Untreated, severe aplastic anemia has a high risk of death. Modern treatment, by drugs or stem cell transplant, has a five-year survival rate that exceeds 85%, with younger age associated with higher survival.

Survival rates for stem cell transplant vary depending on age and availability of a well-matched donor. Five-year survival rates for patients who receive transplants have been shown to be 82% for patients under age 20, 72% for those 20–40 years old, and closer to 50% for patients over age 40. Success rates are better for patients who have donors that are matched siblings and worse for patients who receive their marrow from unrelated donors.

Older people (who are generally too frail to undergo bone marrow transplants), and people who are unable to find a good bone marrow match, undergoing immune suppression have five-year survival rates of up to 75%.

Relapses are common. Relapse following ATG/ciclosporin use can sometimes be treated with a repeated course of therapy. In addition, 10-15% of severe aplastic anemia cases evolve into MDS and leukemia. According to a study, for children who underwent immunosuppressive therapy, about 15.9% of children who responded to immunosuppressive therapy encountered relapse.

Milder disease can resolve on its own.

PNH is rare, with an annual rate of 1-2 cases per million. The prognosis without disease-modifying treatment is 10–20 years. Many cases develop in people who have previously been diagnosed with aplastic anemia or myelodysplastic syndrome. The fact that PNH develops in MDS also explains why there appears to be a higher rate of leukemia in PNH, as MDS can sometimes transform into leukemia.

25% of female cases of PNH are discovered during pregnancy. This group has a high rate of thrombosis, and the risk of death of both mother and child are significantly increased (20% and 8% respectively).

Basically classified by causative mechanism, types of congenital hemolytic anemia include:

- Genetic conditions of RBC Membrane

- Hereditary spherocytosis

- Hereditary elliptocytosis

- Genetic conditions of RBC metabolism (enzyme defects). This group is sometimes called "congenital nonspherocytic (hemolytic) anemia", which is a term for a congenital hemolytic anemia without spherocytosis, and usually excluding hemoglobin abnormalities as well, but rather encompassing defects of glycolysis in the erythrocyte.

- Glucose-6-phosphate dehydrogenase deficiency (G6PD or favism)

- Pyruvate kinase deficiency

- Aldolase A deficiency

- Hemoglobinopathies/genetic conditions of hemoglobin

- Sickle cell anemia

- Congenital dyserythropoietic anemia

- Thalassemia

Aplastic anemia can be caused by exposure to certain chemicals, drugs, radiation, infection, immune disease; in about half the cases, yet a defintive cause is unknown. It is not a familial line hereditary condition, nor is it contagious. It can be acquired due to exposure to other conditions but if a person develops the condition, their offspring would not develop it by virtue of their gene connection.

Aplastic anemia is also sometimes associated with exposure to toxins such as benzene, or with the use of certain drugs, including chloramphenicol, carbamazepine, felbamate, phenytoin, quinine, and phenylbutazone. Many drugs are associated with aplasia mainly according to case reports, but at a very low probability. As an example, chloramphenicol treatment is followed by aplasia in less than one in 40,000 treatment courses, and carbamazepine aplasia is even rarer.

Exposure to ionizing radiation from radioactive materials or radiation-producing devices is also associated with the development of aplastic anemia. Marie Curie, famous for her pioneering work in the field of radioactivity, died of aplastic anemia after working unprotected with radioactive materials for a long period of time; the damaging effects of ionizing radiation were not then known.

Aplastic anemia is present in up to 2% of patients with acute viral hepatitis.

One known cause is an autoimmune disorder in which white blood cells attack the bone marrow.

Short-lived aplastic anemia can also be a result of parvovirus infection. In humans, the P antigen (also known as globoside), one of the many cellular receptors that contribute to a person's blood type, is the cellular receptor for parvovirus B19 virus that causes erythema infectiosum (fifth disease) in children. Because it infects red blood cells as a result of the affinity for the P antigen, Parvovirus causes complete cessation of red blood cell production. In most cases, this goes unnoticed, as red blood cells live on average 120 days, and the drop in production does not significantly affect the total number of circulating red blood cells. In people with conditions where the cells die early (such as sickle cell disease), however, parvovirus infection can lead to severe anemia.

