Peripheral blood smear

Question 1

What is the aim of a peripheral blood smear?

Answer 1

To review a blood sample:

• to count different circulating blood cells
• check their morphology

Question 2

List some indications for a peripheral blood smear?

Answer 2

Indications:
• Detect abnormalities in RBCs, WBCs, platelets
• When automated blood cell counters flag abnormal or immature cells
• Clinical suspicion of certain diseases (e.g. haematological malignancies)

Question 3

Describe the specimen collection for a peripheral blood smear?

Answer 3

Specimen collection:
• 4mL blood collected in EDTA tube (chelates Ca to prevent clotting)
• Inverted 8-10 times
• Instrument automatically preps, fixes, stains and coverslips blood films using wedge method
• Blood sample mixed and aspirated by machine
• Drop of blood added to glass slide
• Wedge prep technology incrporated so smear covers approx. 2/3 of glass slide -> obtain feathered edge
• Sample stained (Wright’s strain), then rinsed with buffer
• Results displayed on monitor
• Slides analysed by haematologist

Question 4

What are some limitations of a peripheral blood smear?

Answer 4

Limitations:
• Slide must be prepped in 2-3 hours (prevent artefacts, e.g. spherocytes)
• Under-filling container can distort morphology (high EDTA: blood ratio -> RBC shrinkage due to hypertonicity)

Question 5

What is microcytic anaemia? Provide examples?

Answer 5

Microcytic anaemia (MCV <80fL):

• iron deficiency (due to bleeding, malnutrition, absorption, GI surgery, increased demand)
• alpha-thalassaemia (alpha-globin gene deletion)
• beta-thalassaemia (b-globin gene point mutation)
• lead poisoning
• Sideroblastic anaemia

Question 6

What is macrocytic anaemia? Provide examples?

Answer 6

Macrocytic anaemia (MCV >100 fL):

1) Megaloblastic: impaired DNA synthesis -> maturation of nucleus precursor cells in BM delayed relative to cytoplasm -> RBC macrocytosis
- folate deficiency
- vit B12 deficiency

2) Non-megaloblastic: DNA synthesis unimpaired, causes included ETOH and liver disease -> RBC macrocytosis
- Diamond-Blackfan anaemia (intrinsic defect erythroid progenitor cells)

Question 7

What is normocytic anaemia? Provide examples?

Answer 7

Def: increased production of normal sized RBCs (80-100fL)

Non-haemolytic: not involving RBC destruction

• anaemia of chronic disease (RA, SLE, neoplasia, CKD)
• aplastic anaemia

Haemolytic: RBC destruction

• Intrinsic: hereditary spherocytosis, sickle cell anaemia
• Extrinsic: autoimmune haemolytic anaemia (AIHA), micro/macroangiopathic anaemia, infections