Blood smears – a step by step guide

Collecting blood is a procedure which takes place in all veterinary practices almost every day of the year. The skills required to take and prepare a good blood smear are vital to being able to examine and interpret the sample accurately, regardless of whether you use your own lab machines, or you choose to send them away.

Posted: 14 January 2020

Blood smears – a step by step guide


They’re essential for examining cell size, morphology and numbers, and are frequently used when trying to find evidence of;

  • Anaemia
  • Haemolytic conditions
  • Blood borne parasites
  • Mycoplasma infections
  • Blood related neoplasia

What equipment do you need to prepare a blood smear?

Fortunately, the only expensive piece of equipment required is a high-quality binocular microscope (you can get away with a monocular microscope, but it’s not as enjoyable). It pays to invest in the best the practice can afford, as the better the quality, the better the image and resolution. Apart from this, all you need is;

  • EDTA tubes – blood for haematology is generally collected in EDTA.
  • Microscope slides - preferably with frosted ends.
  • Spreader slide – use a slide with bevelled edges.
  • Capillary tubes - micro haematocrit tubes are ideal.
  • Pencil – this is used to label the slides on the frosted end of the glass. Pens and felt tip markers should be avoided as the ink usually washes off with the stains.

A step-by-step guide - blood smear preparation

  1. Collect the blood – when possible use a 21G needle, as there is more chance of haemolysis with smaller needles. Fill an EDTA tube to the indicated line, and invert gently to mix and prevent clotting.
  2. Capillary tube – using a micro capillary tube, allow the end of the tube to enter the blood until it fills to half its length.
  3. Allow a drop to form - hold the capillary tube vertically over the slide and allow 1 drop of blood to form on the end of the tube.
  4. Place the drop – transfer the drop to the slide at the frosted end.
  5. Spread the drop - hold the spreader slide at a 30-degree angle in front of the drop and pull back until it touches the drop. Make sure the spreader slide is in even contact with the lower slide.
  6. Push across the slide – allow the blood to spread evenly along the edge of the spreader slide, then in one smooth movement, push the spreader slide forward along the length of the lower slide. Maintain a continuous movement, ensuring the angle and contact is continuous.
  7. Feathered edge – the best smear is one which covers around three-quarters the length of the slide and has a feathered edge.
  8. Air dry - leave the slide to dry and make more smears as required.

Top tips for making great smears

  • Practice – the more you make blood smears, the better you will become.
  • Submit everything – when sending blood away, make sure you send all the smear slides you make. Even the ones you consider to be poor can hold important cellular information.
  • Make them immediately – it’s always best to prepare your slides at the same time as you collect the blood (unless you are out on a farm!). The cells will be fresh and will be the most representative.
  • Use clean slides - these should be of good quality and free from dust, chipped ends and fingerprints.

What can go wrong?

  • Drop size - too small and the smear may not be long enough, but too large and the smear may extend beyond the edge of the slide. This will inevitably result in cells being lost and a failure of the feathered edge.
  • Slide angle – if the angle between the spreader slide and the smear slide is greater that 30-degrees, this can result in a smear which is too thick or short.
  • Speed of spreading – if the slide is spread too fast, this can result in a short and thinner smear, whilst slower spreading can result in a longer or thicker preparation
  • Viscosity – blood which is too thick (e.g. if an animal is dehydrated and has a high haematocrit) can result in a short thick smear. You can reduce the angle between the slides in this situation to help produce a thinner smear. If an animal is anaemic, the viscosity is reduced which produces a thin smear. In this situation, increase the angle of spread and you’ll get a better smear.