Overview of haemoglobin, haematocrit, RBC indices

An increased number of RBCs causes haemoglobin and haematocrit to increase, and once dehydration and diuretic therapy are excluded, may indicate polycythaemia. Polycythaemia can be due to overproduction of RBCs or a loss of plasma volume causing an overall concentration in the volume of RBCs.

An abnormally low haemoglobin, haematocrit or RBC count indicates anaemia. Evaluation of red blood cell indices is helpful in determining the cause. Anaemia can be caused by defective RBCs, decreased production of RBCs, increased loss, or destruction. MCV is useful in classifying it as normocytic, microcytic or macrocytic. The most common cause of microcytic anaemia is iron deficiency. The absence of iron remains the definite test for differentiating iron deficiency from other microcytic states, such as anaemia of chronic disease, thalassaemia and sideroblastic anaemia.

Normocytic anaemia can occur after acute blood loss, with anaemia of chronic disease, haemolytic anaemia, hypothyroidism, bone marrow failure, renal failure, aplastic anaemias or pregnancy.

Mixed anaemias can develop in conditions where absorption from the gut is impaired such as coeliac disease leading to both iron (microcytic) and vitamin B12 (macrocytic anaemia).

Macrocytic anaemia is mainly due to alcoholism, vitamin B12 and folate deficiency, blood disorders (aplastic anaemia, myelodysplastic syndromes, myeloid leukaemia or medication induced).

Haematocrit is determined by the number of RBC and volume of plasma, a raised Hct can result in increased blood viscosity. Severe increases can induce hyperviscosity syndrome causing thrombus formation.

WBC differential

An elevated WCC count, leukocytosis, can have several underlying factors, the most common being infection, inflammatory response, post surgery, burns or trauma, or as a result of steroid use. Acute leukaemias can also present with an acute elevation. Chronically elevated WCC can occur with long standing infection, smoking or pregnancy, or due to myeloproliferative and lymphoproliferative disorders.

Low levels of WCC, leukopaenia, have many underlying causes which depend on the rate of reduction and type of WBC count involved. Infection, medication (chemotherapy, immunosuppressants, anti epileptics), B12, folate or iron deficiency, autoimmune disease or HIV.

Neutrophils account for 40 – 60% of total white blood cells. Neutrophilia most commonly occurs as a result of bacterial infection, inflammation, myeloproliferative disorders or steroid use. Neutropaenia occurs when neutrophils drop below < 1.5 x 10/L. Febrile neutropaenia in the setting of infection warrants in hospital assessment and IV antibiotic treatment.

Lymphocytes account for 20 – 40% of total WBCs in the blood. High levels typically occur as a result of acute viral infections,  however sometimes it may be due to underlying malignancy, lymphoma or lymphoproliferative disorder such as CLL. Low levels occur and are usually transient, occurring with steroid use, SLE, uraemia, HIV infection, immunosuppressants, renal failure and bone marrow disorders.

Monocytes account for approximately 5% of total WBCs in the blood and are associated with immune response, increasing with chronic infection like TB, malaria, brucellosis, malignancy (AML & Hodgkins disease), steroid use or post chemo/radiotherapy.

Eosinophils account for approximately 2% of total WBCs, increased levels are most commonly associated with allergy/atopy, parasitic infections, vasculitis or malignancy.

Basophils account for 1% of total WBCs and like eosinophils, have a role in allergic, atopic, asthmatic conditions or IgE mediated hypersensitivity reactions and parasitic infections. An increased number of basophils, termed basophilia, can occur in myeloproliferative disorders like chronic myeloid leukemia and polycythemia vera.

Blasts are immature cells found in the bone marrow. A release of these premature cells from the bone marrow is abnormal indicating haematological malignancy such as leukaemia or myeloproliferative disorders.

Platelets

Thrombocytopaenia, or low platelet count, occurs as a result of infection or bleeding, acute viral infection, medications such as immunosuppressants, antiepileptics, cytotoxic agents, due to DIC, HIT or ITP or in preeclamptic mothers with HELLP syndrome.

Chronically low counts occur as a result of liver cirrhosis, hypersplenism, excess alcohol intake, medications, ITP, B12/folate deficiency, HIV, hepatitis B/C, bone marrow failure.

A raised count is known as thrombocytosis and can be due to infection, inflammation, after blood loss, iron deficiency states, post splenectomy, myeloproliferative disorders.