Anemia Evaluation and Microcytic and Macrocytic Anemias

Establishing the anemia pattern

The laboratory uses hemoglobin as the main measure of anemia, with hematocrit providing supporting information because it also varies with cell size and plasma volume. In the first review, place the complete blood count (CBC) in the patient's age and physiologic setting, compare current results with prior values, and look for internal inconsistencies or analyzer flags. The red-cell indices are mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC). Red-cell distribution width (RDW) describes variation in cell volume.

For adult pattern recognition, an MCV below approximately 80 fL is microcytic, 80 to 100 fL is normocytic, and above 100 fL is macrocytic. These are teaching intervals. Interpret patient results with the performing laboratory's validated interval. A normal MCV can conceal two populations, such as microcytes and macrocytes whose average falls within range. An increased RDW or a broad, bimodal RBC histogram can reveal that mixture. With a low MCV, increased RDW commonly favors iron deficiency over the usual thalassemia-trait or inflammation pattern. The overlap is large, so an increased RDW does not establish a diagnosis.^{22 Keohane EM, et al. Rodak's Hematology. 2025.}

The absolute reticulocyte count measures current marrow output more directly than the reticulocyte percentage. In anemia, a percentage can appear high because mature RBCs are fewer. A corrected reticulocyte percentage and reticulocyte production index (RPI) can adjust for anemia and early release of larger reticulocytes. The normal hematocrit denominator, maturation factor, and RPI decision limits are conventional and vary among references. An RPI below about 2 supports an inadequate production response. A value above about 3 supports strong regeneration when the marrow has had time to respond.^{22 Keohane EM, et al. Rodak's Hematology. 2025.}

Suppose a patient has hemoglobin 8.2 g/dL, hematocrit 25%, RBC count 4.0 × 10⁶/µL, MCV 62.5 fL, and reticulocytes 2.2%. Using a 45% normal hematocrit and a 2-day maturation factor:

Absolute reticulocyte count = RBC count × reticulocyte fraction = 4.0 × 10⁶/µL × 0.022 = 88 × 10³/µL

Corrected reticulocyte count = 2.2% × (25% ÷ 45%) = 1.22%

RPI = corrected reticulocyte count ÷ maturation factor = 1.22 ÷ 2 = 0.61

Although the absolute count may fall within a general adult interval, the RPI shows an inadequate response for this degree of anemia. Together, the low MCV and the inadequate marrow response describe a hypoproliferative microcytic pattern. Iron studies, markers of inflammation, and hemoglobin analysis are the next tests to narrow the cause.

The immature reticulocyte fraction can rise early when marrow releases less mature reticulocytes. Reticulocyte hemoglobin reflects iron available to the newest cells. Both depend on the analyzer, so interpret them with method-specific intervals.^{33 Obstfeld AE, et al. Report of the International Council for Standardization in… 2024.}

Review the film with a specific question from the CBC in mind. Relevant findings include:

Use your laboratory's procedure for morphology terms and grading. The College of American Pathologists glossary and International Council for Standardization in Haematology recommendations provide common terminology.^{4,54 College of American Pathologists. 2026 Hematology, Clinical Microscopy, and Body Fluids Glossary. 2026.5 Palmer L, et al. ICSH recommendations for the standardization of nomenclature and… 2015.}

Production, destruction, and loss

An inadequate reticulocyte response suggests insufficient or ineffective erythropoiesis. Causes include unavailable iron, impaired DNA or globin synthesis, reduced erythropoietin signaling, marrow failure, or marrow replacement. An increased response suggests blood loss or shortened RBC survival once enough time has passed for marrow output to rise. Bilirubin, lactate dehydrogenase, haptoglobin, urinalysis, and the direct antiglobulin test help separate hemorrhage from hemolysis. The Blood Banking lessons cover immune-hemolysis testing in detail.^{22 Keohane EM, et al. Rodak's Hematology. 2025.}

The hematopoiesis and erythrocyte physiology lesson explains erythropoietin signaling, reticulocyte release, red-cell clearance, and iron recycling.

