Deciding what is normal and interpreting blood counts

Question 1

What is the relationship between altitude and normal ranges of Hb?

Answer 1

The higher the altitude, the higher the Hb

Question 2

How are reference and normal ranges determined and what’s the differences?

Answer 2

• Reference ranges are determined from carefully defined reference populations, healthy with defined characteristics
• Normal ranges is a vaguer concept - should represent people that live in the local area and come to the local hospital. Normal DOES not mean healthy e.g. UK high cholesterol is normal but unhealthy

Question 3

1) How can you determine ‘normal’ ranges with data following a Gaussian distribution?
2) Give an example of a data set following Gaussian distribution

Answer 3

1)

Take the mean and then do 2 S.D. on either side - this covers 95% of the data

2) Hb

Question 4

Give one example of a data set which does not follow Gaussian distribution

Answer 4

WBC count

Question 5

Give some caveats to determining a healthy range using the manipulation of data that follows Gaussian distribution and therefore suggest a good alternative

Answer 5

1. Not all results outside the reference range are abnormal and not all results within the normal range are normal
2. A result deemed ‘normal’ by this determination may still be unhealthy

• A health-related range may be more meaningful

Question 6

What is WBC and what is the units for its measurement?

Answer 6

• White blood cell count
• x10⁹/L

Question 7

What is RBC and what is the units for its measurement?

Answer 7

• Red blood cell count
• x10¹²/L

Question 8

What is Hb and what is the units for its measurement?

Answer 8

• Haemoglobin concentration
• g/L

Question 9

What is PCV and what is the units for its measurement?

Answer 9

• Packed cell volume - the proportion of cells in the blood
• l/l
• As a fraction or percentage

Question 10

What is Hct and what is the units for its measurement?

Answer 10

• Haematocrit - the ratio of RBC to blood
• l/l

Question 11

What is the unit for measurement of platelet count?

Answer 11

10⁹/L

Question 12

How are WBC, RBC and platelet count measured?

Answer 12

Automated machine counters - either through counting the number of electrical impulses generated when cells flow between a light source and a sensor or when cells flow through an electrical field

Question 13

How was Hb initially measured and how is it measured now?

Answer 13

• Initially by spectrophotometry - measuring light absorption at various wavelengths
• Now automated

Question 14

How were Hct and PCV initially measured?

Answer 14

Centrifugation

Question 15

How was MCV initially measured and how is it measured now?

Answer 15

• Initially measured by dividing the total volume of red cells by the number of red cells (PCV / RBC)
• Now determined indirectly by light scattering or interruption of an electrical field

Question 16

You can only measure MCV in a blood film if you what?

Answer 16

If you compare to a reference leukocyte

Question 17

Give a mathematical formula for MCH

Answer 17

MCH = Hb / RBC

Question 18

Give a mathematical formula for MCHC

Answer 18

MCHC = Hb / Hct

Question 19

How do cells with low MCHC appear?

Answer 19

Less red

Question 20

What is MCH and what are its units of measurement?

Answer 20

• The absolute amount of Hb in an individual RBC (Hb/RBC)
• picograms (x10^-12)

Question 21

What is the unit of measurement for MCV?

Answer 21

• Femtolitres (10^-15)

Question 22

What is MCHC, what does it depend on and what are the units of measurement for MCHC?

Answer 22

• Mean cell haemoglobin concentration - this is the concentration of Hb in a cell
• MCHC is related to the shape of the cell
• g/L

Question 23

1) What is polycythaemia?
2) Some markers that indicate polycythaemia?

Answer 23

• Too many RBCs in circulation
• High Hb, RBC, Hct

Question 24

What is pseudopolycythaemia and what is the difference between this and true polycythaemia?

Answer 24

• Pseudopolycythaemia - decrease in plasma volume - so elevated Hb, RBC, Hct as in polycythaemia
• In true polycythaemia, there is actual increase in number of circulating RBCs

Question 25

List the causes of polycythaemia (pseudo and true)

Answer 25

PSEUDOPOLYCYTHAEMIA

• Reduced plasma volume

TRUE POLYCYTHAEMIA

• Increase in circulating RBC due to:

1. Blood doping or overtransfusion
2. Appropriately increased erythropoietin - e.g. secondary to hypoxia
3. Inappropriate increased erythropoietin - e.g. erythropoietin secreting renal tumour, illicit EPO administration
4. Independent of EPO - e.g. bone marrow disorder ‘polycythaemia vera’

Question 26

1) What is polycythaemia vera - what is it a disease of and what class of disease is it?
2) Apart from elevated Hb, RBC and Hct, give one sign that may present with polycythaemia vera

Answer 26

1) Bone marrow - myeloproliferative neoplasm
2) Splenomegaly

Question 27

What might you suspect to be the cause of polycythaemia in the following patients:

1) A young, healthy athlete
2) A breathless, cyanosed patient
3) A patient with splenomegaly

Answer 27

1) Blood doping or illicit EPO use
2) Probably due to hypoxia causing increased erythropoietin
3) Polycythaemia Vera

Question 28

Why could true polycythaemia be a problem?

Answer 28

It can lead to hyperviscosity of the blood and in turn lead to vascular obstruction

Question 29

How to treat true polycythaemia?

Answer 29

• If no physiological reason or if hyperviscosity is extreme - use venesection to remove blood to thin the blood
• If there is an intrinsic bone marrow disease, drugs can be used to reduce the production of RBCs by the bone marrow