2. Deciding what is normal and interpreting a blood count

Question 1

What can “normal” ranges be affected by?

Answer 1

• Age, gender, ethnicity
• Physiological status e.g. pregnancy
• Altitude
• Nutritional status
• Cigarette smoking, alcohol

Question 2

How is a normal or reference range determined?

Answer 2

• Reference - derived from a carefully defined reference population
• Normal - represent people that live in the local area and come to the hospital

Question 3

What is an appropriate statistical technique for a reference range?

Answer 3

• Data with normal (Gaussian) distribution - determine the mean and SD
• Mean ± 2SD as the 95% range

Question 4

Does Hb and WBC show Gaussian distribution?

Answer 4

Hb does but WBC doesn’t

Question 5

Are all results within the normal range normal?

Answer 5

No, serum lipids in the upper end of this range are common in Western populations - still bad

Question 6

Why is a 95% range used to determine “normal” levels?

Answer 6

• Ideally, we would like a laboratory test where the normal and abnormal results are clearly different
• Realistically, there will be a small overlap - compensated for with 95% range rather than 100%

Question 7

How are WBC, RBC and platelet counts carried out?

Answer 7

• Initially counted visually using a microscope
• Now done using large automated instruments
• EDTA used as an anti-coagulant

Question 8

How is Hb measured?

Answer 8

• Initially measure in a spectrometer

* Now measured by an automated instrument (same principle)

Question 9

What is MCH and MCHC?

Answer 9

• Mean cell haemoglobin - amount of Hb in given volume of blood / number of RBCs in same volume … or Hb/RBC
• Mean cell haemoglobin concentration - amount of Hb in given volume / proportion of sample represented by RBCs … or Hb/(PCV/haematocrit)

Question 10

What’s the difference between MCH and MCHC?

Answer 10

• MCH relates to the size of the cell - absolute amount of Hb, tends to be parallel to MCV in micro/macrocytic anaemia
• MCHC relates to the concentration of Hb in the cell - also related to the shape of the cell
• Iron deficiency - low MCH and low MCHC
• Red cell fragmentation - low MCH, normal MCHC

Question 11

How can the MCHC be measured?

Answer 11

• Measure electronically

* On the basis of light scattering

Question 12

What is polycythaemia?

Answer 12

• Too many RBCs in circulation

* Hb, RBC and PVC/Hct are all increased

Question 13

How do you evaluate polycythaemia?

Answer 13

• Clinical history
• Physical examination - splenomegaly, abdominal mass, chronic lung disease or cyanosis
• Compare with appropriate normal range - relevant to age of patient

Question 14

What is pseudopolycythaemia?

Answer 14

• High Hb, RBC and PCV/Hct resulting from a decrease in plasma volume
• Not an increase in the number of circulating RBCs
• aka polycythaemia

Question 15

What are the causes of polycythaemia?

Answer 15

• Blood doping/illicit erythropoietin - blood transfused to improve athletic performance
• Medical negligence - administered too much blood, Hb rises, hypertension
• High erythropoietin
- appropriate elevation
- causes cyanosis and finger clubbing - features of hypoxia
- inappropriate elevation - renal tumour
- people living at high altitudes have mutations that reduce erythropoietin production
• Abnormal function of the bone marrow
• Myeloproliferative neoplasm - mutant stem cell producing too many RBCs

Question 16

What is polycythaemia vera?

Answer 16

• True polycythaemia
• Intrinsic bone marrow disorder - independent of erythropoietin
• A myeloproliferative neoplasm
• Leads to thick blood
• Can cause vascular obstruction
• Very red hands and peripheral gangrene