Acute hemorrhage

Right after an acute bleed, the hemoglobin and hematocrit can remain near baseline, because hemorrhage removes plasma and red cells together. Interstitial fluid movement and intravenous fluids often dilute the remaining blood over approximately 24 to 72 hours, and the measured hemoglobin and hematocrit fall. The timing varies with bleeding, fluid replacement, and physiologic compensation. The early cells are usually normocytic and normochromic.

A catecholamine stress response can produce an early neutrophilic leukocytosis through demargination. A marrow left shift and reactive thrombocytosis are later and variable. If erythropoiesis is intact, polychromasia and an absolute reticulocytosis usually become evident after about 3 to 5 days and peak around 7 to 10 days. Nucleated RBCs can appear after severe loss. Iron studies are initially unchanged after a single acute bleed. Repeated or chronic loss can deplete iron stores.^{22 Keohane EM, et al. Rodak's Hematology. 2025.}

Uncomplicated hemorrhage lacks the biochemical pattern of hemolysis. Concurrent disease can change bilirubin, lactate dehydrogenase, haptoglobin, and direct antiglobulin test results, so interpret those results together.

Microcytic anemia patterns

Microcytosis reflects inadequate hemoglobin assembly. The defect may involve available iron, heme synthesis, or globin-chain production. Several processes can coexist.

Iron deficiency

Iron deficiency progresses from reduced stores to restricted iron delivery and then iron-deficient erythropoiesis. The sequence can overlap, especially when inflammation restricts iron release while storage iron remains present.

• Storage depletion: ferritin falls as macrophage and hepatocyte iron is used.
• Iron-restricted erythropoiesis: transferrin saturation and reticulocyte hemoglobin fall as less iron reaches developing cells.
• Iron-deficiency anemia: hemoglobin falls; microcytosis, hypochromia, anisocytosis, and an inadequate reticulocyte response become more apparent.

A low ferritin strongly supports depleted stores. The World Health Organization uses less than 15 µg/L to indicate deficiency in otherwise healthy adults. In adults with infection or inflammation, WHO conditionally permits a value below 70 µg/L to indicate iron deficiency. The certainty of evidence is low. Clinical laboratories and specialty guidelines may use different limits for the population and setting. A normal or increased ferritin during inflammation can coexist with iron deficiency, so C-reactive protein or another inflammation marker, transferrin saturation, CBC findings, and the clinical source of loss remain part of the interpretation.^{6,76 World Health Organization. WHO Guideline on Use of Ferritin Concentrations to Assess Iron… 2020.7 Fletcher A, et al. Guideline for the laboratory diagnosis of iron deficiency in… 2022.}

Common causes include chronic blood loss, increased need during growth or pregnancy, reduced intake, and impaired absorption. Ongoing hemoglobinuria and repeated phlebotomy or blood donation can also remove iron. The laboratory findings establish iron deficiency. Finding its source requires clinical investigation.

Transferrin saturation is calculated from serum iron and total iron-binding capacity. The core chemistry calculations lesson gives the formula and unit checks.

Anemia of inflammation

Inflammation increases hepatic hepcidin. Hepcidin binds ferroportin and reduces iron export from intestinal cells and iron-recycling macrophages. Plasma iron falls while storage iron remains inside cells. Cytokines also reduce erythropoietin production or response, suppress erythroid progenitors, and can shorten RBC survival. The result is iron-restricted erythropoiesis with an inadequate reticulocyte response.^{88 Weiss G, et al. Anemia of inflammation. 2019.}

