Chiari) → 999
- Major bleeding; digital ischaemia (cold, pale or blue digit) or threatened limb; collapse → 999
- While waiting: manage the thrombosis through the relevant emergency pathway. Do not start aspirin or anticoagulation solely from the platelet count.
1Triage by count and persistence
| Result | Action |
|---|---|
| Clear cause | Treat it (infection, inflammation, iron deficiency, surgery, bleeding); recheck FBC after. |
| Persistent (≥3 months), unexplained | Any count, even <600 → JAK2 V617F per local pathway; Urgent haematology. |
| ≥1000 × 10⁹/L | Prompt review + haematology discussion; Same-day if symptomatic or unstable. Count alone ≠ emergency cytoreduction. |
2Screen — serious causes
- Bleeding with very high count, new or severe erythromelalgia, new or enlarging spleen, film suggesting leukaemia, recent thrombosis → Same-day haematology
- Severe headache or visual disturbance → Same-day assessment; suspected DVT → Same-day (NG158); resolved TIA → specialist ≤24 h
- NG12: ≥40 → consider urgent chest X-ray; women ≥55 + visible haematuria or unexplained vaginal discharge → consider urgent, direct access ultrasound (endometrial); ≥55 + nausea, vomiting, weight loss, reflux, dyspepsia or upper abdominal pain → consider non-urgent OGD (dysphagia, or weight loss with these → Suspected cancer pathway)
- Positive JAK2, CALR, MPL or BCR-ABL1; abnormal film; spleen, B symptoms, high WCC or Hb; unusual-site thrombosis → Urgent haematology
3Assess
☐ Previous FBCs; repeat FBC; film if persistent, unexplained or other FBC abnormalities ☐ Ferritin ± transferrin saturation ☐ CRP/ESR, U&E, LFT ☐ Spleen, nodes, hands/feet, legs, neurology ☐ Prior thrombosis, splenectomy, smoking, drugs
No bone marrow biopsy, no routine broad CT in primary care. Do not restrict JAK2 to counts >600.
4Classify
- Reactive (most): infection, inflammation, iron deficiency, bleeding, surgery, cancer, hyposplenism → treat, recheck.
- Essential thrombocythaemia (ET): persistent, unexplained, may be modestly raised; some triple-negative → specialist diagnosis.
- PV: raised Hb/Hct → polycythaemia pathway + haematology.
- CML or myelofibrosis: high WCC, big spleen, left shift, tear-drop cells → urgent haematology.
- Uncertain — unexplained, disproportionate or progressive → JAK2 per local pathway + haematology.
5Treat today — only these
- Treat the cause: confirmed iron deficiency, and find its source (GI or menstrual); antibiotics only for bacterial infection.
- No aspirin, NSAIDs or cytoreduction for the count (aspirin only for an independent cardiovascular indication).
- Known ET/PV: treatment per haematology; hydroxycarbamide only under local shared care; pregnancy → early haematology + obstetrics.
- Smoking: support + varenicline, cytisinicline, combination NRT or vape (NG209); usual NICE BP, lipid, diabetes targets.
6Refer routinely
- Reactive and settling: recheck FBC, no referral; refer if unexplained, disproportionate or progressive. Timing per the local pathway.
- Cause: respiratory (CXR mass), gastro (IBD), rheumatology (RA, vasculitis).
- Known ET — re-refer: count rising despite cytoreduction, thrombosis on treatment, drug intolerance.
7Never reassure by
- Calling it "reactive" when no adequate cause is found.
- A count below 600 — ET can be only modestly raised.
- Normal CRP/ESR — does not exclude a reactive cause or an MPN.
- A negative JAK2 — some ET is triple-negative.
- Waiting for platelets to settle before treating iron deficiency.
8Safety-net — say it, then document it
999 “Call 999 for face or arm weakness, speech or sudden sight problems, even if they go away; sudden severe headache; chest pain or breathlessness; severe tummy pain with swelling; confusion, a fit, heavy bleeding, a cold or blue finger or toe, or collapse.” Same day “Contact us today if one leg becomes swollen or painful, your hands or feet become burning, red and hot, you have unusual bleeding or bruising, or a bad headache or change in vision.”
Remote result: platelets ≥1000 → prompt review and haematology discussion, same-day if symptoms; an unexplained count at ≥40 → act on NG12, do not file as routine.
✓Safe to manage in primary care only if — all five, documented
No block 0–2 row applies; patient well
Clear reactive cause, being treated
Film, ferritin and CRP reviewed
NG12 routes checked by age and symptoms
Recheck dated, safety-net given
Summary of the full Reasoning GP thrombocytosis algorithm, which holds ET/PV treatment, cytoreduction, monitoring and evidence. Sources: NICE NG12 (updated April 2026), NG128, NG158, NG209; BSH thrombocytosis guidelines (2010, 2025); WHO classification of myeloid neoplasms; local hydroxycarbamide shared-care protocol. Clinical reference — follow local pathways; doses: NICE/SPC, BNF, local formulary.