DWR Diagnostics Laboratory

Preparing a Blood Film

DWR Diagnostics, based at DWR Veterinary Specialists, is a well-established laboratory providing a comprehensive range of diagnostic testing

  • A spreader slide is used. This is narrower than the smear slide to avoid spreading the cells over the edge of the slide. To make a spreader break the corner off a normal slide, having first scored it with a diamond writer or a glass cutter
  • A drop of blood from a microhaematocrit tube is placed onto one end a slide
  • The spreader slide is placed at an angle of about 30° in front of the sample and slid backwards until it comes into contact with the sample drop
  • The sample then spreads out along the width of the spreader slide
  • The spreader slide is advanced forwards, creating a smear with a feathered edge
  • If too much blood is applied to the slide or taken up by the spreader slide the smear will be too long and the cells will be pushed over then end of the slide
  • The angle of the spreader slide can be varied to change the size of the smear – a lower angle produces a longer smear. Care should be taken not to make the smear too long. The smear should extend to the middle or distal 2/3rd of the slide so that the examination area is fairly central
  • Slides with a frosted end are preferred since these are very smooth and produce good smears which can be easily labelled.

Causes of poor smears and their solutions

Problem Cause Solution
Smear too long
(feathered edge has disappeared off the end)
Spreader speed too slow
Low viscosity blood ie anaemia
Excess blood applied to slide
More rapid spreader speed
Apply smaller blood spot
Smear too short/thick Spreader speed too fast
High viscosity blood i.e. high PCV
Slower spreader speed
Feathered edge consists of long streaked tails Uneven contact of spreader with slide
Spreader has roughened edge
Replace spreader
Apply even pressure using index finger on top of spreader
Smear has holes or gaps Grease on slide Clean slides before use
Smear thick at feathered edge end of film Blood in front of spreader Ensure firm contact of spreader and slide when pulling spreader backwards towards blood spot