The usual profile combines low serum iron, low or normal transferrin or TIBC, transferrin saturation that is low or sometimes within the local interval, and normal or increased ferritin. The anemia is often mild to moderate and normocytic, with microcytosis appearing in some cases. Coexisting true iron deficiency can lower ferritin and increase TIBC, but inflammation can mask both changes. Soluble transferrin receptor or an assay-specific reticulocyte hemoglobin result may help in selected cases. Increased erythropoietic drive and thalassemia can also raise soluble transferrin receptor, which limits its specificity.^{88 Weiss G, et al. Anemia of inflammation. 2019.}

Sideroblastic and lead-associated patterns

Sideroblastic anemia results from impaired mitochondrial heme production or iron use. Iron accumulates in erythroblast mitochondria around the nucleus. Perls Prussian blue staining of marrow shows ring sideroblasts. Each cell has at least five iron granules around at least one-third of the nuclear circumference. When you report a percentage, name the erythroblast denominator your laboratory uses. Ring sideroblasts occur in inherited disorders, clonal myeloid neoplasms, and acquired secondary conditions. Current myeloid classifications use genetic and morphologic criteria for the clonal categories.^{99 Khoury JD, et al. The 5th edition of the World Health Organization Classification… 2022.}

Secondary causes include alcohol, lead, copper deficiency, zinc excess, and drugs such as isoniazid, linezolid, and chloramphenicol. The CBC may be microcytic, normocytic, macrocytic, or dimorphic, depending on the cause. Serum iron, ferritin, and transferrin saturation are often increased. Film findings can include basophilic stippling and Pappenheimer bodies. Correlate a marrow ring-sideroblast result with exposure history, medications, nutritional studies, cytogenetic or molecular findings, and other evidence of a myeloid neoplasm.

Lead inhibits aminolevulinate dehydratase and ferrochelatase in heme synthesis. It also interferes with degradation of erythrocyte RNA, contributing to coarse basophilic stippling. Zinc protoporphyrin (ZPP), free erythrocyte protoporphyrin (FEP), and urinary aminolevulinic acid can rise. They are adjuncts. A blood lead concentration establishes exposure. For children, confirm a capillary result at or above the Centers for Disease Control and Prevention blood lead reference value of 3.5 µg/dL with a venous specimen. Lead exposure can harm health below this population-based action threshold.^{1010 Centers for Disease Control and Prevention. Recommended actions based on blood lead level. 2025.}

Thalassemia syndromes

Thalassemias reduce production of a structurally normal globin chain. The unpaired partner chains damage developing or circulating erythrocytes, causing ineffective erythropoiesis, hemolysis, or both. Reduced hemoglobin synthesis produces microcytosis that can be striking relative to the degree of anemia. A β⁰ allele produces no β chain, a β⁺ allele reduces production by a mutation-specific amount, and a β^silent allele has a mild effect.

Macrocytic anemia patterns

Macrocytosis is classified as megaloblastic or nonmegaloblastic. The blood film, reticulocyte count, vitamin studies, liver and thyroid tests, medication history, and other cytopenias guide the next step. Megaloblastic anemia often produces an MCV from about 100 to 150 fL and can exceed 120 fL. Nonmegaloblastic causes often produce an MCV near 100 to 110 fL, although greater values occur. RDW is often normal or only mildly increased in a uniform nonmegaloblastic population.^{22 Keohane EM, et al. Rodak's Hematology. 2025.}

The International Council for Standardization in Haematology defines neutrophil hypersegmentation as at least one neutrophil with six or more lobes or more than 3% of neutrophils with five lobes when 100 neutrophils are examined. This definition helps distinguish true hypersegmentation from occasional five-lobed cells.^{55 Palmer L, et al. ICSH recommendations for the standardization of nomenclature and… 2015.}

Megaloblastic change can lower leukocytes and platelets as well as RBCs. When a film shows macro-ovalocytes, hypersegmented neutrophils, and pancytopenia, the next step is vitamin B12 and folate testing. Similar marrow morphology can occur in myelodysplastic and other dyserythropoietic disorders, so confirm the deficiency biochemically.^{2,52 Keohane EM, et al. Rodak's Hematology. 2025.5 Palmer L, et al. ICSH recommendations for the standardization of nomenclature and… 2015.}

