Deciding what is normal when interpreting a blood count Flashcards
Clinically, why is it important to know what is normal
Before interpreting any clinical sign or any laboratory test we need to know what is normal
Laboratories try to help by providing normal ranges or reference ranges
Compare the difference between a reference range and a normal range
Reference range- derived from a healthy population- more strict
Normal range- lab decides this based on research on lots of different reference ranges- looser
What percentage of the healthy population will have test results falling within a ‘normal range’?
95% will be within normal range
2.5% below, 2.5% above
When deciding what is normal what sample size is best
A larger sample
What factors can affect what is meant by ‘normal’
Age Gender Ethnic origin Physiological status (pregnancy) Altitude (effect on Hb concentration) Nutritional status Cigarette smoking (white cell count and possibly Hb too), alcohol intake (can make RBCs larger
May want to exclude smokers and alcoholics when trying to determine the normal or reference range.
What is important to remember about some factors when trying to establish a normal range
When looking at nutritional status- it is important to exclude the malnourished from the normal range.
However in Africa or India, how do you decide what is ‘normal’ i.e who is malnourished and who isn’t
Describe the effects of altitude on Hb concentration and the impact of this on the reference range
At high altitudes, the partial pressure of O2 decreases such that you enter a mild hypoxia. This stimulates the kidneys to synthesise EPO and thus increase Hb conc
Some cities in the world are located at high altidues- and so we would need to use a healthy population from these samples to determine the reference range for Hb conc in these regions.
Ultimately, what is the difference between a reference range and a normal range
Reference Range = derived from a carefully defined reference population e.g. children 5-10 years
Normal Range = much vaguer – it should represent the people that live in the local area and come to the hospital
Normal ranges are generally harder to define
Outline how a reference range is determined
Samples are collected from healthy volunteers with defined characteristics (e.g height of men >65 who are non-smokers)
They are analysed using the instrument and techniques that will be used for patient samples- to keep variables to a minimum (different equipment can measure the size of platelets differently for example).
The data are analysed by an appropriate statistical 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 (1.96 more accurate)
Between the 2 SDs on either side, you will have 95% of the data
Data with a different distribution must be analysed by an alternative method - or converted to logarithmic scale to make it normal
· Hb shows a GAUSSIAN/NORMAL distribution.
· WBCs show a NON-NORMAL distribution.
What is important to remember about results in the reference or normal range
Not all results outside the reference range are abnormal
Not all results within the normal range are normal (an individual could have a Hb conc of 120- which is in normal range- but may normally be 150 for him- making this result abnormal for this patient
What you really want to know is “Is this result normal for this individual?”
Need to look at history and previous results to determine what is normal for each individual.
i.e. a man with a GI bleed with a normal Hb count is not normal for the situation.
Describe a key caveat to reference and normal ranges
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 (as most are obese/overweight)- therefore being in the upper range of this range will be associated with an increased risk of death
A health-related range may be more meaningful than a 95% range - with a cut-off point between healthy and unhealthy
Compare ideal tests to non-ideal tests
o Ideal and Non-Ideal Tests:
§ Clearly distinct results from a test is ideal but not often viable (different ranges for sick and well)
§ Too much overlap means you can’t determine sick from well.
Best you may achieve in clinical practice is a small overlap between the sick and well ranges
When analysing a full blood count, what should you look at first
WBC
Hb
MCV
platelet count
See if these are normal before looking at other variables to give you more information.
What is meant by WBC and RBC
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)
What is meant by Hb and HCT
Hb – haemoglobin concentration (g/l)
Hct – haematocrit (l/l)
PCV – packed cell volume (% or l/l) (an older name for the Hct)
What is meant by MCV. MCH, MCHC and 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)
What is meant by RDW
Red cell distribution- to look at ansiocytosis
Describe how we measure RBC, WBC and platelet count
Initially counted visually, using a microscope and a diluted sample of blood (not feasible when analysing 300-400 per day)
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
Cells in a thin stream so that each one is counted when they interrupt a light beam or an electric field.
Describe how we measure Hb
Initially measured in a spectrometer, by converting haemoglobin to a stable form and measuring light absorption at a specific wave length (cyanmethaemoglobin)
Now measured by an automated instrument but the principle is the same- converted to a stable form and wavelength measured
Cyanide is dangerous- why it’s no longer used
The spectrum of light absorbance of haemoglobin (as cyanmethaemoglobin) in a spectrophotometer and measure the y-axis at the peak
Describe how we measure PCV or Hct
Initially measured by centrifuging a blood sample (hence PCV)
Measuring height of red column (below buffy coat- white cells are lighter)
Centrifugation is too labour-intensive- so we know have automated measures.
How do we measure MCV
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
Larger cells will cause more scattering
What is the formula for MCV
MCV (fl) = PCV (l/l) × 1000/
RBC (× 10-12/l)
Describe the histograms for MCV
A histogram of red cell size with cell size being determined by interruption of an electrical field
The MCV was 96.9 fl
Number of cells on y-axis
Range of MCVs on x-axis
MCV calculated from this
Peak below 95% range- microcytosis
Peak above 95% range- microcytosis.
What does the MCV correlate with
Cell size, as recognized on a blood film, correlates with the MCV
If MCV is above reference range- you will see macrocytosis on the blood film
How is MCH calculated
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
MCH (pg) = Hb (g/l)
RBC (× 10-12/l)
How is MCHC calculated
Essentially Hb in cells
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
MCHC (g/l) = Hb (g/l)
Hct (l/l)
What is the difference between MCH and 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
MCH tends to parallel MCV (except in iron deficiency, when MCH and hence MCHC drops faster than MCV)
The MCHC is the concentration of haemoglobin in a red cell
The MCHC is related to the shape of the cell
Hypochromia, recognized on a blood film, correlates with the MCHC
so affected by conditions such as thalassaemia (low) and hereditary spherocytosis (high)
Essentially, what does the MCH measure
The MCH measures the average amount of haemoglobin in an individual red cell
Describe how we measure the MCHC
The MCHC is now measured electronically, most accurately on the basis of light scattering
What could a high MCHC indicate
o High MCHC could indicate irregularly contracted cells of spheroidal cells.
