The blood count typically shows decreased numbers of blood cells—including a decreased amount of circulating red blood cells, white blood cells, and platelets.

The bone marrow may show hemophagocytosis.

The liver function tests are usually elevated. A low level of the protein albumin in the blood is common.

The serum C reactive protein, erythrocyte sedimentation rate, and ferritin level are markedly elevated. In children, a ferritin above 10000 is very sensitive and specific for the diagnosis of HLH, however, the diagnostic utility for ferritin is less for adult HLH patients.

The serum fibrinogen level is usually low and the D-dimer level is elevated.

The sphingomyelinase is elevated.

Bone marrow biopsy shows histiocytosis.

The current (2008) diagnostic criteria for HLH are

1. A molecular diagnosis consistent with HLH. These include the identification of pathologic mutations of PRF1, UNC13D, or STX11.

OR

2. Fulfillment of five out of the eight criteria below:

- Fever (defined as a temperature >100.4 °F, >38 °C)

- Enlargement of the spleen

- Decreased blood cell counts affecting at least two of three lineages in the peripheral blood:

- Haemoglobin <9 g/100 ml (in infants <4 weeks: haemoglobin <10 g/100 ml) (anemia)

- Platelets <100×10/L (thrombocytopenia)

- Neutrophils <1×10/L (neutropenia

- High blood levels of triglycerides (fasting, greater than or equal to 265 mg/100 ml) and/or decreased amounts of fibrinogen in the blood (≤ 150 mg/100 ml)

- Ferritin ≥ 500 ng/ml

- Haemophagocytosis in the bone marrow, spleen or lymph nodes

- Low or absent natural killer cell activity

- Soluble CD25 (soluble IL-2 receptor) >2400 U/ml (or per local reference laboratory)

In addition, in the case of familial HLH, no evidence of malignancy should be apparent.

It should be noted that not all five out of eight criteria are required for diagnosis of HLH in adults, and a high index of suspicion is required for diagnosis as delays results in increased mortality. The diagnostic criteria were developed in pediatric populations and have not been validated for adult HLH patients. Attempts to improve diagnosis of HLH have included use of the HScore, which can be used to estimate an individual's risk of HLH.

Typically, a diagnosis of DBA is made through a blood count and a bone marrow biopsy.

A diagnosis of DBA is made on the basis of anemia, low reticulocyte (immature red blood cells) counts, and diminished erythroid precursors in bone marrow. Features that support a diagnosis of DBA include the presence of congenital abnormalities, macrocytosis, elevated fetal hemoglobin, and elevated adenosine deaminase levels in red blood cells.

Most patients are diagnosed in the first two years of life. However, some mildly affected individuals only receive attention after a more severely affected family member is identified.About 20–25% of DBA patients may be identified with a genetic test for mutations in the RPS19 gene.

The only treatment for Omenn syndrome is chemotherapy followed by a bone marrow transplantation. Without treatment, it is rapidly fatal in infancy.

Treatment consists of frequent blood transfusions and chelation therapy. Potential cures include bone marrow transplantation and gene therapy.

All beta thalassemias may exhibit abnormal red blood cells, a family history is followed by DNA analysis. This test is used to investigate deletions and mutations in the alpha- and beta-globin-producing genes. Family studies can be done to evaluate carrier status and the types of mutations present in other family members. DNA testing is not routine, but can help diagnose thalassemia and determine carrier status. In most cases the treating physician uses a clinical prediagnosis assessing anemia symptoms: fatigue, breathlessness and poor exercise tolerance. Further genetic analysis may include HPLC should routine electrophoresis prove difficult.

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 diagnosis is generally suspected when patients from certain ethnic groups (see epidemiology) develop anemia, jaundice and symptoms of hemolysis after challenges from any of the above causes, especially when there is a positive family history.

Generally, tests will include:

- Complete blood count and reticulocyte count; in active G6PD deficiency, Heinz bodies can be seen in red blood cells on a blood film;

- Liver enzymes (to exclude other causes of jaundice);

- Lactate dehydrogenase (elevated in hemolysis and a marker of hemolytic severity)

- Haptoglobin (decreased in hemolysis);

- A "direct antiglobulin test" (Coombs' test) – this should be negative, as hemolysis in G6PD is not immune-mediated;

When there are sufficient grounds to suspect G6PD, a direct test for G6PD is the "Beutler fluorescent spot test", which has largely replaced an older test (the Motulsky dye-decolouration test). Other possibilities are direct DNA testing and/or sequencing of the G6PD gene.

