Haematology lab tests

Haemoglobin

• Reported as the concentration of haemoglobin in whole blood
• Preferred to haematocrit for calculating anaemia because it is directly determined, whereas haematocrit is calculated from other values

Haematocrit

• Also called packed cell volume
• The percentage of blood volume occupied by RBCs
• Usually calculated as RBC count x MCV

RBC count

• The number of RBCs contained in a specified volume of whole blood, usually expressed as millions of cells per microL of whole blood
• Unreliable in microcytosis

Mean corpuscular volume (MCV)

• Average volume of RBC

Mean corpuscular haemoglobin

• Average Hb content in an RBC
• Hypochromia on the blood smear means low MCH - enlarged area of central pallor in RBCs - seen in iron deficiency and thalassaemia

Mean corpuscular haemoglobin concentration (MCHC)

• Average Hb concentration per RBC
• Very low values are typical of IDA
• Very high values reflect spherocytosis or RBC agglutination

Red cell distribution width (RDW)

• Measure of the variation in RBC size
• Reflected in the degree of anisocytosis on the peripheral blood smear
• High RDW implies large variation in RBC size - seen in IDA, B12 or folate deficiency, MDS, haemoglobinopathies, and patients who have received transfusions

Reticulocytes

• The stage in RBC development directly before the mature RBC. Continually produced to replace RBCs cleared from circulation.
• Reticulocyte count reflects the rate of RBC production.
• Reported as a percentage of total RBCs or as an absolute count
• At steady state, should be 1-2% or 25,000 to 1000,000 per microL
• Expected to be high in anaemia - a normal BM can increase the rate of RBC production 5x in adults, 7-8x in children

Lactate dehydrogenase (LDH)

• Abundant in RBCs - high levels can be a marker of haemolysis
• Can be elevated in MI, peaking 3-4 days later
• Reflects overall rate of tissue turnover in the body

Bilirubin

• Created as part of the heme recycling process

Haptoglobin

• Created in liver, binds to haemoglobin breakdown products, then the complex is metabolised in the liver
• Low levels means highly bound to haemoglobin
• Low haptoglobin is likely to be due to haemolysis (specificity of level <25mg/dL is 96%)
• Undetectable almost always due to haemolysis
• Normal or increased haptoglobin does not eliminate haemolysis