deciding what is normal and interpreting blood counts- polycythaemia as an example Flashcards
(24 cards)
‘normal’ can be affected by
Age Gender Ethnic origin Physiological status Altitude Nutritional status Cigarette smoking, alcohol intake
what is the effect of altitude on haemoglobin?
6,500ft: Hb = 0.8-1 g/l
10,000ft: Hb = 2g/l
13,000 ft: Hb = 35 g/l
what is the difference between reference range and normal range?
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)
how is a reference range determined?
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
what is an appropriate statistical technique?
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
Normal does not mean ______
Normal does not necessarily mean that it is healthy
what are the conditions/limitations of the reference and normal range?
-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
what are the current units for Hb and what is the expected range?
Current units for Hb are g/l, e.g. 154 g/l
(134-166
What do the following abbreviations stand for?
WBC – RBC – Hb – Hct – PCV –
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)
What do the following abbreviations stand for?
MCV –
MCH –
MCHC –
Platelet count –
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)
how are the WBC, RBC and platelet counted?
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
How is Hb measured?
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
How is packed cell volume (PCV) or haematocrit (Hct) measured?
Initially measured by centrifuging a blood sample (hence PCV)
How is mean cell volume calculated?
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
how would you calculate the mean cell haemoglobin?
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
How is mean cell haemoglobin concentration calculated?
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
what is the difference between the MCH and the MCHC?
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
summarise the causes of polycytthaemia
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
how to you evaluate ‘polycythaemia’
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
define polycythaemia
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.
how do you know if the polycythaemia is apparent or genuine?
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
how does abnormal function of the bone marrow lead to polycythaemia?
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
what can polycythaemia lead to?
Polycythaemia can lead to ‘thick blood’– more technically known as hyperviscosity
This can lead to vascular obstruction
what can be done to deal with the hyperviscosity with polycythaemia?
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
