Pancytopenia usually requires a bone marrow biopsy in order to distinguish among different causes.

- anemia: hemoglobin < 13.5 g/dL (male) or 12 g/dL (female).

- leukopenia: total white cell count < 4.0 x 10/L. Decrease in all types of white blood cells (revealed by doing a differential count).

- thrombocytopenia: platelet count < 150×10/L.

Pancytopenia is a medical condition in which there is a reduction in the number of red and white blood cells, as well as platelets.

If only two parameters from the full blood count are low, the term bicytopenia can be used. The diagnostic approach is the same as for pancytopenia.

Anemia may lead to malaise, pallor and associated symptoms such as palpitations.

Low platelet counts (thrombocytopenia) if present is associated with an increased risk of hemorrhage, bruising and petechiae. Low white blood cell counts (leukocytopenia) if present leads to an increased risk of infections which can be severe.

Cytopenia is a reduction in the number of mature blood cells. It takes a number of forms:

- Low red blood cell count: resulting in anemia.

- Low white blood cell count: leukopenia or neutropenia. Because neutrophils make up at least half of all white cells, they are almost always low in leukopenia .

- Low platelet count: thrombocytopenia.

- Low granulocyte count: granulocytopenia

- Low red blood cell, white blood cell, and platelet counts: pancytopenia.

Aplastic anemia is a rare disease in which the bone marrow and the hematopoietic stem cells that reside there are damaged. This causes a deficiency of all three blood cell types (pancytopenia): red blood cells (anemia), white blood cells (leukopenia), and platelets (thrombocytopenia). "Aplastic" refers to inability of the stem cells to generate mature blood cells.

It is most prevalent in people in their teens and twenties, but is also common among the elderly. It can be caused by heredity, immune disease, or exposure to chemicals, drugs, or radiation. However, in about half the cases, the cause is unknown.

The definitive diagnosis is by bone marrow biopsy; normal bone marrow has 30–70% blood stem cells, but in aplastic anemia, these cells are mostly gone and replaced by fat.

First line treatment for aplastic anemia consists of immunosuppressive drugs, typically either anti-lymphocyte globulin or anti-thymocyte globulin, combined with corticosteroids and ciclosporin. Hematopoietic stem cell transplantation is also used, especially for patients under 30 years of age with a related matched marrow donor.

Diamond–Blackfan anemia is characterized by normocytic or macrocytic anemia (low red blood cell counts) with decreased erythroid progenitor cells in the bone marrow. This usually develops during the neonatal period. About 47% of affected individuals also have a variety of congenital abnormalities, including craniofacial malformations, thumb or upper limb abnormalities, cardiac defects, urogenital malformations, and cleft palate. Low birth weight and generalized growth delay are sometimes observed. DBA patients have a modest risk of developing leukemia and other malignancies.

In some cases, lymphocytopenia can be further classified according to which kind of lymphocytes are reduced. If all three kinds of lymphocytes are suppressed, then the term is used without further qualification.

- In T lymphocytopenia, there are too few T lymphocytes, but normal numbers of other lymphocytes. It causes, and manifests as, a T cell deficiency. This is usually caused by HIV infection (resulting in AIDS), but may be Idiopathic CD4+ lymphocytopenia (ICL), which is a very rare heterogeneous disorder defined by CD4+ T-cell counts below 300 cells/μL in the absence of any known immune deficiency condition, such as human immunodeficiency virus (HIV) infection or chemotherapy.

- In B lymphocytopenia, there are too few B lymphocytes, but possibly normal numbers of other lymphocytes. It causes, and manifests as, a humoral immune deficiency. This is usually caused by medications that suppress the immune system.

- In NK lymphocytopenia, there are too few natural killer cells, but normal numbers of other lymphocytes. This is very rare.

The primary sign of myelofibrosis is reactive bone marrow fibrosis, but it is often accompanied by:

- Abdominal fullness related to an enlarged spleen (splenomegaly).

- Bone pain

- Bruising and easy bleeding due to inadequate numbers of platelets

- Cachexia (loss of appetite, weight loss, and fatigue)

- Enlargement of both the liver and spleen

- Fatigue

- Gout and high uric acid levels

- Increased susceptibility to infection, such as pneumonia

- Pallor and shortness of breath due to anemia

- In rarer cases, a raised red blood cell volume

- Cutaneous myelofibrosis is a rare skin condition characterized by dermal and subcutaneous nodules.

Lymphocytopenia, or lymphopenia, is the condition of having an abnormally low level of lymphocytes in the blood. Lymphocytes are a white blood cell with important functions in the immune system. The opposite is lymphocytosis, which refers to an excessive level of lymphocytes.

Lymphocytopenia may be present as part of a pancytopenia, when the total numbers of all types of blood cells are reduced.