The "Beutler fluorescent spot test" is a rapid and inexpensive test that visually identifies NADPH produced by G6PD under ultraviolet light. When the blood spot does not fluoresce, the test is positive; it can be falsely negative in patients who are actively hemolysing. It can therefore only be done 2–3 weeks after a hemolytic episode.

When a macrophage in the spleen identifies a RBC with a Heinz body, it removes the precipitate and a small piece of the membrane, leading to characteristic "bite cells". However, if a large number of Heinz bodies are produced, as in the case of G6PD deficiency, some Heinz bodies will nonetheless be visible when viewing RBCs that have been stained with crystal violet. This easy and inexpensive test can lead to an initial presumption of G6PD deficiency, which can be confirmed with the other tests.

Hydroxycarbamide and anagrelide are contraindicated during pregnancy and nursing. Essential thrombocytosis can be linked with a three-fold increase in risk of miscarriage. Throughout pregnancy, close monitoring of the mother and fetus is recommended. Low-dose low molecular weight heparin (e.g. enoxaparin) may be used. For life-threatening complications, the platelet count can be reduced rapidly using platelet apheresis, a procedure that removes platelets from the blood and returns the remainder to the patient.

Corticosteroids can be used to treat anemia in DBA. In a large study of 225 patients, 82% initially responded to this therapy, although many side effects were noted. Some patients remained responsive to steroids, while efficacy waned in others. Blood transfusions can also be used to treat severe anemia in DBA. Periods of remission may occur, during which transfusions and steroid treatments are not required. Bone marrow transplantation (BMT) can cure hematological aspects of DBA. This option may be considered when patients become transfusion-dependent because frequent transfusions can lead to iron overloading and organ damage. However, adverse events from BMTs may exceed those from iron overloading. A 2007 study showed the efficacy of leucine and isoleucine supplementation in one patient. Larger studies are being conducted.

Abdominal pain due to hypersplenism and splenic infarction and right-upper quadrant pain caused by gallstones are major clinical manifestations. However, diagnosing thalassemiæ from symptoms alone is inadequate. Physicians note these signs as associative due to this disease's complexity. The following associative signs can attest to the severity of the phenotype: pallor, poor growth, inadequate food intake, splenomegaly, jaundice, maxillary hyperplasia, dental malocclusion, cholelithiasis, systolic ejection murmur in the presence of severe anemia and pathologic fractures. Based on symptoms, tests are ordered for a differential diagnosis. These tests include complete blood count; hemoglobin electrophoresis; serum transferrin, ferritin, total iron-binding capacity; urine urobilin and urobilogen; peripheral blood smear, which may show codocytes, or target cells; hematocrit; and serum bilirubin.

The gold standard for the diagnosis of Vitamin B deficiency is a low blood level of Vitamin B. A low level of blood Vitamin B is a finding that normally can and should be treated by injections, supplementation, or dietary or lifestyle advice, but it is not a diagnosis. Hypovitaminosis B can result from a number of mechanisms, including those listed above. For determination of cause, further patient history, testing, and empirical therapy may be clinically indicated.

A measurement of methylmalonic acid (methylmalonate) can provide an indirect method for partially differentiating Vitamin B and folate deficiencies. The level of methylmalonic acid is not elevated in folic acid deficiency. Direct measurement of blood cobalamin remains the gold standard because the test for elevated methylmalonic acid is not specific enough. Vitamin B is one necessary prosthetic group to the enzyme methylmalonyl-coenzyme A mutase. Vitamin B deficiency is but one among the conditions that can lead to dysfunction of this enzyme and a buildup of its substrate, methylmalonic acid, the elevated level of which can be detected in the urine and blood.

Due to the lack of available radioactive Vitamin B, the Schilling test is now largely a historical artifact. The Schilling test was performed in the past to help determine the nature of the vitamin B deficiency. An advantage of the Schilling test was that it often included Vitamin B with intrinsic factor.

6-phosphogluconate dehydrogenase (6PGD) deficiency has similar symptoms and is often mistaken for G6PD deficiency, as the affected enzyme is within the same pathway, however these diseases are not linked and can be found within the same person.

