Reasoning GP · Algorithm · 1-page summary
Microcytosis (low MCV) in adults
For: adults with an MCV <80 fL, including the results inbox. Not for: children (paediatric guidance). Pregnancy goes to block 2 and the maternity pathway; anaemia with a normal or high MCV → anaemia algorithm.
Core rule: ferritin is the pivot. Low ferritin = iron deficiency: find the source — in a man or post-menopausal woman that means GI investigation, never iron alone. Normal ferritin with a low MCV is usually not iron deficiency: think thalassaemia trait or chronic disease, and do not give iron.
0Stop — emergency now — on seeing the result or the patient
- Collapse, haemodynamic instability or major active bleeding (haematemesis, melaena, heavy rectal bleeding) → 999
- Severe anaemia with chest pain, breathlessness at rest, syncope or heart failure, or other active significant GI bleeding → Same-day hospital
- Transfusion is a hospital decision on symptoms, haemodynamics and comorbidity, not Hb alone. Identify and control the bleeding source; iron is still needed afterwards.
1Triage by ferritin — read with CRP and TSAT
| Ferritin | Means → next |
|---|---|
| <15 µg/L | Absent stores: iron deficiency → treat and find the source |
| <30 µg/L | Generally supports iron deficiency → as above |
| Normal/high, inflammation | May be falsely normal: TSAT <20% supports iron deficiency (around 45 µg/L more sensitive) → mixed or chronic disease |
| Normal, no inflammation | Persistent microcytosis → haemoglobin analysis/HPLC (thalassaemia trait). No iron. |
2Screen — one row is enough
- IDA in a man of any age or a post-menopausal woman (even without GI symptoms) → Urgent GI investigation, usually OGD + colonoscopy. Offer FIT; ≥10 µg Hb/g → Suspected cancer pathway.
- New GI symptoms on iron → Early review: urgent OGD or colonoscopy
- Very low Hb, marked microcytosis and/or splenomegaly (thalassaemia major/intermedia) → Urgent haematology
- Other cytopenias or abnormal film; lead, alcohol or isoniazid (sideroblastic, MDS); iron overload after transfusions → haematology.
- Pregnancy: Hb <110 g/L (1st trimester), <105 (later); severe, late, unresponsive or intolerant → obstetrics/haematology. Both partners carriers + pregnant → Urgent genetics + obstetrics
3Assess — before prescribing iron
☐ FBC: MCV, MCH, RDW, film ☐ Ferritin; CRP/ESR if inflammation suspected; TSAT where ferritin may mislead; reticulocyte Hb if available ☐ Renal function, coeliac serology (tTG-IgA + total IgA), urine dipstick ☐ Family origin, periods, NSAIDs, diet
Not routine: serum iron alone, sTfR, zinc protoporphyrin, hepcidin, Mentzer index.
4Classify — only if the screen is negative
- Iron deficiency — low ferritin. Blood loss (GI; periods if premenopausal), malabsorption (coeliac, H. pylori, IBD), demand, intake → iron + treat the cause.
- Thalassaemia trait — persistent low MCV, normal ferritin, Hb normal or mildly low → HPLC: beta HbA2 usually ≥3.5% (iron deficiency lowers it); alpha often normal (DNA testing lab-led).
- Chronic disease — normal/high ferritin, raised CRP; usually borderline MCV → treat the condition; check TSAT for co-existing deficiency.
- Sideroblastic / MDS — iron-loaded pattern, other cytopenias → haematology.
- Uncertain — unexplained persistent microcytosis, normal ferritin → haematology.
5Treat today — only these
- Iron deficiency: ferrous sulphate 200 mg once daily (65 mg elemental); alternate days if not tolerated. Alongside GI investigation and FIT, never instead. Continue about 3 months after Hb normalises.
- IV iron — consider if oral fails, is not tolerated or absorbed, or rapid repletion is needed → local pathway.
- The cause: heavy periods (NG88); H. pylori test and treat; coeliac → gluten-free diet; stop NSAIDs if possible.
- Thalassaemia trait: no iron unless deficiency is confirmed. Doses: NICE/SPC, BNF, local formulary.
6Refer routinely — and follow up
- FBC at about 4 weeks (expect Hb up ~10 g/L by 2 weeks). No rise → check adherence, ongoing loss, diagnosis, malabsorption, inflammation, renal disease; then haematology.
- IDA persisting or recurring after good-quality negative endoscopy → gastroenterology (small bowel, renal tract).
- Carrier: document; partner testing (NHS screening programme) if pregnancy planned or ongoing; both carriers → specialist counselling.
7Never reassure by
- A good Hb response to iron in a man or post-menopausal woman.
- A FIT <10 µg Hb/g, or a FIT not returned, against strong clinical concern.
- A normal ferritin with inflammation, infection, liver disease or cancer.
- "Dietary" iron deficiency without investigation.
- One end of the gut only: oesophagitis on OGD does not clear the colon.
- A Mentzer index, to diagnose or exclude thalassaemia trait.
8Safety-net — say it, then document it
999 “Call 999 or go to A&E if you vomit blood, pass a lot of blood or black tarry stools, feel faint or collapse, or have chest pain or severe breathlessness.” Same day: “Contact us today for bloody stools, difficulty swallowing, worse tummy pain, weight loss or worsening breathlessness. Iron turns stools black; tarry, sticky, smelly stools are different.”
Close the loop: action FIT and endoscopy results and confirm the referral was received. If investigation is declined, document the cancer risk explained and the safety-net given.
✓Safe to manage in primary care only if — all five, documented
No block 0–2 row applies
Ferritin read with CRP (± TSAT) before any iron
Source sought: GI investigation + FIT if man or post-menopausal woman; coeliac, urine
Carrier status recorded; partner testing advised
FBC at ~4 weeks booked; safety-net given
Summary of the full Reasoning GP microcytosis algorithm, which holds the detail, iron-study patterns, thalassaemia management and evidence. Sources: NICE NG12 (updated April 2026), NG24, NG88, NG20; BSG 2021 iron deficiency anaemia; BSH 2021 laboratory diagnosis of iron deficiency; BSH haemoglobinopathies 2023 and the NHS Sickle Cell and Thalassaemia Screening Programme; BSH iron deficiency in pregnancy; MHRA; local formulary. Clinical reference — follow local acute-transfer, GI and haematology pathways and check doses: NICE/SPC, BNF.