Myelodysplastic syndrome (MDS) is a diagnosis that requires explicit consideration whenever macrocytosis with unexplained cytopenia (anaemia, neutropenia, or thrombocytopenia) is present in a patient over 60 — it is a clonal bone marrow disorder in which haematopoietic stem cells produce morphologically abnormal cells that fail to mature normally, leading to ineffective erythropoiesis. The macrocytes in MDS are characteristically oval in shape (oval macrocytes) on blood film, unlike the round macrocytes of B12/folate deficiency. MDS affects approximately 5 per 100,000 adults per year, with median diagnosis age approximately 70. The clinical presentation: fatigue from anaemia (the most common symptom), recurrent infections (neutropenia), and bruising/bleeding (thrombocytopenia). The WHO MDS classification (2022) has multiple subtypes determined by cytogenetics and bone marrow morphology. GPs should refer to haematology any patient with unexplained macrocytic anaemia (particularly with cytopenia, oval macrocytes on film, or age over 60) when B12, folate, thyroid, alcohol, and drug causes have been excluded.

Alcohol is the most common cause of isolated macrocytosis in UK primary care — approximately 60-70% of heavy drinkers develop macrocytosis, and in a patient presenting with incidental MCV elevation without anaemia, alcohol should be the first consideration. The mechanism of alcohol-induced macrocytosis is multifactorial: direct toxic effect of acetaldehyde on developing red cell precursors (acetaldehyde is incorporated into red cell membrane lipids, altering membrane fluidity and shape), secondary folate deficiency (poor diet in heavy drinkers + alcohol-induced folate malabsorption), and direct toxic effect on the bone marrow. The key clinical point: alcohol macrocytosis causes large round macrocytes (without hypersegmented neutrophils — unlike megaloblastic macrocytosis), and MCV typically normalises within 2-3 months of alcohol cessation. If the MCV does not normalise after 3 months of confirmed alcohol abstinence, other causes must be sought. The AUDIT-C at every macrocytosis consultation is essential — many patients under-declare alcohol use.

The blood film is the most information-rich investigation in the macrocytosis workup — the morphological findings narrow the differential diagnosis dramatically and guide subsequent targeted investigations. The hypersegmented neutrophil is the single most specific finding for megaloblastic haematopoiesis: any neutrophil with 6 or more lobes, or more than 5% of neutrophils with 5 lobes, constitutes hypersegmentation and is virtually diagnostic of B12 or folate deficiency (or methotrexate toxicity). The oval macrocyte (elliptocyte with one blunt and one pointed end) is the characteristic red cell shape in megaloblastic anaemia — it differs from the round macrocytes of alcohol and liver disease. GPs who request a blood film alongside FBC are getting information that automated cell analysers cannot provide — and the film result should prompt specific targeted investigation rather than a generic 'macrocytosis workup' approach.

The trimethoprim + methotrexate interaction is a potentially fatal combination that every GP must know — trimethoprim (even at standard doses) inhibits dihydrofolate reductase, the same enzyme that methotrexate targets. Co-prescribing trimethoprim to a patient on methotrexate (for rheumatoid arthritis, psoriasis, or cancer) can cause acute severe methotrexate toxicity with: sudden severe pancytopenia, mucositis, severe diarrhoea, and sepsis. Case series show deaths from this interaction. The mechanism: trimethoprim blocks the folate pathway that allows cells to recover from methotrexate toxicity. The safety rule: NEVER prescribe trimethoprim to a patient on methotrexate. Use nitrofurantoin (for UTI) or consult the rheumatologist/oncologist for alternative antibiotic if trimethoprim is truly necessary. This interaction is listed in the BNF under 'interactions' for both trimethoprim and methotrexate, and should be checked at every prescription using the clinical system's drug interaction alert.

