Deciding what is normal and interpreting blood counts

Question 1

What is meant by a normal range?

Answer 1

Conventionally describes the results observed in 95% of a healthy population.

• Doesn’t take into account factors such as age, gender, ethnic origin, altitude etc.
• Therefore is a much vaguer concept

Question 2

State some factors that affect what is ‘normal’.

Answer 2

• Age
• Gender
• Ethnic origin
• Physiological status
• Altitude
• Nutritional status
• Cigarette smoking
• Alcohol intake

Question 3

What is meant by a reference range?

Answer 3

Describes the results observed in 95% of a reference population with characteristics that have been carefully defined with regards to age, gender etc.

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
  
  • i.e. The same instruments and techniques that will be used in hospital laboratories to analyse patient samples
• The data are analysed by an appropriate technique
• Data with a normal (Gaussian) distribution can be analysed by:
  
  • Determining the mean and standard deviation
  • Then taking mean ± 2SD as the 95% range
• Data with a different distribution must be analysed by an alternative method

Question 5

What are some caveats (limitations) with having a normal or reference range?

Answer 5

• Just because the result is outside the 95% range doesn’t mean its abnormal or there’s something wrong
• Same if a result is within the 95% range - doesn’t make it normal
• What you really want to know is: Is this result normal for this individual?

EXAMPLE:

• 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
  
  • So 95% of people in Western population would have high serum lipids but seem apparently healthy
  • So in this population high serum lipids would be ‘normal’
  • But just because it’s normal doesn’t mean it’s not bad for you
• A health-related range may be more meaningful than a 95% range

Question 6

What do the abbreviations in a full blood count (FBC) mean?

Answer 6

• 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**​
• 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 7

How was the WBC, RBC and platelet count initially determined? How has this changed?

Answer 7

Initially:

• Counted visually - using a microscope and a diluted sample of blood

Now:

• Counted in large automated instruments
• Done by enumerating (counting) electronic impulses generated when cells flow:
  
  • Between a light source and a sensor
  • OR through an electrical field

Question 8

How was the Hb initially determined? How has this changed?

Answer 8

Initially:

• Measured in a spectrometer
• Done 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 9

How was the PCV or Hct initially determined? How has this changed now?

Answer 9

Initially:

• Measured by centrifuging a blood sample
  
  • Hence it was given the name PCV (packed cell volume)

Now:

• More modern techniques used
  
  • So now tends to be referred to as Hct

Question 10

Define PCV or Hct.

Answer 10

Ratio of the volume of RBCs to total volume of blood

• Measured in litres/litre
  
  • Your value should always be less than one
  • Total volume of blood = RBCs + WBCs and plateletsvolume + plasma

Question 11

How was the MCV initially determined? How has this changed now?

Answer 11

Initially:

• MCV = mean cell volume
• Calculated be dividing the total volume of red cells in a sample by the number of red cells in a sample
  
  • i.e. PCV ÷ RBC

Now:

• Determined indirectly by light scattering or by interruption of an electrical field

Question 12

What is MCH?

Answer 12

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

• i.e. Hb ÷ RBC
  
  • Units pg - so gives indication of amount

The MCH measures the average amount of haemoglobin in an individual red cell

Question 13

What is MCHC?

Answer 13

The amount of haemoglobin in a given volume of blood divided by the proportion of the sample represented by the red cells

• i.e. Hb ÷ Hct
  
  • ​Hct is also dependent on individual RBC volume as well as total RBC number
    
    • ​Both higher RBC volumes and number will increase Hct
  • ​Units g/l - so gives indication of concentration

Question 14

What is the difference between MCH and MCHC?

Answer 14

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
  
  • So in microcytic anaemia:
    
    • ​MCV is reduced
    • So MCH also tends to be reduced

The MCHC is the concentration of haemoglobin in a red cell

• If MCHC was reduced, the RBC cell would look paler (hypochromia)
• The MCHC is related to the shape of the cell
  
  • e.g. In spherocytosis there is no thinner section where less Hb can be housed so the concentration of Hb in the individual red cells would increase

Question 15

How can the MCHC be determined?

Answer 15

Manually:

• Could be recognised on a blood film
• Hypochromia correlates with low MCHC

Now:

• Measured electronically, most accurately on the basis of light scattering