2: Blood counts

Question 1

Formula for MCV

Answer 1

MCV = PCV x1000 / RBC

Mean cell volume = Packed cell vol. / Red blood cell count

MCV measured in femtolitres

Question 2

Units of Hb count?

Answer 2

g/L

Question 3

MCH vs MCHC? How would they differ when observed under microscope?

Answer 3

Mean cell haemoglobin (measured in picograms) = Hb/RBC

Mean cell haemoglobin concentration (g/L) = Hb/PCV or hematocrit

Reduced MCH + Normal MCHC = cell is smaller than normal (microcytosis) so less haemoglobin BUT color still the same because concentration hasnt changed

Reduced MCH + reduced MCHC = cell is smaller AND concentration is lower so HYPOCHROMIA

Question 4

How do you interpret a blood count?

Answer 4

1. Check for leucocytosis/leucopenia and if so why
2. Check for anaemia
3. Check for size of cells
4. Check for Thromocytosis/thrombocytopenia

Always check for clues in blood count and patient’s clinical history

Question 5

What is polycythaemia and what would its blood count look like?

Answer 5

Too many RBCs in circulation

Hb/RBC/PCV/Hct ALL INCREASED

Question 6

Causes of pseudo/true polycythaemia?

Answer 6

Pseudo = Reduced plasma volume
True = Blood doping/transfusion, increased erythropoietin (hypoxia), inappropriate EP synthesis/use (tumour), cause unrelated to EP (polycythaemia vera = bone marrow disorder)

Question 7

How do you evaluate polycythaemia? Give some clinical signs

Answer 7

Clinical history + examination (splenomegaly suggests polycythaemia vera, abdominal mass suggests kidney carcinoma)
Check blood cell count

Question 8

Treatment for polycythaemia?

Answer 8

Remove blood to thin the blood (if high Hb not physiologically needed)
For intrincic bone marrow disease, use drugs to reduce production of RBCs

Question 9

Reference range vs normal range?

Answer 9

A reference range is derived from a carefully defined reference population

• Samples collected from healthy volunteers with defined characteristics
• Range is analysed using the instrument and techniques that will be used for patient samples
• Data is then analysed by an appropriate technique

A normal range is a much vaguer concept

Question 10

Do Hb and WBC show a normal Gaussian distribution?

Answer 10

Hb = normal
WBC = NON-gaussian

Question 11

Disadvantage of reference range?

Answer 11

A result within the 95% range (mean +- 2SDs) may still be unhealthy

Question 12

Principles of measuring Hb?

Answer 12

Convert Hb into stable form

Measure light absorption at a specific wave length