Blood sample collecting in small mammals

Blood samples from small mammals need to be of high quality and of sufficient volume to be useful. We've got lots of helpful hints

Posted: 11 July 2017

Blood sample collecting in small mammals


We’re all used to running diagnostic blood samples from our dog, cat and rabbit patients, but when it comes to working with smaller mammals, the challenge is greater. The blood samples still need to be of high quality and of sufficient volume to be useful.

No more than 10% blood volume

With very small animals such as mice, hamsters and gerbils, we are limited to how much we can safely collect. A common guideline is to collect no more than 10% of blood volume. The total blood volume is assumed to be no more than 8% of body weight, so we shouldn’t be taking any more than 0.8mls of blood per 100gms body weight. This is assuming the animal is well – for dehydrated or anaemic patients, this volume should be reassessed and reduced.

5 Top Tips

  1. Use heparin to reduce fibrin clot formation.
  2. Use small needles and syringes to prevent collapse of smaller vessels.
  3. Use gentle suction during sample collection to prevent hemolysis.
  4. Blood smears should be made immediately to reduce artefacts.
  5. Practice - sample collection in small exotic mammals needs experience, so take opportunities to practice.

Blood Sample Collection Sites

The choice of sample site is based on clinical experience and your own personal preference.


  • Cephalic vein – this vein is usually small and can be difficult to collect a good quality sample
  • Lateral saphenous vein – easy to visualise, though can be difficult to collect larger volumes
  • Jugular vein – best choice for larger volumes. Access may be restricted in rabbits with large dewlaps
  • Lateral auricular vein – small vein which will compress easily on aspiration
  • Central auricular artery – avoid using this as you can cause tissue damage and necrosis
  • Cranial vena cava – stressful and not recommended

Guinea Pig

  • Lateral saphenous vein – easy to locate and visualise. With good gentle restraint, this makes an ideal site for blood collection.
  • Cephalic vein – in the guinea pig this is very small and obtaining a useful sample is difficult.
  • Cranial vena cava – not recommended. Guinea pig’s get very stressed.


With most of the small rodents, manual restraint is very difficult and blood collection can be extremely difficult. Light chemical restrain is often required to immobilise the patient.

Common sites used include the lateral saphenous vein, the tarsal vein and lateral tail vein.

Risks of Blood Sample Collection

Venipuncture usually carries some degree of risk, including injury or death caused by restraint and handling.

Risks of blood collection itself include;

  • Blood vessel damage – sudden movement of the animal whilst the needle is in the vein can cause significant damage from lacerations
  • Haemorrhage – apply compression where appropriate after venepuncture until all bleeding has stopped
  • Infection – take all the necessary precautions
  • Trauma - inadvertent damage to soft tissue structures adjacent to the site of collection
  • Hypotension – calculate the safe volume to extract as a sudden decreased blood volume will cause dangerous hypotension

A combination of careful assessment, restraint and good clinical technique will all help reduce the risks.

Physical restraint

The method of restraint we use will depend on the species and collection site.

Rabbits are prone to hind limb and spinal injury if restrained inappropriately. Wrapping the animal in a towel or blanket will help minimise these risks.

Guinea pigs and larger rodents can often be restrained by simply grasping them securely around the shoulders and thorax. Alternatively you can use the towel wrap as you would with rabbits.

Manual restraint of species smaller than guinea pigs for blood collection is extremely difficult – chemical restraint is preferred.

Chemical restraint

For animals where manual restraint is difficult, sedation or anaesthesia should be considered. The risk of using chemicals is carefully balanced against the risk of not performing a blood test.