Deciding what is normal and interpreting blood counts

Question 1

Distinguish between the terms ‘reference range’ and ‘normal range’

Answer 1

A reference range is derived from a defined reference population (healthy volunteers with defined characteristics) whereas a normal range is a more vague concept.
They are both 95% ranges

Question 2

State some factors that affect what is ‘normal’

Answer 2

Age
Gender
Ethnic origin
Physiological status
Altitude
Nutritional status
Cigarette smoking
Alcohol intake

Question 3

Hb concentration shows what kind of distribution? Hence, how may it be analysed?

Answer 3

Normal/Gaussian distribution

• It can be analysed with mean and standard deviation:
  95% range = mean ± 2SD

Question 4

White blood cell count shows what kind of distribution?

Answer 4

Non-normal/non-Gaussian distribution (more upper extremes)

Question 5

State an important consideration when comparing a result to the reference range

Answer 5

Not all results outside the reference range are abnormal

Not all results within the normal range are normal

Question 6

How is a WBC, RBC and platelet count obtained? State each of their units of measurement

Answer 6

Counted by large automated instruments, which count electronic impulses generated when cells flow between a light source and a sensor or when cells flow through an electrical field
WBC - (× 10^9/l)
RBC - (× 10^12/l)
Platelet count - (× 10^9/l)

Question 7

How is Hb concentration sometimes measured? State its unit of measurement

Answer 7

Measured in a spectrometer:
- Hb was converted to a stable form (e.g. cyanmethaemoglobin) and its light absorption was measured at a specific wave length

(g/l)

Question 8

How did a haematocrit (Hct) used to be measured?

Answer 8

By centrifuging a blood sample and measuring the packed cell volume as a proportion/percentage

Question 9

How may MCV, MCH and MCHC be measured/calculated? State each of their units of measurement

Answer 9

• MCV is now determined indirectly by light scattering or by interruption of an electrical field/ = Hct divided by RBC. Units: fL
• MCH = Hb divided by RBC. Units: pg
• MCHC is measured electronically, i.e. using light scattering/ = Hb divided by the Hct. Units g/l

Question 10

What is the difference between MCH and MCHC

Answer 10

MCH = absolute amount of Hb in an individual RBC; So, in microcytic and macrocytic anaemias, MCH follows MCV

MCHC is the concentration of haemoglobin in a red cell

Question 11

Briefly outline the method of interpreting a blood count

Answer 11

Is there leucocytosis or leucopenia? Which cell line?

Is there anaemia? Microcytic/Macrocytic?

Is there thrombocytosis or thrombocytopenia?

Check for reasons in the clinical history

Look at blood film

Question 12

What is polycythaemia, and summarise the potential causes?

Answer 12

Abnormally high RBC

• Reduced plasma volume (pseudo)
• Blood doping or over transfusion
• Appropriately increased erythropoietin, e.g. living at high altitude (hypoxic)
• Inappropriate erythropoietin synthesis or use, e.g. self administered, renal or liver tumour
• Cause independent of erythropoietin

Question 13

How is polycythaemia diagnosed and evaluated?

Answer 13

• Take history
• Do physical exam - signs include splenomegaly, abdominal mass or cyanosis (bluish cast to skin)
• Hb and Hct would also be high (but note the inherent differences between adults and kids, and males and females)

Question 14

What is polycythaemia vera? State the cause and describe the signs

Answer 14

Inappropriately increased erythropoiesis independent of erythropoietin.
Caused by a myeloproliferative neoplasm (bone marrow disorder)

• Can lead to blood hyper viscosity => vascular obstruction
• The increases number of red blood cells makes your spleen work harder than normal, which causes it to enlarge (splenomegaly)