Hematologic disorders involve abnormalities in red blood cells (erythrocytes), white blood cells (leukocytes), platelets (thrombocytes), or the coagulation system. The bone marrow — the site of hematopoiesis — is central to many of these conditions. Disorders may be quantitative (too few or too many cells), qualitative (dysfunctional cells), or neoplastic.

SEM of normal human blood cells — Scanning electron micrograph of normal human blood showing red blood cells, lymphocytes, monocytes, and platelets. Hematologic disorders affect any of these cellular components or the coagulation system. Source: Wikimedia Commons.

Anemia

Anemia is defined by reduced hemoglobin concentration or red blood cell mass, resulting in decreased oxygen-carrying capacity. The CBC with red cell indices—MCV, MCH, MCHC, RDW — guides initial classification.

Sickle cell blood smear — Sickle-shaped red blood cells in sickle cell disease. Hematologic disorders encompass a wide range of conditions affecting red blood cells, white blood cells, platelets, and the coagulation system. Source: CDC PHIL.

Microcytic Anemia (Low MCV)

Iron deficiency anemia. The most common anemia worldwide. Causes include inadequate dietary intake, blood loss (GI, menstrual), malabsorption (celiac disease, gastrectomy), and increased requirements (pregnancy, growth). Laboratory findings: low serum ferritin, low serum iron, high TIBC, low transferrin saturation. Peripheral smear shows hypochromic, microcytic red cells with pencil-shaped cells. Treatment is oral iron supplementation. Failure to respond suggests ongoing blood loss, malabsorption, or incorrect diagnosis.

Anemia of chronic disease. A mild to moderate microcytic or normocytic anemia in the setting of acute or chronic inflammation (infection, autoimmune disease, malignancy, CKD). Hepcidin, an acute-phase protein, blocks iron absorption and release from macrophages. Serum ferritin is normal or high (reflecting total body iron). Treatment addresses the underlying inflammatory condition.

Thalassemia. Inherited disorders of globin chain synthesis. Alpha-thalassemia: deletion of alpha-globin genes (silent carrier, trait, hemoglobin H disease, hydrops fetalis). Beta-thalassemia: mutations in beta-globin genes (minor, intermedia, major). Thalassemia major requires lifelong transfusion and iron chelation. Curative therapy: hematopoietic stem cell transplantation and emerging gene therapy.

Macrocytic Anemia (High MCV)

Vitamin B12 deficiency. Causes: pernicious anemia (autoimmune destruction of gastric parietal cells), dietary deficiency (vegans), malabsorption (gastrectomy, Crohn disease, pancreatic insufficiency). B12 is required for DNA synthesis and myelin production. Neurologic symptoms (paresthesias, ataxia, cognitive decline) accompany hematologic findings. Diagnosis: low B12, elevated methylmalonic acid and homocysteine. Treatment is parenteral B12 replacement.

Folate deficiency. Causes: inadequate dietary intake, alcoholism, malabsorption, increased requirements (pregnancy, hemolytic anemia), and methotrexate. Folate is also required for DNA synthesis. Homocysteine is elevated; methylmalonic acid is normal. Treatment is oral folic acid.

Normocytic Anemia

Anemia of chronic disease (discussed above). Acute blood loss. Hemolytic anemias (discussed below). Bone marrow failure (aplastic anemia, MDS, marrow infiltration). Chronic kidney disease — erythropoietin deficiency.

⚠ Clinical Correlation

Hemolysis — the premature destruction of red blood cells — is classified as intravascular (within the circulation) or extravascular (in the spleen, liver, and bone marrow). Laboratory signs: elevated LDH, indirect bilirubin, and reticulocyte count; decreased haptoglobin. Intravascular hemolysis also causes hemoglobinemia and hemoglobinuria. Causes: inherited (sickle cell disease, thalassemia, G6PD deficiency, hereditary spherocytosis), autoimmune (warm or cold antibody), microangiopathic (TTP, HUS, DIC), mechanical (artificial heart valves), and infectious (malaria). Warm autoimmune hemolytic anemia is treated with corticosteroids, rituximab, and splenectomy.

Hemoglobinopathies

Sickle cell disease. Caused by a point mutation in the beta-globin gene (Glu6Val), producing hemoglobin S that polymerizes under deoxygenated conditions. The resulting sickle-shaped red cells cause vaso-occlusion, hemolysis, and end-organ damage. Acute complications: vaso-occlusive pain crises, acute chest syndrome, stroke, splenic sequestration, and aplastic crisis. Chronic complications: pulmonary hypertension, nephropathy, retinopathy, avascular necrosis, and leg ulcers.

Management. Hydroxyurea increases fetal hemoglobin and reduces crisis frequency. L-glutamine, crizanlizumab (P-selectin inhibitor), and voxelotor (HbS polymerization inhibitor) are newer disease-modifying therapies. Transfusion therapy for stroke prevention and acute complications. Curative: hematopoietic stem cell transplantation and gene therapy (Casgevy, Lyfgenia).

Leukemias

Acute Leukemias

Acute myeloid leukemia (AML). A clonal disorder of myeloid progenitors. Presents with cytopenias (anemia, thrombocytopenia, neutropenia), infection, and bleeding. Diagnosis by bone marrow biopsy showing ≥20% blasts. The ELN risk classification (favorable, intermediate, adverse) is based on cytogenetics and molecular mutations (NPM1, FLT3-ITD, CEBPA, RUNX1, TP53). Treatment: intensive induction chemotherapy (7+3: cytarabine + anthracycline) followed by consolidation (chemotherapy or allogeneic transplant). Targeted therapies: FLT3 inhibitors (midostaurin, quizartinib), IDH1/2 inhibitors (ivosidenib, enasidenib), and gemtuzumab ozogamicin.