Also sickle cells (as polymerisation takes place fluid leaks out cell- making the cell more dense and thus the Hb more concentrated).
How do you interpret a blood count when looking at white cells
Is there leucocytosis or leucopenia?
If so, why?
Which cell line is abnormal?
Are there any clues in the clinical history?
SOB and fever with leucocytosis- pneumonia
leucopenia is most likely due to neutrophils- as these have the highest proportion in the circulation
How do you interpret a blood count when looking at anaemia
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?
How do you interpret a blood count when looking at platelets
Is there thrombocytosis or thrombocytopenia?
If so, are there any clues in the blood count?
Are there any clues in the clinical history?
Thrombocytopenia with leucocytosis may indacte leukaemia
Outline a systematic approach for interpreting full blood counts
To start with, learn to interpret
WBC and differential (to look at proportion of different white cells)- Always look at the absolute count not the percentage
Hb
MCV
Platelet count
When you have mastered that, look at the other measurements also
What other things should you look at when interpreting a blood count
The correct interpretation of a blood count may also require you to examine a blood film
The reason for this patient’s anaemia is instantly apparent
Will see sickle cells- sickle cell anaemia
What happens to the platelet count in SCA and why
Infarction of spleen (RBCs can’t pass through the blood vessels)
Splenic inadequacy causes the platelet count to rise (not removed)
What is meant by polycythaemia and which blood cell paramaters are changed
Polycythaemia literally means ‘many cells’ but it refers specifically to too many red cells in the circulation
The Hb, RBC and Hct/PCV are all increased compared with normal subjects of the same age and gender
AKA erythrocytosis
What is meant by psuedopolycythaemia
Reduced plasma volume- therefore increasing Hb, RBC and Hct
Describe true polycythaemia and outline its main causes
Increase in total volume of red cells in the circulation
Due to:
Blood doping or overtransfusion
Appropriately increased erythropoietin (response to hypoxia)
Inappropriate erythropoietin synthesis or use
Independent of erythropoietin
What is meant by overtransfusion
Too much blood given for that’s person size
Hb not checked before subsequent transfusion
How do we evaluate polycythaemia
Start with a clinical history and physical examination (splenomegaly, abdominal mass (could be spleen issues or kindey synthesising too much EPO) or cyanosis (hypoxia) 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
Need ranges to take into account different altitudes
MAKE sure range is appropriate for patient
How do we determine whether the polycythaemia is true or apparent
A high Hb, RBC and PCV/Hct can result from a decrease in plasma volume, referred to as ‘pseudopolycythaemia’ or ‘apparent polycythaemia’ - could be due to dehydration- or a chronic condition where RBC is normal but Hb increased due to reduced plasma volume
I.E. burns draw plasma to the surface.
When the abnormalities result from an increase in the number of circulating red cells there is a true polycythaemia
Look at full clinical picture to determine this
Describe blood doping as a cause of true polycythaemia
o Blood doping – adding blood to your own – too much blood.
This is the first cyclist accused of blood doping after tests suggested cells of two different blood groups
Describe medical negligence as a cause of true polycythaemia
Medical negligence—a bizarre cause of polycythaemia
The annual SHOT (Serious Hazards of Transfusion) report for 2003 gave details of a patient rendered polycythaemic by transfusion
She was a fairly small adult with haematuria and an Hb of 63 g/l
Four units of blood were crossmatched and transfused
Without noting the post-transfusion Hb, a doctor transfused another 4 units
The patient’s Hb rose to 205 g/l and she became hypertensive
She required daily venesections (removal of blood) for 3 days
When giving transfusions what should you take into account
Weight and size of the patient- don’t need as much blood
Hb levels prior- to determine how much blood needs to be given.
Describe physiologically high levels of EPO as a cause of true polycythaemia
when erythropoietin is appropriately raised as a result of hypoxia (may see clubbing of fingers)
– increased blood cell production in response to hypoxia at altitude
However residents of the Tibetan plateau have a 85% prevalence of a mutation that reduces the erythropoietin production in response to hypoxia
As increased production of RBCs will increae viscosity of blood- increasing the risk of strokes and myocardial infarctions.
Describe ilicit EPO as a cause of true polycythaemia
or when erythropoietin is inappropriately administered to haematologically normal subjects — cyclists again
artificially raising RBC
Describe tumour EPO as a cause of true polycythaemia
… or when a renal or other tumour inappropriately secretes erythropoietin
Polycythaemia from an erythropoietin-secreting renal tumour is not surprising since this is the normal site of erythropoietin production
Describe polycythaemia vera
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
Mutation in haematopoietic stem cells
What is a key complication of polycythaemia
Polycythaemia can lead to ‘thick blood’– more technically known as hyperviscosity
This can lead to vascular obstruction
How can we treat 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- these drugs have considerable side effects so only give in polycythaemia vera
What is important to remember when interpreting a blood count
Always interpret a blood count in the context of the clinical history and physical findings
How does clinical context help us with interpreting an FBC that shows polycythaemia?
A young healthy athlete – be very suspicious
A breathless cyanosed patient ‒ probably due to hypoxia
An abdominal mass ‒ it could be carcinoma of the kidney
Splenomegaly ‒ a pointer to polycythaemia vera