Myelophthisic anemia (or myelophthisis) is a severe type of anemia found in some people with diseases that affect the bone marrow. Myelophthisis refers to the displacement of hemopoietic bone-marrow tissue either by fibrosis, tumors or granulomas. The word comes from the roots "myelo-", which refers to bone marrow, and "phthysis", shrinkage or atrophy.

Myelofibrosis, also known as osteomyelofibrosis, is a relatively rare bone marrow cancer. It is currently classified as a myeloproliferative neoplasm, in which the proliferation of an abnormal clone of hematopoietic stem cells in the bone marrow and other sites results in fibrosis, or the replacement of the marrow with scar tissue.

The term "myelofibrosis" alone usually refers to primary myelofibrosis (PMF), also known as chronic idiopathic myelofibrosis (cIMF); the terms idiopathic and primary mean that in these cases the disease is of unknown or spontaneous origin. This is in contrast with myelofibrosis that develops secondary to polycythemia vera or essential thrombocythaemia. Myelofibrosis is a form of myeloid metaplasia, which refers to a change in cell type in the blood-forming tissue of the bone marrow, and often the two terms are used synonymously. The terms agnogenic myeloid metaplasia and myelofibrosis with myeloid metaplasia (MMM) are also used to refer to primary myelofibrosis.

Diamond–Blackfan anemia (DBA) is a congenital erythroid aplasia that usually presents in infancy. DBA causes low red blood cell counts (anemia), without substantially affecting the other blood components (the platelets and the white blood cells), which are usually normal. This is in contrast to Shwachman–Bodian–Diamond syndrome, in which the bone marrow defect results primarily in neutropenia, and Fanconi anemia, where all cell lines are affected resulting in pancytopenia.

A variety of other congenital abnormalities may also occur in DBA.

Myelophthisis can occur in the setting of chronic myeloproliferative disease (e.g. myelofibrosis), leukemia, lymphoma, and metastatic carcinoma or myeloma. It is common in people who have chronic idiopathic myelofibrosis. It has been linked to small-cell lung cancer, breast cancer or prostate cancer that metastasizes to the bone marrow.

Historically, the most common cause of displacement of healthy bone marrow was tuberculosis.

Currently, the most common cause is displacement of bone marrow by metastatic cancer (extramedullary hematopoiesis tends to be modest). Other causes include myeloproliferative disorders (especially late-stage or spent polycythemia vera), granulomatous diseases, and (lipid) storage diseases. Myelofibrosis can occur in all of these.

Factors that may contribute to decreased RBC production include a decreased quantity of functioning hematopoietic tissue, disordered metabolism related to the underlying disorder, and, in some cases, erythrophagocytosis.

The onset of HLH occurs under the age of 1 year in ~70% of cases. Familial HLH should be suspected if siblings are diagnosed with HLH or if symptoms recur when therapy has been stopped. Each full sibling of a child with familial HLH has a 25% chance of developing the disease, a 50% chance of carrying the defective gene (which is very rarely associated with any risk of disease) and a 25% chance of not being affected and not carrying the gene defect.

Patients with HLH, especially when untreated, may need intensive therapy. Therefore, HLH should be included in the differential diagnosis of ICU (Intensive Care Unit) patients with cytopenia and hyperferritinemia.

HLH clinically manifests with fever, enlargement of the liver and spleen, enlarged lymph nodes, yellow discoloration of the skin and eyes, and a rash.

Bone marrow biopsy shows abnormal megakaryocytes, macrocytic erythropoiesis, and defects in neutrophil production and fibrosis of the marrow (myelofibrosis).

Clinically patients present with reduction in the count of all blood cells (pancytopenia), a very few blasts in the peripheral blood and no or little spleen enlargement (splenomegaly).

Cells are usually CD34 positive.

Hemophagocytic lymphohistiocytosis (HLH), also known as haemophagocytic lymphohistiocytosis (British spelling), and hemophagocytic or haemophagocytic syndrome, is an uncommon hematologic disorder seen more often in children than in adults. It is a life-threatening disease of severe hyperinflammation caused by uncontrolled proliferation of activated lymphocytes and macrophages, characterised by proliferation of morphologically benign lymphocytes and macrophages that secrete high amounts of inflammatory cytokines. It is classified as one of the cytokine storm syndromes.

FA is characterized by bone marrow failure, AML, solid tumors, and developmental abnormalities. Classic features include abnormal thumbs, absent radii, short stature, skin hyperpigmentation, including café au lait spots, abnormal facial features (triangular face, microcephaly), abnormal kidneys, and decreased fertility. Many FA patients (about 30%) do not have any of the classic physical findings, but Diepoxybutane chromosome fragility assay showing increased chromosomal breaks can make the diagnosis. . About 80% of FA will develop bone marrow failure by age 20.

The first sign of a hematologic problem is usually petechiae and bruises, with later onset of pale appearance, feeling tired, and infections. Because macrocytosis usually precedes a low platelet count, patients with typical congenital anomalies associated with FA should be evaluated for an elevated red blood cell mean corpuscular volume.

