Anaemia Panel

The CBC quantifies the three formed blood elements and derives erythrocyte morphological indices — MCV (mean corpuscular volume) characterising erythrocyte size; MCH (mean corpuscular haemoglobin) characterising per-cell haemoglobin content; MCHC (mean corpuscular haemoglobin concentration) characterising intracellular haemoglobin density; and RDW (red cell distribution width) quantifying the heterogeneity of erythrocyte sizes, which rises before MCV changes in evolving nutritional deficiencies and haemolytic states. These indices are computed by automated haematology analyser from primary cell counts and light scatter measurements. Haemoglobin is the most clinically used marker of anaemia severity — its sex-specific thresholds reflect the testosterone-driven erythropoietic difference between male and female haematopoiesis.

The CBC anchors the anaemia panel by establishing whether anaemia is present (haemoglobin below the sex-specific lower limit), quantifying its severity, and providing the morphological classification through MCV — microcytic anaemia (MCV <80 fL) directs investigation toward iron deficiency, thalassaemia, anaemia of chronic disease, and sideroblastic anaemia; normocytic anaemia (MCV 80–100 fL) toward haemolysis, anaemia of chronic disease, early mixed deficiency, and bone marrow failure; macrocytic anaemia (MCV >100 fL) toward vitamin B12 deficiency, folate deficiency, hypothyroidism, liver disease, and medication effect. RDW elevation — the earliest haematological change in iron, B12, and folate deficiency — precedes MCV change and provides additional discrimination power, with elevated RDW in a normocytic anaemia suggesting a mixed deficiency pattern.

Serum iron measures transferrin-bound iron in the circulation — representing only a small fraction of total body iron (approximately 0.1% of the total 3,000–5,000 mg body iron pool in adults). Iron is absorbed in the duodenum and proximal jejunum by enterocytes via DMT1 and transported across the basolateral membrane by ferroportin, regulated by hepcidin — the master iron regulatory hormone produced by the liver. Circulating iron is bound almost exclusively to transferrin (a 79 kDa glycoprotein), each molecule carrying up to two iron atoms. Serum iron is subject to pronounced diurnal variation, acute phase response suppression, and recent dietary iron intake effects — making it the least reliable individual iron marker and most useful in the context of transferrin saturation calculation.

Within the anaemia panel, serum iron contributes to the calculation of transferrin saturation — the ratio of serum iron to TIBC — which is the most diagnostically informative single iron panel derived value. Low serum iron with low TIBC and elevated ferritin indicates anaemia of chronic disease; low serum iron with high TIBC and low ferritin confirms iron deficiency. Serum iron in isolation is insufficient to characterise iron status due to its instability — it is always interpreted as part of the full iron panel rather than as a standalone marker.

Total Iron Binding Capacity is an indirect measure of transferrin concentration — it quantifies the maximum amount of iron that serum transferrin can bind. In iron deficiency, the liver upregulates transferrin synthesis to maximise iron capture from the depleted pool, raising TIBC above the reference range. In inflammatory states and chronic disease, transferrin is downregulated as a negative acute phase reactant, reducing TIBC. UIBC (unsaturated iron binding capacity) represents the unoccupied transferrin iron-binding sites — the spare binding capacity — and together with serum iron allows calculation of transferrin saturation: Transferrin Saturation (%) = (Serum Iron ÷ TIBC) × 100.

TIBC and UIBC are essential complements to serum iron and ferritin for distinguishing iron deficiency anaemia from anaemia of chronic disease — two frequently co-occurring conditions with opposite TIBC patterns. Elevated TIBC (>400 µg/dL) with low serum iron and low ferritin confirms iron deficiency. Reduced TIBC (<250 µg/dL) with low serum iron and elevated or normal ferritin confirms anaemia of chronic disease or inflammation. In haemochromatosis, both serum iron and transferrin saturation are elevated with normal or low TIBC. Transferrin saturation below 16% is the threshold for iron-restricted erythropoiesis regardless of haemoglobin level.

Transferrin saturation is a derived value — (Serum Iron ÷ TIBC) × 100 — expressing the fraction of transferrin iron-binding sites that are occupied. It is the most diagnostically informative single iron panel value, reflecting iron delivery to tissues relative to transport capacity. A transferrin saturation below 16% indicates iron-restricted erythropoiesis regardless of absolute haemoglobin or ferritin values; above 45–50% in a non-treated patient raises concern for iron overload and hereditary haemochromatosis (HFE mutation testing is indicated).

Transferrin saturation is the primary discriminator in iron panel interpretation. It is particularly critical in distinguishing iron deficiency from anaemia of chronic disease — both produce low serum iron, but transferrin saturation below 16% with high TIBC confirms iron deficiency while transferrin saturation 16–30% with low TIBC in an inflammatory context confirms anaemia of chronic disease. In chronic kidney disease patients on erythropoiesis-stimulating agents (ESA), transferrin saturation below 20% indicates functional iron deficiency requiring iron supplementation regardless of ferritin level. In haemochromatosis screening, transferrin saturation above 45% on fasting specimens warrants HFE gene testing.

Ferritin is the principal iron storage protein — a 24-subunit nanocage capable of sequestering up to 4,500 iron atoms. Serum ferritin represents a small, glycosylated fraction of cellular ferritin secreted into the bloodstream, with its concentration directly proportional to total body iron stores under stable conditions. Serum ferritin is also an acute phase reactant — rising substantially during systemic inflammation, infection, liver disease, and malignancy through IL-1β, IL-6, and TNF-α-driven upregulation of ferritin synthesis — independent of iron stores. This acute phase reactant behaviour is the primary interpretive limitation of serum ferritin and requires concurrent inflammatory marker assessment for accurate iron status characterisation in patients with chronic disease.