More frequently parvovirus B19 is associated with aplastic crisis which involves only the red blood cells ( despite the name). Aplastic anemia involves all different cell lines.

In some animals, aplastic anemia may have other causes. For example, in the ferret ("Mustela putorius furo"), it is caused by estrogen toxicity, because female ferrets are induced ovulators, so mating is required to bring the female out of heat. Intact females, if not mated, will remain in heat, and after some time the high levels of estrogen will cause the bone marrow to stop producing red blood cells.

Overall, hemoglobin C disease is one of the more benign hemoglobinopathies. Mild-to-moderate reduction in RBC lifespan may accompany from mild hemolytic anemia. Individuals with hemoglobin C disease have sporadic episodes of musculoskeletal (joint) pain. People with hemoglobin C disease can expect to lead a normal life.

Congenital hypoplastic anemia (or constitutional aplastic anemia) is a type of aplastic anemia which is primarily due to a congenital disorder.

Associated genes include "TERC", "TERT", "IFNG", "NBS1", "PRF1", and "SBDS".

Examples include:

- Fanconi anemia

- Diamond-Blackfan anemia

Most pedigrees suggest an autosomal dominant mode of inheritance with incomplete penetrance. Approximately 10–25% of DBA occurs with a family history of disease.

About 25-50% of the causes of DBA have been tied to abnormal ribosomal protein genes. The disease is characterized by genetic heterogeneity, affecting different ribosomal gene loci: Exceptions to this paradigm have been demonstrated, such as with rare mutations of transcription factor GATA1 and advanced alternative splicing of a gene involved in iron metabolism, SLC49A1 (FLVCR1).

In 1997, a patient was identified who carried a rare balanced chromosomal translocation involving chromosome 19 and the X chromosome. This suggested that the affected gene might lie in one of the two regions that were disrupted by this cytogenetic anomaly. Linkage analysis in affected families also implicated this region in disease, and led to the cloning of the first DBA gene. About 20–25% of DBA cases are caused by mutations in the ribosome protein S19 (RPS19) gene on chromosome 19 at cytogenetic position 19q13.2. Some previously undiagnosed relatives of DBA patients were found to carry mutations, and also had increased adenosine deaminase levels in their red blood cells, but had no other overt signs of disease.

A subsequent study of families with no evidence of RPS19 mutations determined that 18 of 38 families showed evidence for involvement of an unknown gene on chromosome 8 at 8p23.3-8p22. The precise genetic defect in these families has not yet been delineated.

Malformations are seen more frequently with DBA6 RPL5 and DBA7 RPL11 mutations.

The genetic abnormalities underpinning the combination of DBA with Treacher Collins syndrome (TCS)/mandibulofacial dysostosis (MFD) phenotypes are heterogeneous, including RPS26 (the known DBA10 gene), TSR2 which encodes a direct binding partner of RPS26, and RPS28.

Anisocytosis is identified by RDW and is classified according to the size of RBC measured by MCV. According to this, it can be divided into

- Anisocytosis with microcytosis – Iron deficiency, sickle cell anemia

- Anisocytosis with macrocytosis – Folate or vitamin B deficiency, autoimmune hemolytic anemia, cytotoxic chemotherapy, chronic liver disease, myelodysplastic syndrome

Increased RDW is seen in iron deficiency anemia and decreased or normal in thalassemia major (Cooley's anemia), thalassemia intermedia

- Anisocytosis with normal RBC size – Early iron, vit B12 or folate deficiency, dimorphic anemia, Sickle cell disease, chronic liver disease, Myelodysplastic syndrome

Hemoglobin C gene is found in 2-3% of US African-Americans while 8% of US African \-Americans have hemoglobin S (Sickle) gene. Thus Hemoglobin SC disease is significantly more common than Hemoglobin CC disease. Hemoglobin C is found in areas of West Africa, such as Nigeria, where Yorubas live.

