RCGP SCA Algorithm — UK Primary Care
Iron deficiency is always a finding, never a diagnosis. Finding the cause is the priority — missing GI malignancy is the most critical pitfall.
Colorectal cancer is found in ~5% of patients presenting with IDA in primary care — making it the most important diagnosis to exclude. Iron deficiency in any post-menopausal woman or any man is a red flag by definition unless there is clear dietary cause (vegan diet, malabsorption). NICE NG12 mandates urgent 2WW referral for IDA in all patients aged ≥60, and for any adult with IDA + GI symptoms. The key clinical error is treating the iron deficiency without investigating the cause — this risks delayed cancer diagnosis.
Iron deficiency exists in 3 stages: depleted stores (low ferritin, normal Hb), iron-deficient erythropoiesis (low ferritin + low MCV), and iron deficiency anaemia (all low + low Hb).
Ferritin <15 µg/L is essentially diagnostic of iron deficiency (specificity ~99%), but levels of 15–100 can be misleading. Inflammatory states (infection, cancer, autoimmune disease) raise ferritin as an acute phase protein — a patient with active inflammatory bowel disease and cancer-related IDA may have a "normal" ferritin of 50. The CRP correction is essential: in any patient where iron deficiency is clinically suspected, check CRP alongside ferritin. Isolated iron deficiency without anaemia is clinically significant — symptoms (fatigue, reduced exercise tolerance, restless legs) occur before Hb drops.
Coeliac disease is the most commonly missed cause of IDA — it presents with iron deficiency in 50% of cases with no GI symptoms. Every patient with IDA of unclear cause should have TTG-IgA checked before GI endoscopy (the duodenal biopsy at endoscopy may be the only test that confirms coeliac if TTG is falsely negative due to IgA deficiency). H. pylori eradication alone resolves IDA in ~70% of cases where it is the cause. In women of reproductive age, menorrhagia accounts for 75% of IDA — but this does not mean GI pathology should be excluded, especially ≥45 years.
Thalassaemia trait is very common in South Asian, Mediterranean, and African populations (carrier frequency up to 5%) and produces microcytic anaemia that is completely benign and requires no iron treatment. Incorrectly treating thalassaemia trait with iron causes iron overload. The Mentzer index (MCV ÷ RBC count) helps discriminate: <13 suggests thalassaemia, >13 suggests IDA. Mixed B12 and iron deficiency is particularly seen in coeliacs and post-gastrectomy patients — always check B12/folate in unexplained normocytic or macrocytic picture.
TTG-IgA should be checked in all patients with IDA of unclear cause before GI endoscopy — if positive, the gastroenterologist can combine diagnostic gastroscopy with duodenal biopsy in one procedure. H. pylori stool antigen test is the most practical primary care test (avoid urea breath test if on PPIs — falsely negative). A Hb response to oral iron of ≥10 g/L at 4 weeks confirms the diagnosis retrospectively. Non-response at 4 weeks requires reassessment — most commonly due to poor adherence (side effects), malabsorption (coeliac, post-surgery), or ongoing occult bleeding.
The 2WW cancer referral rules for IDA are explicit in NICE NG12 (2015, updated 2023): IDA in men of any age or post-menopausal women requires urgent GI investigation. This reflects the ~5% colorectal cancer detection rate in this group. A common mistake is delaying endoscopy because the patient is on anticoagulants or iron treatment — neither prevents endoscopy, and both are insufficient reasons to defer investigation. Coeliac disease identified through TTG-IgA screening requires formal duodenal biopsy confirmation — a positive serology alone is not sufficient to start a lifelong gluten-free diet.
The BSH 2021 guidelines changed practice: alternate-day iron dosing (e.g. Monday/Wednesday/Friday) produces equivalent iron absorption to daily dosing via hepcidin regulation (hepcidin rises after each dose and suppresses next-day absorption), but causes significantly fewer GI side effects — improving adherence. This is now recommended practice. IV iron (Ferinject/Monofer) is first-line in IBD and post-bariatric surgery — oral iron worsens inflammation in IBD and is poorly absorbed after Roux-en-Y bypass. IV iron produces faster Hb response (4 weeks vs 8–12 weeks oral) and is safe in primary care with a brief observation period.
Dietary advice is particularly relevant for vegetarians, vegans, and adolescent girls — groups where dietary iron is insufficient relative to requirements. The vitamin C co-ingestion tip is evidence-based and practical: a glass of orange juice with an iron-rich meal or tablet can triple iron absorption. Tea and coffee are the most commonly overlooked absorption inhibitors — many patients with "refractory" IDA are drinking tea with every meal. The Mirena IUS is the most effective long-term intervention for menorrhagia-related IDA — a single device prevents 5–10 years of excessive blood loss.
The most important safety signal is failure to respond to oral iron at 4 weeks — this should always trigger reassessment, not dose escalation. Common causes of non-response: poor adherence (GI side effects — switch to alternate-day dosing or IV), coeliac (undiagnosed — check TTG-IgA), ongoing blood loss (GI cancer — arrange endoscopy), H. pylori (suppresses absorption). The 3-month ferritin check after Hb normalisation is critical — stopping iron when Hb normalises but ferritin is still low leads to early recurrence. H. pylori eradication should always be confirmed with test-of-cure at 4 weeks post-antibiotics (not 2 weeks — PPIs can cause false negative).