The following revised diagnostic criteria for essential thrombocythaemia were proposed in 2005. The diagnosis requires the presence of both A criteria together with B3 to B6, or of criterion A1 together with B1 to B6. The criteria are as follows:

- A1. Platelet count > 450 × 10/µL for at least 2 months.

- A2. Acquired V617F JAK2 mutation present

- B1. No cause for a reactive thrombocytosis

- normal inflammatory indices

- B2. No evidence of iron deficiency

- stainable iron in the bone marrow or normal red cell mean corpuscular volume

- B3. No evidence of polycythemia vera

- hematocrit < midpoint of normal range or normal red cell mass in presence of normal iron stores

- B4. No evidence of chronic myeloid leukemia

- But the Philadelphia chromosome may be present in up to 10% of cases. Patients with the Philadelphia chromosome have a potential for the development of acute leukemia, especially acute lymphocytic leukemia.

- B5. No evidence of myelofibrosis

- no collagen fibrosis and ≤ grade 2 reticulin fibrosis (using 0–4 scale)

- B6. No evidence of a myelodysplastic syndrome

- no significant dysplasia

- no cytogenetic abnormalities suggestive of myelodysplasia

CDA type IV is characterized by mild to moderate splenomegaly. Hemoglobin is very low and patients are transfusion dependent. MCV is normal or mildly elevated. Erythropoiesis is normoblastic or mildly to moderately megaloblastic. Nonspecific erythroblast dysplasia is present.

Omenn syndrome is an autosomal recessive severe combined immunodeficiency associated with hypomorphic missense mutations in immunologically relevant genes of T-cells (and B-cells) such as recombination activating genes (RAG1 and RAG2), IL-7 Receptor α gene (IL7Rα), DCLRE1C-Artemis, RMRP-CHH, DNA-Ligase IV, common gamma chain, WHN-FOXN1, ZAP-70 and complete DiGeorge anomaly (DiGeorge Syndrome; CHARGE).

Once a diagnosis is made, each individual's treatment is based on an individual’s clinical condition. Hematopoietic stem cell transplant is a possible treatment of this condition but its effectiveness is unproven.

Additionally, magnesium supplementation is a promising potential treatment for XMEN. One of the consequences of loss of "MAGT1" function is a decreased level of unbound intracellular Mg2+. This decrease leads to loss of expression of an immune cell receptor called "NKG2D", which is involved in EBV-immunity. Remarkably, Mg2+ supplementation can restore "NKG2D" expression and other functions that are abnormal in patients with XMEN. Early evidence suggests continuous oral magnesium threonate supplementation is safe and well tolerated. Nonetheless, further research is needed to evaluate the use of Mg2+ as a treatment for XMEN. It remains unclear if such supplementation will protect against the development of lymphoma in patients with XMEN. Investigators at the National Institute of Allergy and Infectious Diseases at the US National Institutes of Health currently have clinical protocols to study new approaches to the diagnosis and treatment of this disorder.

McLeod syndrome is one of only a few disorders in which acanthocytes may be found on the peripheral blood smear. Blood evaluation may show signs of hemolytic anemia. Elevated creatine kinase can be seen with myopathy in McLeod syndrome.

The first line of therapy is androgens and hematopoietic growth factors, but only 50-75% of patients respond. A more permanent cure is hematopoietic stem cell transplantation. If no potential donors exist, a savior sibling can be conceived by preimplantation genetic diagnosis (PGD) to match the recipient's HLA type.

Hereditary spherocytosis is the most common disorder of the red cell membrane and affects 1 in 2,000 people of Northern European ancestry. According to Harrison's Principles of Internal Medicine, the frequency is at least 1 in 5,000.

Experimental gene therapy exists to treat hereditary spherocytosis in lab mice; however, this treatment has not yet been tried on humans due to all of the risks involved in human gene therapy.

A typical patient with severe McLeod syndrome that begins in adulthood lives for an additional 5 to 10 years. Patients with cardiomyopathy have elevated risk for congestive heart failure and sudden cardiac death. The prognosis for a normal life span is often good in some patients with mild neurological or cardiac sequelae.

Physical exam findings are non-specific, but may include enlarged liver or spleen, plethora, or gouty nodules. The diagnosis is often suspected on the basis of laboratory tests. Common findings include an elevated hemoglobin level and hematocrit, reflecting the increased number of red blood cells; the platelet count or white blood cell count may also be increased. The erythrocyte sedimentation rate (ESR) is decreased due to a decrease in zeta potential. Because polycythemia vera results from an essential decrease in erythrocyte production, patients have a low erythropoietin (EPO) level.

