CLL (Chronic Lymphocytic Leukaemia) is the most common adult leukaemia in the Western world — it presents as an incidental finding of lymphocytosis on routine FBC in approximately 70% of cases. The blood film helps distinguish reactive from suspicious morphology: smear cells (Gumprecht shadows or smudge cells — fragile lymphocytes that disintegrate when spread on the slide) may support CLL in the appropriate clinical and morphological context but are not diagnostic; confirmation requires immunophenotyping and specialist interpretation. CLL is diagnosed by persistent (at least 3 months) clonal B lymphocytes ≥5 × 10⁹/L with a characteristic immunophenotype (typically CD19+, CD5+, CD23+ with light-chain restriction), usually confirmed by peripheral-blood flow cytometry. A total lymphocyte count alone does not diagnose CLL, as it includes T cells and other lymphocytes. The majority of CLL patients are elderly (median age at diagnosis 72) and have indolent disease that requires no treatment initially — 'watchful waiting' (active monitoring without treatment) is appropriate for patients without active-disease criteria. The GP's role: arrange haematology advice or referral per the local pathway, provide reassurance that CLL is often slow-growing, and avoid over-alarming the patient.

Pertussis (whooping cough) classically causes a very pronounced lymphocytosis — absolute lymphocyte counts of 15-30 × 10⁹/L are characteristic and are caused by pertussis toxin inhibiting lymphocyte migration out of the blood into lymphoid tissue (the cells accumulate in peripheral blood). This lymphocytosis pattern in a child or unvaccinated adult with a prolonged cough (the catarrhal phase lasting 1-2 weeks, followed by the paroxysmal phase with inspiratory whoop) should prompt consideration of pertussis, although marked lymphocytosis alone does not establish the diagnosis — manage via the separate whooping cough pathway (notification, PCR timing, antibiotic indications, infectious-period advice and contact prophylaxis per current UKHSA guidance). PCR on a nasopharyngeal swab is the test early in the illness (roughly the first 2–3 weeks of cough); once the cough has lasted over 2 weeks, oral fluid testing (ages 2 to under 17, via the HPT) or serum serology. Treatment: azithromycin 500 mg once daily for 3 days in non-pregnant adults (erythromycin in pregnancy) (UKHSA pertussis guidance) (reduces transmission, minimal effect on illness duration if paroxysmal phase already begun). Pertussis is a notifiable disease — notify the UKHSA Health Protection Team. The whooping cough immunisation programme (6-in-1 at 8, 12, 16 weeks; a fourth 6-in-1 dose at 18 months for children born on or after 1 July 2024; pre-school booster at 3 years 4 months; maternal vaccination in every pregnancy from 16 weeks, ideally 20–32 weeks, and still worth giving later up to labour — UKHSA routine schedule, Green Book ch. 11 and ch. 24) has significantly reduced but not eliminated pertussis. Large granular lymphocyte (LGL) leukaemia is another important and frequently delayed diagnosis — the triad of: neutropenia (recurrent bacterial infections) + splenomegaly + positive RF/ANA in a middle-aged adult should prompt haematology assessment (persistent LGL expansion, often with T-cell receptor clonality testing).

Flow cytometry for lymphocyte immunophenotyping is the gold standard investigation for distinguishing reactive from clonal lymphocytosis — it measures the surface antigen profile of lymphocytes, identifying: clonal B-cell populations (CD5+/CD19+/CD23+ = CLL; CD5+/CD19+/cyclin D1+ = mantle cell lymphoma; CD10+ = follicular lymphoma), T-cell receptor clonality (T-LGL), and the ratio of kappa to lambda immunoglobulin light chains on B-cells (a skewed kappa:lambda ratio indicates a monoclonal population — clonal is not necessarily malignant, e.g. MBL). Many UK laboratories use reflex or clinician-requested lymphocyte immunophenotyping pathways for persistent absolute lymphocytosis — request through the local laboratory or haematology pathway according to local criteria. GPs should refer promptly when the blood film and clinical picture suggest clonal lymphocytosis, rather than waiting further — early haematology assessment allows proper characterisation and staging, which determines management (watchful waiting vs treatment initiation).

The CLL watchful waiting approach requires specific patient communication skills — patients who are told they have 'leukaemia' but are then told nothing will be done about it understandably find this distressing and confusing. The key explanation: 'CLL is a type of blood cancer, but it behaves very differently from most cancers. In most people, it grows very slowly — so slowly that we can watch it for years, or even decades, without needing to treat it. The research shows clearly that treating it early (when it is not causing symptoms) does not help and can harm — it does not extend life or prevent the disease from progressing. We will monitor you closely with regular blood tests, and the moment it starts to cause symptoms or affect your other blood cells, we will act.' This reframing of 'watchful waiting' as active evidence-based management (not neglect) significantly improves patient acceptance. Many CLL patients live normal lives for 10-15 years without ever needing treatment.

