Reasoning GP · Algorithm · 1-page summary
Leucocytosis (raised white cell count)
For: adults with a raised WCC on FBC in primary care. Not for: children; known CLL, CML or another haematological disorder (follow the specialist monitoring plan); eosinophilia detail (use the eosinophilia pathway).
Core rule: the clinical state, differential, blood film, Hb, platelets, trend and organomegaly set the urgency, not the total WCC. Any blasts, or a persistent unexplained leucocytosis, needs haematology, not another antibiotic course.
0Stop — emergency assessment now — any count
- Shock or sepsis, reduced consciousness, severe respiratory or cardiac symptoms, active significant bleeding, febrile neutropenia, suspected acute leukaemia with instability → 999 / emergency
- Suspected leucostasis: headache, confusion, visual changes, breathlessness, chest pain or priapism with marked leucocytosis → Emergency assessment; management is haematology-led.
- WCC ≥100 × 10⁹/L but stable → Urgent acute haematology, usually hospital assessment; transport by clinical state, not an automatic 999 ambulance.
1Triage by film and counts — not the WCC alone
| Finding | Action |
|---|---|
| Blasts / immature cells | Same-day haematology; emergency if unwell, bleeding, infection or cytopenias |
| New cytopenias | Anaemia, low platelets, leukoerythroblastic film → Same-day haematology |
| Marked spleen; bleeding | Spleen with abnormal counts; petechiae, gum bleeding, haematuria → Same-day haematology |
| Well, clear cause | Mild, reactive, reassuring differential and film → treat cause; repeat FBC once resolved. |
2Screen — serious causes by line
- Neutrophilia + left shift, basophilia or spleen (?CML) → discuss with haematology.
- Persistent lymphocytosis → film; CLL needs clonality on immunophenotyping (local pathway).
- Unexplained lymphadenopathy or splenomegaly, especially with B symptoms, itch or breathlessness → consider Suspected cancer pathway (NG12).
- Unexplained fever, night sweats or weight loss → blood film.
- Monocytosis with anaemia, low platelets, splenomegaly or dysplasia (?CMML) → discuss with haematology.
- Eosinophilia ≥1.5 × 10⁹/L persistent → Urgent haematology advice; with organ involvement → Same-day
3Assess
☐ Original FBC: absolute differential, Hb, platelets, indices, trend, lab comment ☐ Infection, steroids, G-CSF, beta-agonists, smoking, splenectomy, inflammatory disease, surgery or exercise, travel ☐ Obs, nodes, spleen, liver, skin, chest
Tests: repeat FBC + differential; film if persistent, unexplained or abnormal; CRP/ESR, U&E, LFTs, bone profile; LDH if haematological concern.
Not in primary care: bone marrow, JAK2, BCR-ABL on the WCC alone, flow for every lymphocytosis, CT without indication, uric acid for a high WCC.
4Classify — by the dominant line (× 10⁹/L)
- Neutrophils >7.5 (commonest) — infection, inflammation, steroids, smoking, obesity, stress.
- Lymphocytes >4 — viral (EBV, CMV, pertussis; may persist after infection), CLL, lymphoma.
- Monocytes >1 — chronic infection (TB, endocarditis), SLE, sarcoid, recovery from neutropenia, CMML.
- Eosinophils >0.5 — allergy, drugs, parasites, EGPA → eosinophilia pathway.
- Basophils >0.1 (rare) — CML, PV, myelofibrosis, hypothyroidism.
- Mixed or uncertain — say so; myeloproliferative vs reactive → film, discuss with haematology.
5Treat today — the cause, not the count
- Infection: treat a clinically diagnosed source per current antimicrobial guidance and local formulary. A raised WCC alone is not an indication for antibiotics.
- Steroids: review as a cause; do not taper solely for the WCC; never stop abruptly when clinically required.
- Smoking: offer cessation (varenicline or NRT, local stop-smoking service) for general health. Doses: NICE/SPC, BNF, local formulary.
- Known CLL, CML or MPN: follow the specialist plan; treat infections promptly; flu and pneumococcal vaccines; no live vaccines if immunosuppressed.
6Follow up and refer
- Repeat FBC once the reactive cause has resolved, per lab comment; normal → no further routine testing.
- Persistent unexplained or rising count after reactive causes addressed → Urgent haematology via local pathway.
- Abnormal film (left shift, atypical cells) without urgent features → Routine haematology
- Persistent fever, travel, ?TB or HIV → infectious diseases; suspected EGPA (ANCA may be negative) → rheumatology.
7Never reassure by
- A modest total WCC: blasts, cytopenias or an abnormal film matter more.
- Another antibiotic course for a persistent unexplained leucocytosis.
- "Neutrophilia means bacterial infection."
- Each value "not alarm-level" when the trend is rising (12 → 15 → 20).
- "Just viral" lymphocytosis with nodes, spleen, cytopenias or B symptoms.
- Steroids for unexplained eosinophilia with travel exposure before Strongyloides testing or advice.
8Safety-net — say it, then document it
999 “Call 999 for a sudden severe headache, confusion, loss of vision, chest pain or breathlessness, a painful erection that will not go down, severe bleeding, or fever with shaking chills or feeling very unwell.”
Same day “New lumps in your neck, armpit or groin, a temperature over 38°C, a swelling tummy, drenching night sweats or weight loss you can’t explain — get seen today.”
✓Safe to leave primary care only if — all five, documented
Well; no block 0 feature; no blasts or new cytopenias
Differential, film (if indicated) and trend reviewed
No unexplained nodes, spleen or B symptoms
Clear reactive cause being treated
Repeat FBC date set; safety-net given
Summary of the full Reasoning GP leucocytosis algorithm, which holds the causes, investigations and evidence. Sources: NICE NG12 Suspected cancer (updated April 2026); BSH CLL guideline (2025); BSH eosinophilia guideline (2017); UKHSA Green Book chapter 25; local antimicrobial formulary; local haematology referral and laboratory pathways. Clinical reference — check doses: NICE/SPC, BNF.