There have been reports of pulmonary venous thromboembolism in pregnant women with sickle cell trait, or men during prolonged airflight, and mild strokes and abnormalities on PET scans in children with the trait.

Sickle cell trait appears to worsen the complications seen in diabetes mellitus type 2 (retinopathy, nephropathy and proteinuria) and provoke hyperosmolar diabetic coma nephropathy, especially in male patients.

Thalassemia can coexist with other hemoglobinopathies. The most common of these are:

- Hemoglobin E/thalassemia: common in Cambodia, Thailand, and parts of India, it is clinically similar to β thalassemia major or thalassemia intermedia.

- Hemoglobin S/thalassemia: common in African and Mediterranean populations, is clinically similar to sickle-cell anemia, with the additional feature of splenomegaly.

- Hemoglobin C/thalassemia: common in Mediterranean and African populations, hemoglobin C/β thalassemia causes a moderately severe hemolytic anemia with splenomegaly; hemoglobin C/β thalassemia produces a milder disease.

- Hemoglobin D/thalassemia: common in the northwestern parts of India and Pakistan (Punjab region).

The American College of Obstetricians and Gynecologists recommends all people thinking of becoming pregnant be tested to see if they have thalassemia. Genetic counseling and genetic testing are recommended for families who carry a thalassemia trait.

A screening policy exists in Cyprus to reduce the rate of thalassemia, which, since the program's implementation in the 1970s (which also includes prenatal screening and abortion), has reduced the number of children born with the disease from one of every 158 births to almost zero.

In Iran as a premarital screening, the man's red cell indices are checked first, if he has microcytosis (mean cell hemoglobin < 27 pg or mean red cell volume < 80 fl), the woman is tested. When both are microcytic, their hemoglobin A2 concentrations are measured. If both have a concentration above 3.5% (diagnostic of thalassemia trait) they are referred to the local designated health post for genetic counseling.

Large scale awareness campaigns are being organized in India both by government and non-government organizations in favor of voluntary premarital screening to detect carriers of thalassemia and marriage between both carriers are strongly discouraged.

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

The thalassemia trait may confer a degree of protection against malaria, which is or was prevalent in the regions where the trait is common, thus conferring a selective survival advantage on carriers (known as heterozygous advantage), thus perpetuating the mutation. In that respect, the various thalassemias resemble another genetic disorder affecting hemoglobin, sickle-cell disease.

The disorder affects all genders but is more prevalent in certain ethnicities and age groups. 20 people die per year causing thalassemia to be listed as a “rare disease”. In the United States, thalassemia’s prevalence is approximately 1 in 272,000 or 1,000 people. There have been 4,000 hospitalized cases in England in 2002 and 9,233 consultant episodes for thalassemia. Men accounted for 53% of hospital consultant episodes and women accounted for 47%. The mean patient age is 23 with only 1% of consultants the patient is older than 75 and 69% were 15-59 year olds. The Children’s Hospital Oakland formed an international network to combat thalassemia. “It is the world’s most common genetic blood disorder and is rapidly increasing”. 7% of the world’s population are carriers and 400,000 babies are born with the trait annually. It is usually fatal in infancy if blood transfusion are not initiated immediately.

Hypochromic anemia occurs in patients with hypochromic microcytic anemia with iron overload. The condition is autosomal recessive and is caused by mutations in the SLC11A2 gene. The condition prevents red blood cells from accessing iron in the blood, which causes anemia that is apparent at birth. It can lead to pallor, fatigue, and slow growth. The iron overload aspect of the disorder means that the iron accumulates in the liver and can cause liver impairment in adolescence or early adulthood.

It also occurs in patients with hereditary iron refractory iron-deficiency anemia (IRIDA). Patients with IRIDA have very low serum iron and transferrin saturation, but their serum ferritin is normal or high. The anemia is usually moderate in severity and presents later in childhood.

Hypochromic anemia is also caused by thalassemia and congenital disorders like Benjamin anemia.

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

Hypochromic anemia may be caused by vitamin B6 deficiency from a low iron intake, diminished iron absorption, or excessive iron loss. It can also be caused by infections (e.g. hookworms) or other diseases (i.e. anemia of chronic disease), therapeutic drugs, copper toxicity, and lead poisoning. One acquired form of anemia is also known as Faber's syndrome. It may also occur from severe stomach or intestinal bleeding caused by ulcers or medications such as aspirin or bleeding from hemorrhoids.

In terms of epidemiology, worldwide distribution of inherited alpha-thalassemia corresponds to areas of malaria exposure, suggesting a protective role. Thus, alpha-thalassemia is common in sub-Saharan Africa, the Mediterranean Basin, and generally tropical (and subtropical) regions. The epidemiology of alpha-thalassemia in the US reflects this global distribution pattern. More specifically, HbH disease is seen in Southeast Asia and the Middle East, while Hb Bart hydrops fetalis is acknowledged in Southeast Asia only.

The data indicate that 15% of the Greek and Turkish Cypriots are carriers of beta-thalassaemia genes, while 10% of the population carry alpha-thalassaemia genes.

Sickle cell trait provides a survival advantage over people with normal hemoglobin in regions where malaria is endemic. The trait is known to cause significantly fewer deaths due to malaria, especially when "Plasmodium falciparum" is the causative organism. This is a prime example of natural selection, evidenced by the fact that the geographical distribution of the gene for hemoglobin S and the distribution of malaria in Africa virtually overlap. Because of the unique survival advantage, people with the trait become increasingly numerous as the number of malaria-infected people increases. Conversely, people who have normal hemoglobin tend to succumb to the complications of malaria.