About 1 out of every 40 African-Americans has hemoglobin C trait. The trait also affects people whose ancestors came from Italy, Greece, Africa, Latin America, and the Caribbean region. However, it is possible for a person of any race or nationality to have hemoglobin C trait. In terms of geographic distribution, the hemoglobin C allele is found at the highest frequencies in West Africa, where it has been associated with protection against malaria. Hemoglobin C disease is present at birth, though some cases may not be diagnosed until adulthood. Both sexes, male and female, are affected equally.

Acquired hemolytic anemia may be caused by immune-mediated causes, drugs and other miscellaneous causes.

- Immune-mediated causes could include transient factors as in "Mycoplasma pneumoniae" infection (cold agglutinin disease) or permanent factors as in autoimmune diseases like autoimmune hemolytic anemia (itself more common in diseases such as systemic lupus erythematosus, rheumatoid arthritis, Hodgkin's lymphoma, and chronic lymphocytic leukemia).

- Spur cell hemolytic anemia

- Any of the causes of hypersplenism (increased activity of the spleen), such as portal hypertension.

- Acquired hemolytic anemia is also encountered in burns and as a result of certain infections (e.g. malaria).

- Lead poisoning resulting from the environment causes non-immune hemolytic anemia.

- Runners can suffer hemolytic anemia due to "footstrike hemolysis", owing to the destruction of red blood cells in feet at foot impact.

- Low-grade hemolytic anemia occurs in 70% of prosthetic heart valve recipients, and severe hemolytic anemia occurs in 3%.

Hemolytic anemia affects nonhuman species as well as humans. It has been found, in a number of animal species, to result from specific triggers.

Some notable cases include hemolytic anemia found in black rhinos kept in captivity, with the disease, in one instance, affecting 20% of captive rhinos at a specific facility. The disease is also found in wild rhinos.

Dogs and cats differ slightly from humans in some details of their RBC composition and have altered susceptibility to damage, notably, increased susceptibility to oxidative damage from consumption of onion. Garlic is less toxic to dogs than onion.

Causes of sideroblastic anemia can be categorized into three groups: congenital sideroblastic anemia, acquired clonal sideroblastic anemia, and acquired reversible sideroblastic anemia. All cases involve dysfunctional heme synthesis or processing. This leads to granular deposition of iron in the mitochondria that form a ring around the nucleus of the developing red blood cell. Congenital forms often present with normocytic or microcytic anemia while acquired forms of sideroblastic anemia are often normocytic or macrocytic.

- Congenital sideroblastic anemia

- X-linked sideroblastic anemia: This is the most common congenital cause of sideroblastic anemia and involves a defect in ALAS2, which is involved in the first step of heme synthesis. Although X-linked, approximately one third of patients are women due to skewed X-inactivation (lyonizations).

- Autosomal recessive sideroblastic anemia involves mutations in the SLC25A38 gene. The function of this protein is not fully understood, but it is involved in mitochondrial transport of glycine. Glycine is a substrate for ALAS2 and necessary for heme synthesis. The autosomal recessive form is typically severe in presentation.

- Genetic syndromes: Rarely, sideroblastic anemia may be part of a congenital syndrome and present with associated findings, such as ataxia, myopathy, and pancreatic insufficiency.

- Acquired clonal sideroblastic anemia

- Clonal sideroblastic anemias fall under the broader category of myelodysplastic syndromes (MDS). Three forms exist and include refractory anemia with ringed sideroblasts (RARS), refractory anemia with ringed sideroblasts and thrombocytosis (RARS-T), and refractory cytopenia with multilineage dysplasia and ringed sideroblasts (RCMD-RS). These anemias are associated with increased risk for leukemic evolution.

- Acquired reversible sideroblastic anemia

- Causes include excessive alcohol use (the most common cause of sideroblastic anemia), pyridoxine deficiency, lead poisoning, and copper deficiency. Excess zinc can indirectly cause sideroblastic anemia by decreasing absorption and increasing excretion of copper. Antimicrobials that may lead to sideroblastic anemia include isoniazid, chloramphenicol, cycloserine, and linezolid.

Congenital hemolytic anemia (or hereditary hemolytic anemia) refers to hemolytic anemia which is primarily due to congenital disorders.