The pernicious anaemia gastroscopy referral for macrocytic anaemia is an important quality standard — whenever a patient is diagnosed with pernicious anaemia as the cause of their macrocytic anaemia (positive anti-intrinsic factor antibody or positive anti-parietal cell antibody + B12 deficiency + appropriate clinical context), a single gastroscopy should be arranged. This is because PA causes atrophic gastritis, which increases the risk of both gastric cancer (approximately 1-3% lifetime risk — 2-3x higher than general population) and type I gastric carcinoid tumours (neuroendocrine tumours — relatively indolent but require monitoring). GPs who diagnose PA and initiate B12 injections without arranging gastroscopy are missing an important quality step. The referral should be to gastroenterology for routine OGD; the macrocytic anaemia itself has been treated, but the structural gastric pathology needs assessment.

Folic acid dose selection is important — the standard prescribed dose for treating folate deficiency or B12 deficiency haematological complications is 5 mg OD (high-dose) for 4 months. This is different from: (1) the pregnancy prevention dose (400 mcg/day — low-dose for neural tube defect prevention in normal-risk pregnancy); (2) the high-risk pregnancy dose (5 mg/day — for women with prior NTD, on antiepileptics, BMI >30, or diabetes); and (3) the co-prescription with methotrexate (5 mg weekly, on a different day to methotrexate — not daily). Prescribing the wrong dose is a common error: prescribing 400 mcg for treatment of established deficiency (insufficient); prescribing 5 mg daily indefinitely for a woman taking methotrexate (exceeds the weekly requirement and may reduce methotrexate efficacy). GPs should also recall that folic acid must never be prescribed alone in a B12-deficient patient without simultaneously treating the B12 deficiency.

Azacitidine (azacytidine) is the most important disease-modifying treatment for high-risk MDS that GPs should know about — it is a DNA methyltransferase inhibitor (hypomethylating agent) that, in the pivotal AZA-001 trial, doubled median overall survival from approximately 15 months to approximately 25 months compared to conventional care in high-risk MDS. It is administered as SC injections on days 1-7 of each 28-day cycle, typically for at least 6 cycles (response may not be apparent for 4-6 cycles). GPs may encounter patients on azacitidine: key monitoring points are: FBC 2-weekly during treatment (myelosuppression is expected — nadir at day 14-21), infection vigilance (neutropenic fever = 999 regardless of ANC), and renal function monitoring. Azacitidine does not cure MDS but delays progression to AML and improves quality of life and survival in eligible patients.

The folic acid pre-conception advice is one of the most important primary prevention interventions in UK primary care — neural tube defects (NTDs: spina bifida, anencephaly) affect approximately 1,000 pregnancies per year in the UK, and folic acid supplementation (400 mcg/day) started at least 1 month before conception and continued through the first trimester reduces NTD risk by approximately 70%. Despite decades of public health messaging, only approximately 30% of women take folic acid pre-conception because many pregnancies are unplanned. Mandatory fortification of flour with folic acid was implemented in the UK in 2024 (following decades of campaigning by RCOG and others) — this will reduce NTD incidence by an estimated 20-30% at population level. GPs should still advise folic acid supplementation to all women planning pregnancy, noting that food fortification does not substitute for supplementation in the pre-conception period.

The absence of a reticulocyte response 7-10 days after starting B12 (or folate) treatment is an important clinical signal — if the reticulocyte count does not rise above normal within 5-10 days of IM hydroxocobalamin injection in a patient with megaloblastic anaemia, the diagnosis must be reconsidered. Possible explanations: concurrent iron deficiency (iron is required for new red cell production — supplement simultaneously), concurrent infection or inflammation suppressing the reticulocyte response, incorrect diagnosis (MDS rather than megaloblastic anaemia — MDS may present with macrocytosis and fail to respond to B12/folate), co-existing coeliac disease with malabsorption, or very severe bone marrow failure requiring longer recovery. An absent or minimal reticulocyte response at day 7-10 should prompt blood film review and haematology referral.