Reasoning GP · Algorithm · 1-page summary
Lymphocytosis (raised lymphocyte count)
For: adults and children with a raised absolute lymphocyte count on FBC in primary care. Not for: known CLL or MBL under a written haematology plan (follow the plan); glandular fever and whooping cough care (use those protocols).
Core rule: the film, other cell lines, nodes, spleen, symptoms and trend set the urgency, not the count. A lymphocytosis that persists after recovery needs a film and flow cytometry via the local pathway; a total count alone never diagnoses CLL.
0Stop — act now — any count
- Blasts on the film, or film/FBC reported as acute leukaemia → Immediate haematology discussion + hospital assessment. Do not arrange a bone-marrow biopsy from primary care.
- Severe cytopenias with deterioration, severe infection or bleeding, rapidly progressive count with B symptoms, rapidly enlarging nodes with systemic illness → Same-day haematology
- EBV: stridor, respiratory distress or unable to swallow saliva; sudden or severe left upper abdominal or left shoulder-tip pain, dizziness or collapse (splenic rupture) → 999
- Infant with failure to thrive, persistent or severe infections, chronic diarrhoea, thrush or unusual organisms → Same-day paediatrics
1Triage — not the count alone
| Finding | Action |
|---|---|
| Acute illness, well | Clear viral cause → support; repeat FBC after recovery. |
| Persists, unexplained | Film, old FBCs, examine; flow cytometry via the local pathway. |
| Concerning, stable | Progressive cytopenias, rapid rise, recurrent infection, bruising, growing nodes or spleen → Urgent haematology (local) |
| Blasts or unwell | Block 0. |
2Screen — serious causes
- Adult, unexplained nodes or splenomegaly, weigh B symptoms, itch, breathlessness → consider Suspected cancer pathway (NG12)
- Abnormal lymphocytes on film + mediastinal mass on CXR → Suspected cancer pathway; haematology stages.
- Neutropenia + splenomegaly + RA-like picture (?LGL leukaemia) → haematology assessment.
- Paroxysmal cough ± whoop → pertussis: notify the Health Protection Team on suspicion; whooping cough pathway.
- Fever, sore throat, nodes ± spleen → EBV likely; think CMV, HIV.
3Assess
☐ Absolute count (not %), previous FBCs, timing ☐ Hb, neutrophils, platelets ☐ Nodes, liver, spleen, bruising, pallor, B symptoms ☐ Drugs, smoking, splenectomy, recent infection or vaccination
Tests: repeat FBC + differential, film; CRP/ESR, LFTs, renal function as indicated. HIV, EBV (VCA IgM/IgG + EBNA) or pertussis PCR only if the picture fits.
Not in primary care: bone marrow; CT staging (haematology arranges); diagnosing CLL from the count or smear cells.
4Classify
- Reactive viral — EBV, CMV (atypical lymphocytes), HIV, pertussis, adenovirus, hepatitis A → settles on repeat.
- Other reactive — smoking, post-splenectomy (mild, stable: no action), Addison's, hyperthyroidism, some antiepileptics.
- CLL — clonal B cells ≥5 × 10⁹/L on flow cytometry, persistent ≥3 months; usually over 60; mostly indolent.
- MBL — clonal B cells <5 × 10⁹/L, no nodes, organomegaly or cytopenias → follow the report's plan.
- Lymphoma, LGL leukaemia, ALL — film + flow cytometry; haematology.
- Uncertain — say so; film + flow cytometry via the local pathway.
5Treat today
- EBV: paracetamol or NSAID, fluids; no routine steroids or antivirals. No amoxicillin or ampicillin; if needed, phenoxymethylpenicillin or clarithromycin. Doses: NICE/SPC, BNF, local formulary.
- EBV sport: no contact sport for at least 4 weeks from onset (CKS).
- Smoker, reactive and stable: stop-smoking advice.
- Known CLL: treat infections promptly; vaccines per Green Book, no live vaccines unless haematology advises.
6Follow up and refer
- Persistent unexplained after reactive causes excluded, abnormal flow, unexplained cytopenias → Haematology advice / routine
- MBL or CLL: GP monitoring only with a written haematology plan; re-refer for new nodes, spleen, B symptoms, cytopenias or rising count.
- New B symptoms in known CLL → Urgent haematology; same-day if nodes enlarging fast or unwell.
- Post-EBV fatigue over 3 months with functional impairment → ME/CFS service.
7Never reassure by
- A count under 20 × 10⁹/L: serious disease occurs at lower counts.
- "Just viral" when it persists after recovery, or with nodes, spleen, cytopenias or B symptoms.
- Smear cells or a total count as the diagnosis: flow cytometry confirms CLL or MBL.
- A negative Monospot early in the illness or in a young child.
- An adult lymphocyte range in a young child: use the age-specific range.
- GP follow-up of CLL or MBL without a written haematology plan.
8Safety-net — say it, then document it
999 “Call 999 for noisy or difficult breathing, being unable to swallow your saliva, or sudden severe pain in the upper left tummy or left shoulder tip, dizziness or collapse.”
Same day (CLL) “A temperature over 38°C, a worsening cough or breathlessness, spreading skin redness, or new bruising or bleeding — get seen today; call 999 if you feel very unwell.”
✓Safe to leave primary care only if — all five, documented
No block 0 feature; no blasts or new cytopenias
Absolute count, old FBCs and film reviewed
No unexplained nodes, spleen or B symptoms
Clear reactive cause, or flow / haematology route set
Repeat FBC after recovery booked; safety-net given
Summary of the full Reasoning GP lymphocytosis algorithm, which holds CLL staging, shared care and the evidence. Sources: NICE NG12 Suspected cancer (updated April 2026); BSH CLL guideline (2025); UKHSA pertussis guidance and Green Book; local haematology referral and reflex flow-cytometry pathway. Clinical reference — check doses: NICE/SPC, BNF.