Microcytosis (MCV below 80 fl) combined with an iron deficiency pattern in a man or post-menopausal woman is a red flag for gastrointestinal malignancy that must not be attributed to dietary causes without investigation — the BSG guidelines are explicit: iron deficiency anaemia in a post-menopausal woman or a man of any age mandates bidirectional GI investigation (OGD + colonoscopy) regardless of whether there are GI symptoms (FIT, offered per NICE NG12, guides colorectal cancer referral but does not replace bidirectional endoscopy). A significant minority of IDA in these groups has an underlying GI malignancy (colorectal, gastric, or oesophageal cancer). The investigation must be bidirectional because some patients have lesions in both the upper and lower GI tract. Coeliac disease should also be excluded in every patient with IDA (anti-tTG IgA + total IgA). The GP who prescribes iron without investigating the cause in a post-menopausal woman or man has made a clinical error, regardless of whether there are GI symptoms.

The Mentzer index (MCV ÷ RBC count) is a screening heuristic with limited diagnostic performance — it cannot reliably distinguish iron deficiency from thalassaemia trait in an individual patient and should not be used alone to diagnose or exclude thalassaemia, or to trigger DNA testing. Persistent microcytosis with normal ferritin and no evidence of inflammation should prompt haemoglobinopathy investigation through the local laboratory, interpreting red-cell indices, MCH, Hb analysis/HPLC, HbA2, HbF and family origin together. The clinical importance: giving iron to a patient with thalassaemia trait (who has normal iron stores) is ineffective and may cause unnecessary investigation. Conversely, missing IDA in a patient thought to have thalassaemia trait can cause ongoing investigation for a 'genetic condition' while GI blood loss goes untreated.

The iron studies pattern is one of the most useful and underutilised diagnostic discriminators in haematology — the combination of serum ferritin, serum iron, TIBC, and transferrin saturation helps distinguish iron deficiency anaemia from anaemia of chronic disease, although no single test should be interpreted in isolation and serum iron is highly variable. The key distinguishing feature: in IDA, the body depletes iron stores (low ferritin) and upregulates iron absorption by producing more transferrin (high TIBC). In ACD, inflammation-driven hepcidin blocks iron release from macrophage stores — iron is actually trapped in the reticuloendothelial system (ferritin is elevated as an acute phase protein; TIBC is low or normal because the liver reduces transferrin synthesis in response to IL-6). The transferrin saturation (serum iron ÷ TIBC × 100%) is below 20% in both conditions — it does not discriminate. The clinical trap: in a patient with both IDA and ACD (common in IBD, RA with GI blood loss from NSAID use, or malignancy with nutritional iron deficiency), the ferritin may appear normal because the ACD component elevates it while the IDA component depletes it — interpret ferritin with transferrin saturation, CRP and reticulocyte haemoglobin where available (a ferritin threshold around 45 mcg/L may improve sensitivity); sTfR, zinc protoporphyrin and hepcidin are not routinely recommended (BSH).

Partner thalassaemia testing after a carrier is identified in primary care is a critical step that is frequently not completed — the clinical consequence of missed partner testing is a couple where both partners are thalassaemia trait carriers who conceive without knowing their risk, and present in pregnancy at 20+ weeks with a thalassaemia major fetus on anomaly scan. This is avoidable: if thalassaemia trait is identified in a patient of reproductive age, their partner must be tested before the next pregnancy, and ideally before conception. The GP's responsibility: document thalassaemia trait clearly in the clinical record, provide patient education, and advise partner testing. If the patient is pregnant at the time of diagnosis, the antenatal team will coordinate partner testing — but if identified outside pregnancy, the GP must take proactive action. The UK Thalassaemia Society (ukts.org) and the Sickle Cell Society (sicklecellsociety.org) provide patient information resources.