Sideroblastic anemias are often described as responsive or non-responsive in terms of increased hemoglobin levels to pharmacological doses of vitamin B.

1- Congenital: 80% are responsive, though the anemia does not completely resolve.

2- Acquired clonal: 40% are responsive, but the response may be minimal.

3- Acquired reversible: 60% are responsive, but course depends on treatment of the underlying cause.

Severe refractory sideroblastic anemias requiring regular transfusions and/or that undergo leukemic transformation (5-10%) significantly reduce life expectancy.

Bone marrow failure in both children and adults can be either inherited or acquired. Inherited bone marrow failure is often the cause in young children, while older children and adults may acquire the disease later in life. A maturation defect in genes is a common cause of inherited bone marrow failure. The most common cause of acquired bone marrow failure is aplastic anemia. Working with chemicals such as benzene could be a factor in causing the illness. Other factors include radiation or chemotherapy treatments, and immune system problems.

Typical causes of microcytic anemia include:

- Childhood

- Iron deficiency anemia, by far the most common cause of anemia in general and of microcytic anemia in particular

- Thalassemia

- Adulthood

- Iron deficiency anemia

- Sideroblastic anemia, In congenital sideroblastic anemia the MCV (mean corpuscular volume) is either low or normal. In contrast, the MCV is usually high in the much more common acquired sideroblastic anemia.

- Anemia of chronic disease, although this more typically causes normochromic, normocytic anemia. Microcytic anemia has been discussed by Weng et al.

- Lead poisoning

- Vitamin B (pyridoxine) deficiency

Other causes that are typically thought of as causing normocytic anemia or macrocytic anemia must also be considered, and the presence of two or more causes of anemia can distort the typical picture.

There are five main causes of microcytic anemia forming the acronym TAILS. Thalassemia, Anemia of chronic disease, Iron deficiency, Lead poisoning and Congenital sideroblastic anemia. Only the first three are common in most parts of the world. In theory, these three can be differentiated by their red blood cell (RBC) morphologies. Anemia of chronic disease shows unremarkable RBCs, iron deficiency shows anisocytosis, anisochromia and elliptocytosis, and thalessemias demonstrate target cells and coarse basophilic stippling. In practice though elliptocytes and anisocytosis are often seen in thalessemia and target cells occasionally in iron deficiency. All three may show unremarkable RBC morphology. Coarse basophlic stippling is one reliable morphologic finding of thalessemia which does not appear in iron deficiency or anemia of chronic disease. The patient should be in an ethnically at risk group and the diagnosis is not confirmed without a confirmatory method such as hemoglobin HPLC, H body staining, molecular testing or another reliable method. Course basophlic stippling occurs in other cases as seen in Table 1

Congenital dyserythropoietic anemia type IV is an autosomal dominant inherited red blood cell disorder characterized by ineffective erythropoiesis and hemolysis resulting in anemia. Circulating erythroblasts and erythroblasts in the bone marrow show various morphologic abnormalities. Affected individuals with CDAN4 also have increased levels of fetal hemoglobin.

The issue is thought of as representing any of the following:

- a decreased production of normal-sized red blood cells (e.g., anemia of chronic disease, aplastic anemia);

- an increased production of HbS as seen in sickle cell disease (not sickle cell trait);

- an increased destruction or loss of red blood cells (e.g., hemolysis, posthemorrhagic anemia);

- an uncompensated increase in plasma volume (e.g., pregnancy, fluid overload);

- a B2 (riboflavin) deficiency

- a B6 (pyridoxine) deficiency

- or a mixture of conditions producing microcytic and macrocytic anemia.

