The first step in the identification of red blood cell pathology is to look at the haemoglobin, haematocrit and red cell count. A reduction in each of these indices is used to diagnose anaemia, while an increase in these values is used to diagnose erythrocytosis.

Haemoglobin (Hb) is a protein within red blood cells which is composed of a haem molecule and four subunits. Haemoglobin transports oxygen and other molecules in blood. This is the most commonly used marker of anaemia.

The haematocrit (Hct), also known as the packed cell volume (PCV), is the volume of red blood cells as a percentage of total blood volume. This value is often calculated by an automatic analyser based on the red cell count and mean cell volume (MCV).

The red cell count reflects the number of circulating red blood cells. The red cell count is particularly useful in identifying erythrocytosis; a normal red cell count with elevated haemoglobin / haematocrit suggests relative erythrocytosis (e.g. dehydration), while an elevated red cell count suggests absolute erythrocytosis (e.g. polycythaemia vera).

The mean cell volume (MCV) is a measure of the average volume of a red blood cell - whether the cells are microcytic, normocytic or macrocytic. 

The mean corpuscular haemoglobin (MCH) is the average amount of haemoglobin per red blood cell, while the mean corpuscular haemoglobin concentration (MCHC) refers to the average concentration of haemoglobin within red blood cells - these two measures are used to determine whether red cells are hypochromic or hyperchromic.

A microcytic, hypochromic anaemia contains predominantly small and pale cells. This is classically a sign of iron deficiency anaemia, however can also occur with thalassaemia, lead poisoning, congenital sidroblastic anaemia and hyperthyroidism.

A normocytic, normochromic anaemia is where the red blood cells are normal in size and colour. This may be dilutional or occur in the context of acute haemorrhage, chronic kidney disease, bone marrow pathology or haemolytic anaemia.

A macrocytic, hyperchromic anaemia refers to large red cells with loss of central pallor. This is classically a sign of megaloblastic anaemia (B12 / folate deficiency or drugs), though may also occur with myelodysplastic syndrome, alcohol abuse and severe hypothyroidism.

An elevated RDW is referred to as anisocytosis, and indicates that red blood cells are a wide range of sizes. This can suggest a developing microcytic / macrocytic pathology such as early iron, B12 or folate deficiency; it may also be seen in haemolysis or following blood transfusion.

Reticulocytes are non-nucleated immature red blood cells. They are not normally present in the circulation in high numbers, though may be seen if the bone marrow is producing large numbers of red cells.

Elevated reticulocytes are indicative of compensatory erythropoiesis due to haemolysis, recovery from anaemia or increased red blood cell requirement. Near absence of reticulocytes is caused by bone marrow suppression, such as due to aplastic anaemia or cytotoxic chemotherapy.

Leukocytes are a variety of cells of myeloid and lymphoid lineage that perform a variety of immune functions.

Leukocytosis refers to an increase in circulating white blood cells. This may be due to infection, inflammation, malignancy or certain other conditions. Leukopaenia refers to a reduction in circulating white blood cells; this is almost always caused by neutropaenia, lymphopaenia, or both.

An increase in neutrophils is known as neutrophilia. This is commonly caused by acute infection (particularly bacterial), though is also seen with inflammation, myeloproliferative neoplasms, corticosteroid administration and with heavy smoking. Pseudoneutrophilia is where there is shift of granulocytes into circulation without an increase in the total number of neutrophils - this can be seen in situations such as vigorous exercise and emotional stress.

A reduction in neutrophils is referred to as neutropaenia. This may be congenital or be acquired due to viral infection, autoimmune conditions, bone marrow pathology, dietary deficiency and certain drugs.

Lymphocytosis is an increase in circulating lymphocytes. This may occur in the context of acute viral infection, adrenocortical insufficiency, post splenectomy, chronic tuberculosis or chronic lymphocytic leukaemia (CLL).

Lymphopaenia refers to a reduction in the number of circulating lymphocytes. This is seen with primary immunodeficiency syndromes, certain infections, certain medications, bone marrow pathology and with dietary deficiency.

Platelets are very small non-nucleated cells that derive from fragmentation of megakaryocytes. They play an important role in clot formation and haemostasis.

Thrombocytosis is an increase in circulating platelets. Primary thrombocytosis is seen with myeloproliferative neoplasms such as essential thrombocytosis or CML, while secondary thrombocytosis may occur in reaction to infection, inflammation or malignancy.

Thrombocytopaenia is a reduction in platelets. This may occur within the context of bone marrow failure, suppression or infiltration; dietary deficiency; splenomegaly (e.g. due to cirrhosis); viral infections and immune-mediated thrombocytopaenias (e.g. ITP, HIT, DIC, TTP, HUS). Thrombocytopaenia may also be congenital.