Vitamin B12 and folate

Folate supplies one-carbon units used to make thymidylate for DNA. Vitamin B12 supports methionine synthase, which regenerates tetrahydrofolate from 5-methyltetrahydrofolate, and methylmalonyl-CoA mutase, which converts methylmalonyl-CoA to succinyl-CoA. During B12 deficiency, folate becomes trapped as 5-methyltetrahydrofolate and methylmalonic acid accumulates. Impaired DNA synthesis produces nuclear-cytoplasmic asynchrony and ineffective erythropoiesis.^{14,1514 National Institutes of Health Office of… Vitamin B12: fact sheet for health professionals.15 National Institutes of Health Office of… Folate: fact sheet for health professionals.}

The testing relationships and limitations follow current NIH and NICE guidance.^{14,15,1614 National Institutes of Health Office of… Vitamin B12: fact sheet for health professionals.15 National Institutes of Health Office of… Folate: fact sheet for health professionals.16 National Institute for Health and Care… Vitamin B12 Deficiency in Over 16s. 2024.}

Many laboratories consider total B12 below approximately 200 to 250 pg/mL subnormal and use methylmalonic acid when total B12 is indeterminate. These values are assay dependent. Neurologic B12 deficiency can occur with normal hemoglobin and MCV. Clinical suspicion therefore remains relevant when the CBC is normal.^{14,1614 National Institutes of Health Office of… Vitamin B12: fact sheet for health professionals.16 National Institute for Health and Care… Vitamin B12 Deficiency in Over 16s. 2024.}

Autoimmune gastritis damages gastric parietal cells and reduces intrinsic factor, impairing B12 absorption. Pernicious anemia is the hematologic expression of this process. Anti-intrinsic-factor antibody is relatively specific but insensitive. If suspicion remains after a negative result, the applicable diagnostic pathway may include parietal-cell antibody, gastrin, or additional clinical evaluation. Gastrin can rise with acid-suppressing medication and other conditions, so it is an adjunct.^{1616 National Institute for Health and Care… Vitamin B12 Deficiency in Over 16s. 2024.}

Other causes of B12 deficiency include inadequate intake, food-cobalamin malabsorption, gastric or ileal surgery, ileal disease, pancreatic insufficiency, bacterial overgrowth, and inherited transport defects. Folate deficiency can follow inadequate intake, alcohol use disorder, increased need, malabsorption, dialysis, or medications that block folate metabolism. Interpret the laboratory pattern alongside the medication and clinical history.

Nonmegaloblastic macrocytosis and spurious results

Reticulocytes are larger than mature RBCs, so hemolysis, recovery after hemorrhage, or response to effective therapy can raise MCV. Liver disease and alcohol-related changes can produce round macrocytes, sometimes with target cells or stomatocytes. Hypothyroidism, marrow disease, and drugs that affect DNA synthesis or cell turnover are additional causes. Persistent macrocytosis with other cytopenias or dysplastic cells warrants evaluation for a marrow disorder.^{22 Keohane EM, et al. Rodak's Hematology. 2025.}

Before you release a disease interpretation of an implausible CBC, check it for analytic interference:

• Cold agglutinins clump RBCs, producing a falsely low RBC count and falsely high MCV and MCHC. Warming the specimen to 37 °C and repeating the count usually resolves the pattern.
• Marked hyperglycemia can cause analyzer-dependent in-vitro RBC swelling after dilution and a spuriously increased MCV.
• Lipemia, icterus, marked leukocytosis, hemolysis, clots, specimen age, and intravenous-fluid contamination can disturb selected CBC measurements.

The correction follows the analyzer's validated procedure and may require warming, dilution, plasma replacement, repeat analysis, or a fresh uncontaminated specimen. Resolve any interference before you interpret the anemia.^{1717 Gulati G, et al. Unreliable automated complete blood count results. 2022.}

Practice