Deciding what is normal and interpreting a blood count

Question 1

Define reference range.

Answer 1

Range derived from a healthy population.

Question 2

Define normal range?

Answer 2

Looser definition than reference range. 95% population falls within normal range.

Question 3

What can effect whether a measurement is considered normal with regards to haematology?

Answer 3

• Age
• Gender
• Ethnic origin
• Physiological status
• Altitude
• Nutritional status
• Cigarette smoking, alcohol intake

Question 4

How does haemoglobin change with altitude? How might this effect normal values?

Answer 4

13,000ft - 35g/l

10,000ft - 20g/l

6500ft - 0.8-1g/l

Question 5

How is a referenc range determined?

Answer 5

• Samples are collected from healthy volunteers with defined characteristics
• They are analysed using the instrument and techniques that will be used for patient samples
• The data are analysed by an appropriate technique

Question 6

What is the difference between MCH and MCHC?

Answer 6

MCH = absolute amount of Hb in an RBC.

MCHC = concentration of Hb in a red cell

Question 7

What is an appropriate statistical technique for establishing reference range?

Answer 7

Normal (Gaussian) distribution - analysed by taking mean and sd. mean +/- 2SD = 95% range.

Different distribution needs alternative method.

Question 8

Do Hb and WBC show Gaussian distributions?

Answer 8

Hb - yes

WBC - no, positive skew.

Question 9

Why might the use of normal/reference ranges be misleading?

Answer 9

Healthy person could be outside range and vice versa.

95% isn’t the cut off for whats healthy - e.g. for blood lipids.

Question 10

Explain the abbreviations MCV, MCH, MCHC, Platelet count, WBC, RBC, Hb, Hct, PCV. What units are used?

Answer 10

• WBC – white blood cell count in a given volume of blood (× 109/l)
• RBC – red blood cell count in a given volume of blood (× 1012/l)
• Hb – haemoglobin concentration (g/l)
• Hct – haematocrit (l/l)
• PCV – packed cell volume (% or l/l) (an older name for the Hct)
• MCV – mean cell volume (fl)
• MCH – mean cell haemoglobin (pg)
• MCHC – mean cell haemoglobin concentration (g/l)
• Platelet count – the number of platelets in a given volume of blood (× 109/l)

Question 11

How are WBC, RBC, and platelet counts taken?

Answer 11

Either manually with microscope or counted by large automated instruments - flow of cells interfere with light source/electrical field –> impulse generation

Question 12

How is Hb measured?

Answer 12

Hb converted to stable form and light absorption measured at specific wavelength (spectrometer or automated).

Question 13

How is MCV determined?

Answer 13

Divide volume of red cells by number of RBC in sample (PCV/RBC).

Or determined indirectly by light scattering or interruption of electrical field.

Question 14

How is PCV/Hct measured?

Answer 14

Centrifuging blood sample.

Question 15

How is MCH calculated?

Answer 15

amount of Hb in given volume of blood/number of RBC in same volume.

Also measured electronically on basis of light scattering.

Question 16

How is MCHC calculated?

Answer 16

Amount of Hb in given blood volume/proportion of sample represented by red cells.

(Hb/Hct)

Question 17

Why is do spherocytic cells have increased MCHC?

Answer 17

Lower cell volume due to spherical shape but Hb remains the same so MCHC rises.

Question 18

Does hypochromia correlate with MCHC?

Answer 18

yes

Question 19

How is a blood count interpreted?

Answer 19

• Is there leucocytosis or leucopenia?
• If so, why?
• Which cell line is abnormal?
• Are there any clues in the clinical history?
• Is there anaemia?
• If so, are there any clues in the blood count?
• Are the cells large or small?
• Are there any clues in the clinical history?
• Is there thrombocytosis or thrombocytopenia?
• If so, are there any clues in the blood count?
• Are there any clues in the clinical history?

Question 20

Why does platelet count rise in sickle anaemia?

Answer 20

Splenic dysfunction reduces platelet clearing.

Question 21

Why is a blood film examination heplful when interpreting a blood count?

Answer 21

It makes anaemia immediately apparent.

Question 22

Define polycythaemia.

Answer 22

Too many red cells in circulation compared to expected normal

Question 23

Give some causes of polycythaemia.

Answer 23

Blood doping/overtransfusion.

Appropriately increased erythropoietin (blue tinge + clubbing of nails).

Innappropriate erythropoietin synthesis or use (e.g. tumour in kidney)

Independent of erythropoietin.

Reduced plasma volume (pseudopolycythaemia).

Question 24

How can abnormal function of bone marrow cause polycythaemia?

Answer 24

Polycythaemia vera - myeloproliferative neoplasm. Increased RBC production independent of erythropoetin at expense of other blood cells.

Question 25

Name a problem associated with polycythaemia.

Answer 25

hyperviscosity.

Question 26

How can polycythaemia be treated?

Answer 26

Remove blood if hyperviscous. For polycythaemia vira, use drugs to reduce bone marrow RBC production.

Question 27

What clinical pointers indicate to different causes of polycythaemia?

Answer 27

young athlete - likely blood doping. Breathless, cyanosed patient - likely due to hypoxia. Abdomial mass - could be kidney carcinoma. Splenomegaly - indicatvie of polycythaemia vera.