Ferritin is the most sensitive single marker of iron deficiency — it falls below normal before haemoglobin, MCV, serum iron, or transferrin saturation become abnormal, allowing detection of depleted iron stores in the pre-anaemic phase. A ferritin below 12 ng/mL confirms absent iron stores and requires no further confirmatory testing for iron deficiency. A ferritin below 30 ng/mL is highly specific for iron deficiency even in the presence of inflammatory disease. In functional medicine, a ferritin below 50–70 ng/mL — technically 'within the reference range' — is associated with symptomatic iron depletion including fatigue, hair loss, cold intolerance, restless legs, and impaired cognitive function, particularly in premenopausal women with ongoing menstrual blood loss.

Reticulocytes are immature, anucleate erythrocytes that have recently extruded their nucleus — characterised by the presence of residual ribosomal RNA detectable by supravital staining or automated fluorescence cytometry. They circulate in peripheral blood for approximately 1–2 days before losing their residual RNA and maturing into fully functional erythrocytes. The reticulocyte count reflects the rate of bone marrow erythropoietic output — rising when the marrow is stimulated (haemolysis, haemorrhage, effective erythropoiesis after deficiency correction) and falling when marrow production is impaired (nutritional deficiency, aplastic anaemia, myelosuppression). The Reticulocyte Production Index (RPI) corrects the reticulocyte percentage for both the degree of anaemia-driven reticulocyte release stimulus and the extended peripheral maturation time that occurs during severe anaemia — providing a bone marrow production capacity estimate independent of haematocrit.

The reticulocyte count is the single most informative add-on to a CBC in anaemia evaluation — it immediately classifies anaemia as hyperproliferative (elevated reticulocytes, RPI ≥2: haemolysis or haemorrhage — the marrow is responding) or hypoproliferative (low or normal reticulocytes, RPI <2: nutritional deficiency, bone marrow failure, or anaemia of chronic disease — the marrow is not responding adequately). This classification is the pivotal second diagnostic step following MCV-based morphological classification and directs the subsequent investigation pathway. A rising reticulocyte count following iron, B12, or folate repletion therapy is the earliest objective evidence of treatment response — preceding haemoglobin improvement by 5–10 days.

Vitamin B12 (cobalamin) is a water-soluble, cobalt-containing organometallic vitamin obtained exclusively from animal-source dietary protein — meat, fish, eggs, and dairy — that serves as an essential cofactor for two critical enzymatic reactions: methionine synthase (converting homocysteine to methionine, requiring methylcobalamin) and methylmalonyl-CoA mutase (converting methylmalonyl-CoA to succinyl-CoA in the mitochondria, requiring adenosylcobalamin). Dietary B12 is released by gastric acid and pepsin, bound to intrinsic factor (IF) secreted by gastric parietal cells, and absorbed via cubilin receptors in the terminal ileum. Serum B12 is bound primarily to haptocorrin (transcobalamin I, approximately 80%) and transcobalamin II (approximately 20%) — transcobalamin II-bound B12 is the metabolically active, cellular delivery fraction (holotranscobalamin, holoTC). B12 is stored primarily in the liver — total body stores of 2–5 mg support 2–4 years of dietary absence before clinical deficiency manifests.

Serum B12 is the standard first-line screening marker for cobalamin deficiency but has significant limitations at near-normal concentrations. B12 deficiency causes macrocytic megaloblastic anaemia through impaired DNA synthesis in rapidly dividing erythroid precursors; subacute combined degeneration of the spinal cord from myelin synthesis impairment; and hyperhomocysteinaemia increasing cardiovascular and thrombotic risk. In the anaemia panel, B12 is a mandatory component for investigating macrocytic anaemia — particularly when MCV is above 100 fL alongside hypersegmented neutrophils on differential. Functional B12 deficiency — with neurological or haematological manifestations despite a serum B12 within the reference range — is identified by elevated methylmalonic acid and/or homocysteine, which are more sensitive functional biomarkers warranting consideration when B12 is borderline (200–400 pg/mL) in a symptomatic patient.

Folate (vitamin B9) is a water-soluble B-vitamin found in leafy green vegetables, legumes, fortified cereals, and liver. Dietary folates are reduced and methylated in the intestinal mucosa before entering portal circulation as 5-methyltetrahydrofolate (5-MTHF) — the predominant circulating form. Folate serves as the methyl group donor in a series of one-carbon transfer reactions essential for DNA synthesis (thymidylate synthesis via dihydrofolate reductase), DNA methylation, and amino acid interconversion (including homocysteine remethylation to methionine in conjunction with vitamin B12). Serum folate is highly sensitive to recent dietary intake — a single folate-restricted meal can lower serum folate within 24–48 hours; RBC folate reflects the integrated folate status over the 90–120 day erythrocyte lifespan and is the more reliable marker of tissue folate stores.

Folate deficiency causes megaloblastic macrocytic anaemia morphologically identical to B12 deficiency — both impair thymidylate synthesis and produce the characteristic hypersegmented neutrophils and macro-ovalocytes on blood film. The critical clinical distinction is that folate supplementation can correct the haematological manifestations of B12 deficiency while leaving the neurological manifestations (subacute combined degeneration) to progress undetected — making simultaneous B12 and folate assessment mandatory before folate supplementation is initiated in any patient with macrocytic anaemia. Folate deficiency is particularly relevant in pregnancy (neural tube defect risk), alcohol use disorder (impaired absorption and intake), methotrexate therapy (DHFR inhibition), and patients on anticonvulsants. In the anaemia panel, folate assessment closes the macrocytic anaemia workup alongside B12.