The bidirectional GI investigation requirement for IDA — OGD plus colonoscopy — must both be performed, not performed sequentially only if the first is normal. The BSG IDA guideline states clearly that both investigations should be arranged simultaneously in men and post-menopausal women with IDA, because some patients have relevant pathology in both the upper and lower GI tract. Common clinical errors: (1) arranging OGD only (finding oesophagitis and attributing IDA to it, missing a concurrent right-sided colorectal cancer); (2) arranging colonoscopy only (finding left-sided polyps but missing gastric cancer); (3) arranging FIT alone without endoscopy — FIT is a screening tool, not a diagnostic tool; a negative FIT does not exclude upper GI pathology and does not eliminate the need for OGD in a man with IDA. The GP referral should request both OGD and colonoscopy simultaneously, with a cover letter explaining: IDA confirmed (ferritin level + Hb), sex and menopausal status, GI symptoms if any, family history of CRC or gastric cancer.

IV ferric carboxymaltose (Ferinject) management in primary care is an area where UK pathways are actively expanding — many ICBs now have pathways for GP-initiated IV iron infusion in day units without specialist referral for specific indications (confirmed IDA with failed/intolerable oral iron, IBD, pre-operative anaemia, obstetric anaemia). GPs who identify patients meeting these criteria should: (1) check local ICB formulary for the IV iron pathway; (2) arrange pre-infusion blood tests (FBC, ferritin, CRP, renal function); (3) refer to the appropriate day unit with a completed referral form; (4) arrange post-infusion monitoring (FBC + ferritin at 4-6 weeks; serum phosphate selectively according to repeated dosing, risk factors and symptoms, per MHRA). IV iron in pregnancy is arranged under obstetric/haematology supervision, not routinely initiated in general practice. The GIRFT (Getting It Right First Time) programme has highlighted that IV iron is significantly underutilised in UK primary care, resulting in avoidable hospital admissions for severe IDA and unnecessary delays in surgical procedures.

Iron supplementation must not be given to patients with thalassaemia trait without confirming concurrent iron deficiency — this is a common and important prescribing error in primary care. Thalassaemia trait causes microcytosis and a mildly low MCV that looks superficially identical to early IDA. However, in thalassaemia trait, the iron stores are normal (ferritin is normal or elevated), and the microcytosis is caused by reduced haemoglobin chain synthesis rather than iron deficiency. Prescribing iron to a thalassaemia trait patient who does not also have IDA: (1) is ineffective (MCV and Hb will not improve); (2) exposes the patient to unnecessary GI side effects; and (3) if continued long-term, may cause iron overload (particularly in alpha-thalassaemia carriers who may have slightly elevated ferritin even at baseline). The safe practice: always check ferritin before prescribing iron for microcytosis. If ferritin is normal and microcytosis persists without inflammation: arrange haemoglobin analysis/HPLC before starting iron (do not rely on the Mentzer index).

The duration of iron supplementation after Hb normalisation is one of the most important patient education points that prevents IDA relapse — most patients stop taking iron when their symptoms resolve (usually 4-8 weeks into treatment). However, at the point when the Hb normalises, the iron stores (ferritin) are still depleted. The bone marrow's priority is to restore the Hb to normal first, using all available iron for haemoglobin synthesis — only after this is the remaining iron stored. Stopping iron when Hb normalises leaves a patient with replete Hb but near-empty stores (ferritin often still below 10-15 mcg/L), meaning the next episode of increased demand (heavy period, minor illness, dietary restriction) will cause IDA to return rapidly. The message to patients: 'We will check your blood test in 3 months — you need to keep taking the iron tablets until then, even when you feel completely better. This is because it takes your body longer to refill its iron stores than it takes to fix the anaemia.'

The clinical governance requirement for closed-loop referral management applies critically in the IDA context — IDA is one of the most common diagnoses associated with missed cancer diagnoses in UK medicolegal claims. The typical claim pattern: GP identifies IDA in a 62-year-old man → prescribes iron without arranging GI investigation → patient's Hb improves on iron → GP assumes problem resolved → patient returns 8 months later with symptomatic stage IV colorectal cancer. Prevention: every diagnosis of IDA in a man or post-menopausal woman must be followed by: (1) arrangement of bidirectional GI investigation (OGD + colonoscopy); (2) documentation that the referral was sent; (3) a system to confirm the referral was received and acted upon; and (4) review of the results when available. If a patient declines investigation, this must be documented: 'Patient fully informed of investigation requirement and cancer risk. Patient declines investigation — reasons documented. Patient safety-netted to attend urgently if symptoms develop.'