Presentation: Neutropenia/Lymphocytopenia

Total white blood cell count alone can be misleading — always look at the differential. Neutrophils are the most numerous leukocytes and the front-line defence against bacteria and fungi.

Normal neutrophil range roughly 2.0–8.0×10⁹/L (about 40–60% of the WBC, or 1500–8000 cells/µL). The standard lower limit of a normal circulating neutrophil count is about 1.5×10⁹/L, and recurrent infections become more common once the count falls below this.

Severity bands: mild neutropenia 1.0–2.0×10⁹/L, moderate 0.5–1.0×10⁹/L, severe < 0.5×10⁹/L; agranulocytosis is < 0.2×10⁹/L with absent marrow neutrophil precursors. Neutropenia is called acute if under ~3 months and chronic beyond 3–6 months.

Significant levels: < 1.0×10⁹/L carries a high risk of infection, rising steeply below 0.5×10⁹/L.

Only a small fraction of the body’s neutrophils circulate — the rest are a marrow and marginated reserve — so if marrow reserves are adequate the infection risk from an isolated low peripheral count is lower than the number alone suggests.

Most common causes

Grouped by mechanism:

• Reduced marrow production (primary): congenital/genetic neutropenias (ELANE, HAX1 and others), cyclic neutropenia, myelodysplasia, marrow infiltration by leukaemia or high-grade lymphoma, and nutritional deficiency of folate, vitamin B12 or copper.
• Peripheral loss or destruction (secondary, normal marrow reserves): viral infection (often overt or occult, including viral hepatitis B and C), bacterial sepsis (increased consumption), autoimmune/idiopathic neutropenia, hypersplenism, and drugs.
• Autoimmune: more common in women aged 25–40; associated with SLE, Sjögren syndrome, rheumatoid arthritis (Felty syndrome = rheumatoid arthritis + splenomegaly + neutropenia), thyroid disease and inflammatory bowel disease. A positive ANA is suggestive but not diagnostic.
• Idiopathic: no cause is found despite full workup in a substantial minority — around half in one series.
• Benign ethnic (Duffy-null) neutropenia: a constitutionally lower count, common in people of African ancestry, with no increase in infection risk and a good prognosis.
• Drugs: chemotherapy and immunosuppressants; antithyroid drugs (carbimazole, propylthiouracil); antibiotics (beta-lactams, sulfamethoxazole–trimethoprim, vancomycin); sulfasalazine; antirheumatics; antipsychotics (notably clozapine); anticonvulsants (lamotrigine, carbamazepine); hydroxychloroquine; procainamide.

Red flags

Person particularly unwell; severity of neutropenia; rate of change; lymphadenopathy or hepatosplenomegaly; mouth ulcers or other mucosal infection; constitutional symptoms (fever, night sweats, weight loss, bone pain).

Approach

Neutrophils < 1: bacterial infection risk rises as neutrophils drop below 1, most markedly below 0.5. Unwell and/or febrile patients with a count < 1 need ED assessment (febrile neutropenia is a medical emergency). If well, advise the patient to seek urgent care if they become unwell, and arrange follow-up bloods in 48 hours. Discuss with haematology if neutropenia persists or progresses.

Neutrophils 1–2: if an isolated abnormality, repeat bloods in 1–2 weeks. If neutropenia persists beyond 6 weeks, further investigation is needed — discuss with or refer to haematology.

Drug-induced neutropenia: stop the likely offending drug where possible, since counts usually recover once it is stopped. Clozapine has its own mandatory haematologic monitoring protocol (it causes severe neutropenia in roughly 4% and agranulocytosis in about 1% of patients). If a drug cause is suspected and the neutropenia is moderate–severe, discuss with haematology.

Assessment

• History: frequency/severity of infections, mouth ulcers, recent viral illness, drug/toxin exposure, malabsorption symptoms, symptoms of reduced immunity, family history of neutropenia, constitutional symptoms.
• Medication history (including over-the-counter and recent courses).
• Examination: mouth ulcers, fever, signs of infection, jaundice, lymphadenopathy, hepatomegaly, splenomegaly, signs of autoimmune/connective tissue disease.
• Investigations: is the rest of the FBC normal? A blood film to look for blasts, dysplasia or left shift. In persistent moderate neutropenia without an infective/drug cause, consider ANA, B12, folate, HIV, liver enzymes, HBV, rheumatoid factor; add immunoglobulins and lymphocyte subsets if an immune deficiency is suspected. Bone marrow examination is reserved for an unclear cause or suspected primary marrow disorder.
• Treatment is directed at the cause. G-CSF is the mainstay when the marrow response is defective or infections are recurrent; immunosuppression is used for some autoimmune/idiopathic cases; stem cell transplant is reserved for G-CSF-refractory congenital neutropenia or malignant transformation. Congenital neutropenias (ELANE, HAX1, WAS, GATA2, SBDS and others) carry a raised risk of myelodysplasia and acute myeloid leukaemia and need periodic marrow surveillance.

Lymphocytes

Normal lymphocyte range roughly 1.0–4.0×10⁹/L (about 20–40% of the WBC). Lymphocytopenia is a total lymphocyte count < 1.0×10⁹/L in adults (< 3.0×10⁹/L in children under 2).

Because lymphocytes are a minority of the WBC, a low count is easily missed if only a total WBC is checked. About 75% of blood lymphocytes are T cells, so lymphocytopenia usually reflects a fall in T cells, particularly CD4+ T cells.

A mild isolated lymphocytopenia is often transient and not clinically significant — common with viral and bacterial infection, sepsis, corticosteroids and stress responses.

Persistent or marked lymphocytopenia warrants a cause: protein-energy undernutrition (the commonest cause worldwide), HIV (routine, from CD4+ T-cell destruction) and other viral infections including COVID-19 and EBV, autoimmune disease (SLE, rheumatoid arthritis, myasthenia gravis, sarcoidosis), lymphoma and other malignancy, protein-losing enteropathy, and iatrogenic causes (cytotoxic chemotherapy, radiotherapy, glucocorticoids, rituximab and other lymphocyte antibodies). Inherited lymphocytopenia is rare — severe combined immunodeficiency and Wiskott–Aldrich syndrome are the main examples.

Persistent lymphocytopenia carries an increased risk of opportunistic infection and of autoimmune and malignant disorders; investigate with immunodeficiency testing and lymphocyte subset analysis once any acute illness has resolved, and treat the underlying disorder.