The pertussis notification obligation for GPs is a legal requirement under UK public health legislation — pertussis is a notifiable disease and GPs who suspect or diagnose pertussis must notify the local Health Protection Team (HPT) on suspicion — in writing within 3 days, or by phone immediately if urgent (for example a vulnerable infant in the household) — without waiting for laboratory confirmation. The UK still experiences significant pertussis outbreaks — annual confirmed case numbers vary widely, with a marked resurgence in 2024. The maternal pertussis vaccination programme (ADACEL (Tdap) since 1 July 2024, replacing Boostrix-IPV; offered from 16 weeks, ideally 20–32 weeks) has dramatically reduced infant pertussis deaths by providing passive immunity to newborns before their own vaccination at 8 weeks. GPs who identify pertussis in a pregnant woman or a close contact of a newborn should: phone the UKHSA Health Protection Team, follow its advice on antibiotic prophylaxis and vaccination for close contacts in priority groups, and inform the obstetric team.

EBV infectious mononucleosis post-acute fatigue is one of the most important patient communication challenges in primary care — a minority of patients develop prolonged fatigue after the acute illness (post-infectious fatigue), and a subset meet the criteria for myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS); persistent or severe fatigue warrants reassessment for alternative diagnoses. GPs should: (1) at the initial consultation, warn patients that a period of prolonged fatigue is possible after EBV; (2) avoid reassurance that they will 'be back to normal in 2 weeks' — this creates unrealistic expectations that damage the patient-GP relationship when prolonged fatigue develops; (3) repeat FBC (+ LFTs if they were abnormal) after recovery, commonly 4–6 weeks, to confirm resolution of acute haematological changes; (4) screen with PHQ-9 (depression is a common co-morbidity in prolonged post-EBV fatigue); (5) refer to CFS/ME specialist services if fatigue persists beyond 3 months with significant functional impairment.

The CLL vaccination strategy requires specific planning — patients with CLL are immunosuppressed (particularly those who have received treatment) and should receive: annual influenza vaccine (inactivated IM), pneumococcal vaccination (per Green Book chapter 25), COVID-19 boosters, Shingrix (recombinant zoster), and hepatitis B vaccine if not immune. However, vaccine responses may be suboptimal because CLL B-cells cannot produce adequate antibody responses to vaccine antigens. Specialist teams may measure antibody responses to vaccines (anti-pneumococcal IgG, anti-HBs) in CLL patients to confirm adequate protection. Patients who fail to respond to standard doses may benefit from higher-dose or repeat vaccination schedules. Avoid live vaccines (e.g. MMR, yellow fever, varicella/ZVL) unless specifically advised by haematology, and review the schedule against the current Green Book.

The CLL and skin cancer connection is clinically important and frequently underemphasised — CLL patients have an increased risk of skin cancer, particularly aggressive or recurrent keratinocyte cancers such as cutaneous squamous cell carcinoma (SCC), driven by: T-cell immune surveillance impairment (failing to eliminate UV-damaged keratinocytes), B-cell immune abnormalities, and treatment-related immunosuppression. GPs monitoring CLL patients in watchful waiting should incorporate: skin surveillance per local dermatology/haematology arrangements (including sun-exposed areas), advice about daily SPF50+ sunscreen and sun protection clothing, prompt biopsy of any non-healing, crusting, or growing skin lesion. Dermatology referral (suspected cancer pathway referral if suspicious) for any concerning skin lesion in a CLL patient. The management of SCC in immunosuppressed patients is more aggressive than in immunocompetent patients because of the significantly higher risk of metastasis.

Smear cells (Gumprecht shadows) represent fragile lymphocytes that rupture during preparation of the blood film. They may support CLL in the appropriate clinical and morphological context but are not diagnostic — they also occur in other lymphoproliferative and reactive conditions and never replace flow cytometry. A film reported as showing smear cells or suspicious lymphocytes without a prior diagnosis should prompt peripheral-blood flow cytometry through the local laboratory or haematology pathway; the report should state the clonal B-cell count (≥5 × 10⁹/L = CLL; below 5 × 10⁹/L without nodes, organomegaly or cytopenias = MBL) and a recommended follow-up plan. The finding of smear cells is a morphological cue to request haematology assessment — it is not a wait-and-watch result.