In primary polycythemia, there may be 8 to 9 million and occasionally 11 million erythrocytes per cubic millimeter of blood (a normal range for adults is 4-6), and the hematocrit may be as high as 70 to 80%. In addition, the total blood volume sometimes increases to as much as twice normal. The entire vascular system can become markedly engorged with blood, and circulation times for blood throughout the body can increase up to twice the normal value. The increased numbers of erythrocytes can cause the viscosity of the blood to increase as much as five times normal. Capillaries can become plugged by the very viscous blood, and the flow of blood through the vessels tends to be extremely sluggish.

As a consequence of the above, people with untreated polycythemia vera are at a risk of various thrombotic events (deep venous thrombosis, pulmonary embolism), heart attack and stroke, and have a substantial risk of Budd-Chiari syndrome (hepatic vein thrombosis), or myelofibrosis. The condition is considered chronic; no cure exists. Symptomatic treatment (see below) can normalize the blood count and most patients can live a normal life for years.

The disease appears more common in Jews of European extraction than in most non-Jewish populations. Some familial forms of polycythemia vera are noted, but the mode of inheritance is not clear.

A mutation in the JAK2 kinase (V617F) is strongly associated with polycythemia vera. "JAK2" is a member of the Janus kinase family and makes the erythroid precursors hypersensitive to erythropoietin (EPO). This mutation may be helpful in making a diagnosis or as a target for future therapy.

Following history and examination, the British Committee for Standards in Haematology (BCSH) recommend the following tests are performed:

- full blood count/film (raised haematocrit; neutrophils, basophils, platelets raised in half of patients)

- JAK2 mutation

- serum ferritin

- renal and liver function tests

If the JAK2 mutation is negative and there is no obvious secondary causes the BCSH suggest the following tests:

- red cell mass

- arterial oxygen saturation

- abdominal ultrasound

- serum erythropoietin level

- bone marrow aspirate and trephine

- cytogenetic analysis

- erythroid burst-forming unit (BFU-E) culture

Other features that may be seen in polycythemia vera include a low ESR and a raised leukocyte alkaline phosphatase.

The diagnostic criteria for polycythemia vera have recently been updated by the BCSH. This replaces the previous Polycythemia Vera Study Group criteria.

JAK2-positive polycythaemia vera - diagnosis requires both criteria to be present:

JAK2-negative polycythemia vera - diagnosis requires A1 + A2 + A3 + either another A or two B criteria:

Conventionally, a leukocytosis exceeding 50,000 WBC/mm with a significant increase in early neutrophil precursors is referred to as a leukemoid reaction. The peripheral blood smear may show myelocytes, metamyelocytes, promyelocytes, and rarely myeloblasts; however, there is a mix of early mature neutrophil precursors, in contrast to the immature forms typically seen in acute leukemia. Serum leukocyte alkaline phosphatase is normal or elevated in leukemoid reaction, but is depressed in chronic myelogenous leukemia. The bone marrow in a leukemoid reaction, if examined, may be hypercellular but is otherwise typically unremarkable.

Leukemoid reactions are generally benign and are not dangerous in and of themselves, although they are often a response to a significant disease state (see "Causes" below). However, leukemoid reactions can resemble more serious conditions such as chronic myelogenous leukemia (CML), which can present with identical findings on peripheral blood smear.

Historically, various clues including the leukocyte alkaline phosphatase score and the presence of basophilia were used to distinguish CML from a leukemoid reaction. However, at present the test of choice in adults to distinguish CML is an assay for the presence of the Philadelphia chromosome, either via cytogenetics and FISH, or via PCR for the BCR/ABL fusion gene. The LAP (Leukocyte Alkaline Phosphatase) score is high in reactive states but is low in CML. In cases where the diagnosis is uncertain, a qualified hematologist or oncologist should be consulted.

There was a study on a three year old that was a carrier of the hemoglobin variant of Hopkins-2. The child had mild anemia and reticulocytosis, which is commonly seen in anemia. There were, however, no sickled cells found in the blood and they had no symptoms relating to sickle cell. There was also a reduced mean corpuscular volume (MCV), which is the average volume of red blood cell count.