Acute lymphoblastic leukemia (ALL). A clonal disorder of lymphoid progenitors. The most common cancer in children. B-cell ALL (85%) and T-cell ALL (15%). The Philadelphia chromosome (BCR-ABL) is an adverse risk factor. Treatment: multi-agent chemotherapy, tyrosine kinase inhibitors (for Ph+ ALL), blinatumomab (bispecific T-cell engager), inotuzumab ozogamicin, and CAR T-cell therapy (tisagenlecleucel).

Chronic Leukemias

Chronic myeloid leukemia (CML). Driven by the BCR-ABL fusion gene (Philadelphia chromosome). Three phases: chronic, accelerated, and blast crisis. Tyrosine kinase inhibitors (imatinib, dasatinib, nilotinib, bosutinib, ponatinib) produce deep molecular responses with near-normal life expectancy.

Chronic lymphocytic leukemia (CLL). The most common adult leukemia. Monoclonal B-cell lymphocytosis evolves to progressive lymphocytosis, lymphadenopathy, and cytopenias. Rai and Binet staging systems. Treatment: BTK inhibitors (ibrutinib, acalabrutinib), BCL2 inhibitors (venetoclax), and anti-CD20 antibodies (rituximab, obinutuzumab).

Lymphomas

Hodgkin lymphoma. Characterized by Reed-Sternberg cells on a background of reactive immune cells. Nodular sclerosis is the most common subtype. Presents with painless lymphadenopathy (often cervical), B symptoms (fever, night sweats, weight loss), and pruritus. PET-CT guides staging and response assessment. Treatment: ABVD chemotherapy (adriamycin, bleomycin, vinblastine, dacarbazine) with or without involved-field radiation. Cure rates exceed 80%.

Non-Hodgkin lymphoma. A diverse group of lymphoid malignancies. Diffuse large B-cell lymphoma (DLBCL) is the most common. Follicular lymphoma is indolent but incurable. Mantle cell lymphoma, marginal zone lymphoma, and Burkitt lymphoma are distinct entities. Treatment varies from watchful waiting (indolent) to immunochemotherapy (R-CHOP for DLBCL), CAR T-cell therapy, and bispecific antibodies.

Plasma Cell Disorders

Multiple myeloma. A clonal proliferation of plasma cells producing monoclonal immunoglobulin. CRAB features: hyperCalcemia, Renal insufficiency, Anemia, lytic Bone lesions. Diagnosis: bone marrow >10% clonal plasma cells, serum or urine M-protein, and end-organ damage. The Revised ISS (R-ISS) incorporates beta-2 microglobulin, albumin, LDH, and high-risk cytogenetics (del17p, t(4;14), t(14;16)). Treatment: proteasome inhibitors (bortezomib, carfilzomib), immunomodulatory drugs (lenalidomide, pomalidomide), corticosteroids, and anti-CD38 antibodies (daratumumab, isatuximab). Autologous stem cell transplant is used for eligible patients. CAR T-cell therapy and bispecific antibodies are newer options.

Bleeding Disorders

Hemophilia A and B. X-linked deficiencies of factor VIII (hemophilia A) and factor IX (hemophilia B). Severity correlates with factor activity. Spontaneous hemarthroses and soft tissue bleeding. Treatment: factor replacement, emicizumab (bispecific antibody for hemophilia A), and gene therapy (valoctocogene roxaparvovec for hemophilia A, etranacogene dezaparvovec for hemophilia B).

von Willebrand disease. The most common inherited bleeding disorder. Types 1 (mild quantitative), 2 (qualitative), and 3 (severe quantitative). Treatment: desmopressin (type 1), VWF-containing concentrates, and tranexamic acid.

★ Key Concept

VTE — encompassing deep vein thrombosis (DVT) and pulmonary embolism (PE) — is the third most common cardiovascular disease. Provoked VTE occurs after surgery, trauma, cancer, pregnancy, or prolonged immobility. Unprovoked VTE warrants evaluation for thrombophilia (factor V Leiden, prothrombin G20210A, antithrombin deficiency, protein C and S deficiency, antiphospholipid syndrome). Direct oral anticoagulants (apixaban, rivaroxaban, edoxaban, dabigatran) are first-line for treatment and prevention. Duration of anticoagulation is 3 months for provoked VTE, indefinite for unprovoked or recurrent VTE.

Transfusion Medicine

Blood components are transfused to correct specific deficits: packed red blood cells (anemia), platelets (thrombocytopenia), fresh frozen plasma (coagulopathy), and cryoprecipitate (fibrinogen deficiency). Massively transfusion protocols are used for hemorrhagic shock. Risks: transfusion reactions (acute hemolytic, febrile, allergic, TRALI, TACO), infectious transmission (rare), and iron overload (chronic transfusion). Patient blood management programs minimize unnecessary transfusions.

Summary

Hematologic disorders range from common anemias to rare coagulation disorders to hematologic malignancies. The CBC and peripheral smear are foundational diagnostic tools. Iron deficiency is the most prevalent anemia worldwide. The hemoglobinopathies — sickle cell disease and thalassemia — affect millions globally. Leukemias and lymphomas are increasingly curable with targeted therapies, immunotherapies, and cellular therapies. Venous thromboembolism is a major cause of morbidity and mortality. The integration of molecular diagnostics, targeted therapy, and cellular engineering is transforming hematology.