Although the precise mechanism for this phenomenon is not known, a several factors are believed to be responsible.

- Infected erythrocytes (red blood cells) tend to have lower oxygen tension, because it is significantly reduced by the parasite. This causes sickling of that particular erythrocyte, signalling the phagocytes to get rid of the cell and hence the parasite within.

- Since the sickling of parasite-infected cells is higher, these selectively get removed by the reticulo-endothelial system, thus sparing the normal erythrocytes.

- Excessive vacuole formation occurs in those parasites infecting sickle cells.

- Sickle trait erythrocytes produce higher levels of the superoxide anion and hydrogen peroxide than normal erythrocytes do, both are toxic to malarial parasites.

The sickle cell trait was found to be 50% protective against mild clinical malaria, 75% protective against admission to the hospital for malaria, and almost 90% protective against severe or complicated malaria.

A potential complication that may occur in children that suffer acute anemia with a hemoglobin count below 5.5 g/dl is silent stroke A silent stroke is a type of stroke that does not have any outward symptoms (asymptomatic), and the patient is typically unaware they have suffered a stroke. Despite not causing identifiable symptoms a silent stroke still causes damage to the brain, and places the patient at increased risk for both transient ischemic attack and major stroke in the future.

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

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

Microcytic anemia is not caused by reduced DNA synthesis.

Thalassemia can cause microcytosis. Depending upon how the terms are being defined, thalassemia can be considered a cause of microcytic anemia, or it can be considered a cause of microcytosis but not a cause of microcytic anemia.

There are many causes of microcytosis, which is essentially only a descriptor. Cells can be small because of mutations in the formation of blood cells (hereditary microcytosis) or because they are not filled with enough hemoglobin, as in iron-deficiency-associated microcytosis.

Red blood cells can be characterised by their haemoglobin content as well as by their size. The haemoglobin content is referred to as the cell's colour. Therefore, there are both "normochromic microcytotic red cells" and "hypochromic, microcytotic red cells". The normochromic cells have a normal concentration of haemoglobin, and are therefore 'red enough' while the hypochromic cells do not; thus the value of the mean corpuscular hemoglobin concentration.

Microcytic anaemia is any of several types of anaemia characterized by small red blood cells (called microcytes). The normal mean corpuscular volume (abbreviated to MCV on full blood count results) is 80-100 fL, with smaller cells (100 fL) as macrocytic (the latter occur in macrocytic anemia).The MCV is the average red blood cell size.

In microcytic anaemia, the red blood cells (erythrocytes) are usually also hypochromic, meaning that the red blood cells appear paler than usual. This is reflected by a lower-than-normal mean corpuscular hemoglobin concentration (MCHC), a measure representing the amount of hemoglobin per unit volume of fluid inside the cell; normally about 320-360 g/L or 32-36 g/dL. Typically, therefore, anemia of this category is described as "microcytic, hypochromic anaemia".

An individual with delta-beta thalassemia is usually asymptomatic, however microcytosis can occur where the red blood cells are abnormally small.

People who have hemoglobin E/β-thalassemia have inherited one gene for hemoglobin E from one parent and one gene for β-thalassemia from the other parent. Hemoglobin E/β-thalassemia is a severe disease, and it still has no universal cure. It affects more than a million people in the world. The consequences of hemoglobin E/β-thalassemia when it is not treated can be heart failure, enlargement of the liver, problems in the bones, etc.

There is a variety of genotypes depending on the interaction of HbE and α-thalassemia. The presence of the α-thalassemia reduces the amount of HbE usually found in HbE heterozygotes. In other cases, in combination with certain thalassemia mutations, it provides an increased resistance to malaria ("P. falciparum").

Sickle cell-beta thalassemia is caused by inheritance of a sickle cell allele from one parent and a beta thalassemia allele from the other.

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)

Microcytosis is a condition in which red blood cells are unusually small as measured by their mean corpuscular volume.

It is also known as "microcythemia". When associated with anemia, it is known as microcytic anemia.

Hemoglobin E is most prevalent in mainland Southeast Asia (Thailand, Myanmar, Cambodia, Laos, Vietnam), where its prevalence can reach 30 or 40%, and Northeast India, where in certain areas carrier rates reach 60% of the population. In Thailand the mutation can reach 50 or 70%, and it is higher in the northeast of the country. In Sri Lanka, it can reach up to 40% and affects those of Sinhalese and Vedda descent. It is also found at high frequencies in Bangladesh and Indonesia. The trait can also appear in people of Turkish, Chinese and Filipino descent. The mutation is estimated to have arisen within the last 5,000 years. In Europe there have been found cases of families with hemoglobin E, but in these cases, the mutation differs from the one found in South-East Asia. This means that there may be different origins of the βE mutation.

Delta-beta thalassemia is a form of thalassemia, and is autosomal recessive in terms of heredity. It is associated with "hemoglobin subunit delta"

A sickle allele is always the same mutation of the beta-globin gene (glutamic acid to valine at amino acid six). In contrast, beta-thalassemia alleles can be created by many different mutations including both deletion and non-deletion forms.

Those homozygous (Hb LeporeLepore; a very rare situation) or compound heterozygous (Hb Lepore-Β-thalassaemia) might suffer from a severe anaemia. They should be managed in a comprehensive multi-disciplinary program of care. Management includes a regular course of blood transfusions, although the clinical severity in compound (double) heterozygotes can range from minor to major, depending on the combination of genes that have caused the condition.