Fanconi anaemia (FA) is a rare genetic disease resulting in impaired response to DNA damage. Although it is a very rare disorder, study of this and other bone marrow failure syndromes has improved scientific understanding of the mechanisms of normal bone marrow function and development of cancer. Among those affected, the majority develops cancer, most often acute myelogenous leukemia, and 90% develop bone marrow failure (the inability to produce blood cells) by age 40. About 60–75% of people have congenital defects, commonly short stature, abnormalities of the skin, arms, head, eyes, kidneys, and ears, and developmental disabilities. Around 75% of people have some form of endocrine problems, with varying degrees of severity.

FA is the result of a genetic defect in a cluster of proteins responsible for DNA repair.

Treatment with androgens and hematopoietic (blood cell) growth factors can help bone marrow failure temporarily, but the long-term treatment is bone marrow transplant if a donor is available. Because of the genetic defect in DNA repair, cells from people with FA are sensitive to drugs that treat cancer by DNA crosslinking, such as mitomycin C. The typical age of death was 30 years in 2000.

FA occurs in about one per 130,000 births, with a higher frequency in Ashkenazi Jews in Israel and Afrikaners in South Africa. The disease is named after the Swiss pediatrician who originally described this disorder, Guido Fanconi. It should not be confused with Fanconi syndrome, a kidney disorder also named after Fanconi.

The most common symptoms of AEL are related to pancytopenia (a shortage of all types of blood cells), including fatigue, infections, and mucocutaneous bleeding. Almost half of people with AEL exhibit weight loss, fever and night sweats at the time of diagnosis. Almost all people with AEL are anemic, and 77% have a hemoglobin level under 10.0 g/dl. Signs of thrombocytopenia are found in about half of people with AEL.

Controversy remains today whether this disorder is a subtype of acute myeloid leukemia or myelodysplastic syndromes; however, it is currently classified as a form of AML.

Acute megakaryoblastic leukemia (AMKL) is a form of leukemia where a majority of the blasts are megakaryoblastic.

It is classified under AML-M7 category of the French-American-British classification.

The latest WHO classification (2008, Lyon), classifies Acute Myeloid Leukemia into distinct subtypes, based on clinico-pathological and molecular features. Acute megakaryoblastic leukemia is placed under the AML-Not Otherwise Specified subcategory.

Diagnosis requires more than 20% Blasts in the marrow/ peripheral blood with more than 50% demonstrating megakaryocytic derivation by morphology, immunophenotypic or electron microscopic studies.

In hairy cell leukemia, the "hairy cells" (malignant B lymphocytes) accumulate in the bone marrow, interfering with the production of normal white blood cells, red blood cells, and platelets. Consequently, patients may develop infections related to low white blood cell count, anemia and fatigue due to a lack of red blood cells, or easy bleeding due to a low platelet count. Leukemic cells may gather in the spleen and cause it to swell; this can have the side effect of making the person feel full even when he or she has not eaten much.

Hairy cell leukemia is commonly diagnosed after a routine blood count shows unexpectedly low numbers of one or more kinds of normal blood cells, or after unexplained bruises or recurrent infections in an otherwise apparently healthy patient.

Platelet function may be somewhat impaired in HCL patients, although this does not appear to have any significant practical effect. It may result in somewhat more mild bruises than would otherwise be expected for a given platelet count or a mildly increased bleeding time for a minor cut. It is likely the result of producing slightly abnormal platelets in the overstressed bone marrow tissue.

Patients with a high tumor burden may also have somewhat reduced levels of cholesterol, especially in patients with an enlarged spleen. Cholesterol levels return to more normal values with successful treatment of HCL.

The most common symptoms of CAEBV include:

- Fever

- Hepatitis

- Pancytopenia

- Spleen enlargement

- Hypersensitivity to mosquito bites

Complications include:

- Interstitial pneumonia

- Lymphoma, including B-cell, T-cell and NK-cell lymphomas

- Haemophagocytic syndrome

- Coronary artery aneurysms

- Liver failure

- Nasopharyngeal carcinoma

- Gastric adenocarcinoma

- CNS

- Intestinal perforation

- Myocarditis

- Peripheral neuropathy

Acute erythroid leukemia or Di Guglielmo syndrome is a rare form of acute myeloid leukemia (less than 5% of AML cases) where the myeloproliferation is of erythroblastic precursors. It is defined as type "M6" under the FAB classification.

The disease in the lungs is characterized by enlargement of the tracheobronchial lymph nodes and infiltration of the lungs, sometimes leading to lung lobe consolidation and pleural effusion. Signs and symptoms include cough, loss of appetite, weight loss, anemia, and difficulty breathing. Seizures and rear limb weakness can be seen. Invasion of the bone marrow can cause pancytopenia. Diagnosis requires a biopsy.