Haem 2 - Deciding what is normal and interpreting blood counts - polycythaemia as an example

Question 1

What can normal be affected by?

Answer 1

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

Question 2

What are the effects of altitude on normal Hb?

Answer 2

6500 ft Hb = 0.8-1 g/L
10,000 ft Hb = 2g/L
13000 ft Hb = 35 g/L

Question 3

How is a normal or reference range determined?

Answer 3

A reference range is derived from a carefully defined reference population. E.g children 5-10 yrs

A normal range is a much vaguer concept - it should represent people that live in the local area and come to the hospital

Question 4

How is a reference range determined? How is data that shows a normal (Gaussian) distribution and non normal distribution analysed?

Answer 4

Samples are collected from healthy volunteers with defined characteristic. They are analysed using the instrument and techniques that will be used for patient samples.

The data is then analysed appropriately whether it shows a normal distribution or not.

Normal distribution - Take the mean and take 2 SD on each side. By taking 2 SD on each side it will include 95% of the data. Hb

Non-normal distribution - Must be analysed in another way. WBC

Not all results outside the reference range are abnormal and not all results inside the reference range are normal. A result in the 95% range determined from apparently healthy people may still be bad for your health - serum lipids in western populations. Health related range may be more meaningful than a 95% range.

See notes

Question 5

Full blood count (FBC) - abbreviations used:
WBC
RBC
Hb
PCV
Hct
MCV
MCH
MCHC
Platelet count

Answer 5

White blood cell count in a given volume of blood (x10^9/l)
RBC count in a given volume of blood (x10^12/l)
Haemoglobin concentraion (g/l)
Packed cell volume (l/l)
Haematocrit (l/l)
Mean cell volume (fL)
Mean cell haemoglobin (pg)
Mean cell haemoglobin concentration (g/l)
Platelet count - the number of platelets in a given volume of blood (x10^9/l)

Question 6

How was Hb initially measured?

Answer 6

Spectrometer - Haemoglobin is converted to a stable form and you measure the light absorption at a specific wave length.

Question 7

How is Hct or PCV initially measured?

Answer 7

Centrifuging a blood sample

Question 8

How was MCV initially measured?

What is the formula?

Answer 8

Calculated by dividing the total volume of red cells in a sample by the number of red cells in a sample - PCV/RBC.

Now determined indirectly by light scattering or by interruption of an electrical field.

You can’t compare MCV by looking at a blood film unless you have a reference leukocyte. MCV is useful for differentiating between different types of anaemia

Question 9

Define MCH

Answer 9

The amount of haemoglobin in a given volume of blood divided by the number of red cells in the same volume, i.e the Hb divided by the RBC

Question 10

Define MCHC

Answer 10

The amount of haemoglobin in a given volume of blood divided by the proportion of the sample represented by the red cells, i.e the Hb divided by the PCV or haematocrit

Question 11

What is the difference between MCH and MCHC?

Answer 11

The MCH is the absolute amount of haemoglobin in an individual red cell.
The MCHC is the concentration of haemoglobin in a red cell.
In microcytic and macrocytic anaemias, the MCH tends to parallel the MCV.
See diagram/lecture notes

Question 12

What correlates with the MCHC?

Answer 12

Hypochromia - paler RBC means less Hb concentration in the middle which leans lower MCHC. MCHC is related to the shape of the cell.

Question 13

How do you interpret a blood count?

Answer 13

See lecture slides

To start with lean to interpret:
WBC and differentials, Hb, MCV and platelet count
Examine a blood film

Question 14

Define polycythaemia

Answer 14

Too many red cells in the circulation - the Hb, RBC, and PCV/Hct are all increased compared to normal subjects of the same age and gender

Question 15

What are the causes of polycythaemia?

Answer 15

Pseudo = reduced plasma volume
True = increase in total volume of red cells in the circulation --> blood doping or over transfusion (cyclists), appropriately increased erythropoietin (elevated at altitude and raised as a result of hypoxia), innappropriate erythropoietin synthesis or use (cyclists or when renal or other tumour secretes erythropoietin), increased erythropoiesis independent of erythropoietin (intrinsic bone marrow disorder; polycythaemia vera) - increase in the number of circulating RBC = true

Question 16

How do you evaluate polycythaemia?

Answer 16

1) Clinical history + physical examination (splenomegaly, abdominal mass or cyanosis could be relevant)
2) Compare with an appropriate normal range
Note - Hb, RBC and Hct are higher in neonate compared to other times of life, lower in children than in adults and lower in women than in men.

Question 17

Describe polycythaemia vera

Answer 17

Intrinsic bone marrow disorder classified as a myeloproliferative neoplasm - can cause hyperviscosity which leads to vascular obstruction

Splenomegaly is often a pointer towards to polycythaemia vera

Question 18

How can you treat polycythaemia?

Answer 18

Blood can be removed to thin the blood if there is no physiological reason for a high haemoglobin, or if hyperviscosity is extreme.
Intrinsic bone marrow disease - drugs to reduce bone marrow production of red cells

See slide on clinical context

Question 19

What is the difference between polycythaemia and erthyrocytosis?

Answer 19

Erythrocytosis is an increase in RBCs relative to the volume of blood.

Polycytheamias an increase in both RBC concentration and haemoglobin