Blood loss, suppressed production of RBCs or hemolysis represent most cases of normocytic anemia. In blood loss, morphologic findings are generally unremarkable except after 12 to 24 hrs where polychromasia appears. For reduced production of RBCs, like with low erythropoietin, the RBC morphology is unremarkable. Patients with disordered RBC production, e.g. myelodysplastic syndrome, may have a dual population of elliptocytes, teardrop cells, or other poikilocytes as well as a nucleated RBCs. Hemolysis will often demonstrate poikilocytes specific to a cause or mechanism. E.g. Bite cells and/or blistor cells for oxidative hemolysis, Acanthocytes for pyruvate kinase deficiency or McLeod phenotype, Sickle cells for sickle cell anemia, Spherocytes for immune-mediated hemolysis or hereditary spherocytosis, Elliptocytosis for iron deficiency or hereditary elliptocytosis and schistocytes for intravascular hemolysis. Many hemolytic anemias show multiple poikilocytes such as G6PD deficiency which may show blister and bites cells as well as shistocytes. Neonatal hemolysis may not follow the classic patterns as in adults

Many patients eventually develop acute myelogenous leukemia (AML). Older patients are extremely likely to develop head and neck, esophageal, gastrointestinal, vulvar and anal cancers. Patients who have had a successful bone marrow transplant and, thus, are cured of the blood problem associated with FA still must have regular examinations to watch for signs of cancer. Many patients do not reach adulthood.

The overarching medical challenge that Fanconi patients face is a failure of their bone marrow to produce blood cells. In addition, Fanconi patients normally are born with a variety of birth defects. A good number of Fanconi patients have kidney problems, trouble with their eyes, developmental retardation and other serious defects, such as microcephaly (small head).

The phenotype of DBA patients suggests a hematological stem cell defect specifically affecting the erythroid progenitor population. Loss of ribosomal function might be predicted to affect translation and protein biosynthesis broadly and impact many tissues. However, DBA is characterized by dominant inheritance, and arises from partial loss of ribosomal function, so it is possible that erythroid progenitors are more sensitive to this decreased function, while most other tissues are less affected.

Certain gastrointestinal disorders can cause anemia. The mechanisms involved are multifactorial and not limited to malabsorption but mainly related to chronic intestinal inflammation, which causes dysregulation of hepcidin that leads to decreased access of iron to the circulation.

- "Helicobacter pylori" infection.

- Gluten-related disorders: untreated celiac disease and non-celiac gluten sensitivity. Anemia can be the only manifestation of celiac disease, in absence of gastrointestinal or any other symptoms.

- Inflammatory bowel disease.

The two most common signs and symptoms of bone marrow failure are bleeding and bruising. Blood may be seen throughout the gums, nose or the skin, and tend to last longer than normal. Children have a bigger chance of seeing blood in their urine or stools, which results in digestive problems with an unpleasant scent. Individuals with this condition may also encounter tooth loss or tooth decay. Chronic fatigue, shortness of breath, and recurrent colds can also be symptoms of bone marrow failure.

Anisocytosis is a medical term meaning that a patient's red blood cells are of unequal size. This is commonly found in anemia and other blood conditions. False diagnostic flagging may be triggered by an elevated WBC count, agglutinated RBCs, RBC fragments, giant platelets or platelet clumps. In addition, it is a characteristic feature of bovine blood.

The red cell distribution width (RDW) is a measurement of anisocytosis and is calculated as a coefficient of variation of the distribution of RBC volumes divided by the mean corpuscular volume (MCV)

Drug induced hemolysis has large clinical relevance. It occurs when drugs actively provoke red blood cell destruction. It can be divided in the following manner:

- Drug-induced autoimmune hemolytic anemia

- Drug-induced nonautoimmune hemolytic anemia

A total of four mechanisms are usually described, but there is some evidence that these mechanisms may overlap.

Asplenia is the absence of normal spleen function. It predisposes to some septicemia infections. Therefore, vaccination and antibiotic measures are essential in such cases. There are multiple causes:

- Some people congenitally completely lack a spleen, although this is rare.

- Sickle-cell disease can cause a functional asplenia (or autosplenectomy) by causing infarctions of the spleen during repeated sickle-cell crises.

- It may be removed surgically (known as a splenectomy), but this is rarely performed, as it carries a high risk of infection and other adverse effects. Indications include following abdominal injuries with rupture and hemorrhage of the spleen, or in the treatment of certain blood diseases (Idiopathic thrombocytopenic purpura, hereditary spherocytosis, etc.), certain forms of lymphoma or for the removal of splenic tumors or cysts.