L6: Interpreting the full blood count Flashcards
What do we mean by abnormal results?
Results outside the ‘normal’ range
‘Normal’ only 95% population
–> some people outside will be normal, some people inside will be normal
Look for significant fall in value
What causes the normal range to change?
Age, sex, ethnicity, co-morbidities
What do abnormal results normally show?
Often reactive rather than reflective
Reactive–> to another physiological situation in the body
Reflective–> doesn’t usually reflect true haematological result
How would you know if the result is reactive or reflective?
Interpret–> clinical context and previous FBC if known
- -> acute/ chronic change
- -> is it explained by disease?
What errors can occur in pathology results?
- Specimen collection –> mix up, wrong bottle, pooling samples, poor technique, wrong blood in tube (incorrect labelling)
- Delivery of specimen to lab–> delayed/ not delivered, wrong delivery method
- Specimen analysis and result reporting–> Mix up, incorrect clinical details, wrong test requested/ perfomed, inherent variability (analysers not perfect), technical error
- Responsive action–> result not reviewed, reflex tests not carried out, right result wrong patient!
How is a full blood count performed?
Automated test–> high sample throughput, greater accuracy
What are the essential parameters?
Red cells–> indices, RCC, haemoglobin
Platelets–> Count
White cells–> count, full differential
What are the analyser techniques?
Spectrophotometry
Flow cytometry
How does spectrophotometry work?
Spectrophotometry–> amount of light absorbed proportional to amount of absorbent compound within–> measure amount of haemoglobin
- -> hypotonic solution–> lyse cells
- -> light of appropriate wavelength
- -> calibration curve to determine sample concentration
How does flow cytometry work?
Hydrodynamic focussing –> single file line of cells
- -> passes through light beam - perpendicular angle
- -> impedance counting –> broken beam, one cell, stops beam hitting detector
- -> forward scatter–> size of cell –> more scatter= bigger cell
How does flow cytometry differential work?
Same as normal flow cytometry
Look at side scatter–> bounces back >90 degree angle
Tells you complexity of environment–> mononucleated, polynucleated
Plot graph of forward scatter against side scatter–> shows size and complexity of cell–> work out what each cells is
What does myeloperoxidase activity show?
Present in granules (granulocytes)
If activity present –> granulocyte!
What does the packed cell volume (PVC/Hct) show?
The proportion of blood that is made up of RBC (L/L)
- centrifuge down
Assess anaemia and polycythemia (increased hematocrit)
Polycythemia reduced by venesection or drug treatment
What is the difference between true polycythemia and pseudopolycythemia?
True polycythemia–> increased red cell count
Pseudopolycythemia–> reduction in circulating plasma volume–> reduced plasma volume makes it look like more red cells
What other parameters can be tested to show if it is a true polycythemia or not?
Haemoglobin
RCC–> single RBC passed through detector
Mean cell volume –> amount of light scattered measured
Mean cell haemoglobin
Mean cell haemoglobin concentration
Red cell distribution width
What does haemoglobin show? What are the problems associated?
Amount of Hb mass/ plasma volume
Reference range–> varies sightly between labs but
>135g/L adult men
>115g/L adult women
>110g/L children (3/12- puberty)
>150g/L newborns
However not perfect as it is a concentration –> lower plasma level looks like increased RBC count therefore looks like more haemoglobin (Dehydration, acute bleed can affect result)
Turbidity of plasma–> affects amount of light absorbed–> over estimate
Heamolysis in vitro–> RBC destroyed whilst sample is being transported –> ↓ Heamoglobin
What is red cell count (RCC)?
Number of red blood cells in a given volume of blood
(x10^12/L)
Assessment of anaemia, erythrocytosis etc
Microcytic anaemia
–> iron deficiency anaemia–> RCC reduced no iron less RBC produced
–> Thalassemia –> ↓ heamoglobin –> compensates by ↑RCC make more RBC to carry O2 (inherited)
Erythrocytosis
–> too many RBC made–> indicates true polycythemia
What does the mean cell volume (MCV) show?
Mean size of RBC
High or low can give you lots of information
Most important parameter used to screen the cause of anaemia
What does the mean cell haemoglobin (MCH) show?
Average amount of Hb in each RBC (pg)
MCH= Total amount of Hb/ Number fo RBC
Anaemia assessment
What does the mean cell haemoglobin concentration (MCHC) show?
Mean concentration of Hb in RBC in a given volume
MCHC= (Hb/MCV) X RCC
Not common
Useful for cold agglutinins (RBC stick together when cold)
What is the red cell distribution width (RDW) show?
Variation in size of the RBC
If increased = anisocytosis (patient RBC of unequal size)
Help assess cause of anaemia;
–> ↑ in iron deficiency
–> normal in thalassaemia trait
–> increased following transfusion (pt RBC and someone else RBC)
What does the reticulocyte count show?
Measurement of the number of young erythrocytes (X10^9/L)
Identified using size and RNA count
Measure anaemia–> increased in lots of cases to compensate
decreases in haematinic deficiency–> not got what they need to make RBC and bone marrow deficiency
What is a blood film? Why would it be done?
Thin layer of blood smeared onto a slide looked at under microscope
- Significant result outside normal range
- Significant change within normal range
- Analysers think there are abnormal cells (immature cell, analyser unable to identify)
What are some of the RBC terminology used to describe the size and colour of the RBC?
Microcytic --> MCV --> small Macrocytic--> MCV --> large Hypochromic--> MCH --> pale, less Hb Hyperchromic--> MCH --> Dense, more Hb in given volume more on slide 25!
