Blood Smears – Make Them A Routine Test!

Patient with elevated white blood cells caused by leukaemia
Patient with elevated white blood cells caused by leukaemia


Blood smear evaluation is an often overlooked but a very important aspect for in-house haematology. With the advancement in haematology analysers that can now detect reticulocytes and even band neutrophils, some practitioners are beginning to rely solely on the numerical data alone in evaluating the patient’s blood. The art of blood smear interpretation is on the decline. However, it is an extremely valuable skill that needs to be practiced and perfected and should be part of every in-house haematology.

What are the benefits of understanding blood smears?

  • Identifying of a regenerative response, looking for reticulocytes (polychromatophils)
  • Looking for the possible causes of an anaemia – eg. heinz bodies, infectious microorganisms
  • Looking for spherocytes which can indicate an immune mediated haemolytic anaemias
  • Confirming thromobocytopaenias, as frequently platelet clumping can be reported as a thromocytopaenia.
  • Assessment of the nature of a leukocytosis. High leucocyte counts do not always mean infection!! Neutrophilias can be caused by other things apart from infection, stress, corticosteroids, neoplastic leukaemias.
  • Normal leucocyte counts do not always mean the patient is ok, patients can have severe left shifts but normal leucocyte counts

Blood smear evaluation begins with becoming accomplished at producing great diagnostic smears – this of course takes practice. Poorly performed smears can be non-diagnostic and frustrating to assess for both yourself and an external pathologist.

A few tips on the technique:

  • Use a very small drop of blood, if you have picked up too much blood with the ‘spreader’ slide, then lift off and start the smear away from that drop of blood
  • Angle the ‘spreader’ slide about 30 degrees. The bigger the angle, the shorter your smear.
  • The smear should end at about half to 3/4 of the way down the slide and must have a ‘feathered edge’.

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