deciding what is normal and interpreting blood counts- polycythaemia as an example

Question 1

‘normal’ can be affected by

Answer 1

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

Question 2

what is the effect of altitude on haemoglobin?

Answer 2

6,500ft: Hb = 0.8-1 g/l
10,000ft: Hb = 2g/l
13,000 ft: Hb = 35 g/l

Question 3

what is the difference between reference range and normal range?

Answer 3

A reference range is derived from a carefully defined reference population (eg children 10-15 years)

A normal range is a much vaguer concept (eg represents people that live in the local area)

Question 4

how is a reference range determined?

Answer 4

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 5

what is an appropriate statistical technique?

Answer 5

Data with a normal (Gaussian) distribution can be analysed by determining the mean and standard deviation and taking mean ± 2SD as the 95% range
Data with a different distribution must be analysed by an alternative method

Question 6

Normal does not mean ______

Answer 6

Normal does not necessarily mean that it is healthy

Question 7

what are the conditions/limitations of the reference and normal range?

Answer 7

-Not all results outside the reference range are abnormal
-Not all results within the normal range are normal
What you really want to know is “Is this result normal for this individual?”
-A result within the 95% range determined from apparently healthy people may still be bad for your health
-Serum lipids in the upper end of this range are common in Western populations
-A health-related range may be more meaningful than a 95% range

Question 8

what are the current units for Hb and what is the expected range?

Answer 8

Current units for Hb are g/l, e.g. 154 g/l

(134-166

Question 9

What do the following abbreviations stand for?

WBC –
RBC – 
Hb –
Hct – 
PCV –

Answer 9

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)

Question 10

What do the following abbreviations stand for?

MCV –
MCH –
MCHC –
Platelet count –

Answer 10

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 the WBC, RBC and platelet counted?

Answer 11

Initially counted visually, using a microscope and a diluted sample of blood
Now counted in large automated instruments, by enumerating electronic impulses generated when cells flow between a light source and a sensor or when cells flow through an electrical field

Question 12

How is Hb measured?

Answer 12

Initially measured in a spectrometer, by converting haemoglobin to a stable form and measuring light absorption at a specific wave length
Now measured by an automated instrument but the principle is the same

Question 13

How is packed cell volume (PCV) or haematocrit (Hct) measured?

Answer 13

Initially measured by centrifuging a blood sample (hence PCV)

Question 14

How is mean cell volume calculated?

Answer 14

Initially calculated be dividing the total volume of red cells in a sample by the number of red cells in a sample, i.e. by dividing the PCV by the RBC
Now determined indirectly by light scattering or by interruption of an electrical field

Question 15

how would you calculate the mean cell haemoglobin?

Answer 15

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 16

How is mean cell haemoglobin concentration calculated?

Answer 16

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 Hct

Question 17

what is the difference between the MCH and the MCHC?

Answer 17

The MCH is the absolute amount of haemoglobin in an individual red cell
In microcytic and macrocytic anaemias, the MCH tends to parallel the MCV
The MCHC is the concentration of haemoglobin in a red cell
The MCH measures the average amount of haemoglobin in an individual red cell

Question 18

summarise the causes of polycytthaemia

Answer 18

pseudo = reduced plasma volume

true = increase in total volume of red cells in the circulation

• blood doping or overtransfusion
• appropriately increased erythropoietin (eg as a result of hypoxia or high altitude)
• inappropriate erythropoietin synthesis or use
• independent of erythropoietin

Question 19

how to you evaluate ‘polycythaemia’

Answer 19

Start with a clinical history and physical examination (splenomegaly, abdominal mass or cyanosis could be relevant)
Next compare with an appropriate normal range
Note: the Hb, RBC and Hct are higher in the neonate than at other times of life, lower in children than in adults and lower in women than in men

Question 20

define polycythaemia

Answer 20

an abnormally increased concentration of haemoglobin in the blood, either through reduction of plasma volume or increase in red cell numbers. It may be a primary disease of unknown cause, or a secondary condition linked to respiratory or circulatory disorder or cancer.

Question 21

how do you know if the polycythaemia is apparent or genuine?

Answer 21

A high Hb, RBC and PCV/Hct can result from a decrease in plasma volume, referred to as ‘pseudopolycythaemia’ or ‘apparent polycythaemia’
When the abnormalities result from an increase in the number of circulating red cells there is a true polycythaemia

Question 22

how does abnormal function of the bone marrow lead to polycythaemia?

Answer 22

Polycythaemia can also result from
Inappropriately increased erythropoiesis that is independent, or largely independent, of erythropoietin
This condition is an intrinsic bone marrow disorder called polycythaemia vera
It is classified as a myeloproliferative neoplasm

Question 23

what can polycythaemia lead to?

Answer 23

Polycythaemia can lead to ‘thick blood’– more technically known as hyperviscosity
This can lead to vascular obstruction

Question 24

what can be done to deal with the hyperviscosity with polycythaemia?

Answer 24

If there is no physiological need for a high haemoglobin, or if hyperviscosity is extreme, blood can be removed to thin the blood
If there is intrinsic bone marrow disease, drugs can be used to reduce bone marrow production of red cells