The volume of blood in a healthy guinea pig is equivalent to about 7% of the animal’s body weight; 10% of the total blood volume can be safely taken from a healthy guinea pig, but it is wise not to take more than half this amount from debilitated subjects.
It is difficult to take blood samples from a guinea pig, particularly without sedation, because of the animal’s stubby shape. The veins used for taking blood samples are the lateral saphenous, jugular and caval veins. The volumes of blood which can be withdrawn from the first two veins are very small, while sampling of the caval vein usually necessitates sedation (brief, direct induction with isoflurane can be used); the feasibility of taking blood from this vein does, therefore, depend on the general state of the animal and the risks and benefits must be weighed. The needle is inserted cranially at the level of the first rib, laterally to the sternal manubrium, at an angle of 45° and directed towards the contralateral posterior limb. In order to take blood from a jugular vein, the animal’s head must be placed in marked extension, which could worsen any respiratory problems present. In severely debilitated and stressed subjects, it is better to avoid worsening the animal’s condition by taking a blood sample, postponing this procedure until the animal’s condition has stabilised.
The prothrombin time of the blood of guinea pigs is longer than that of rabbits and, therefore, guinea pig blood tends to clot more slowly. Guinea pig lymphocytes are not very sensitive to cortisone, which does not alter the peripheral blood lymphocyte count. The guinea pig has a particular type of white blood cell called the Kurloff cell which is produced by the thymus under the effect of oestrogens. The function of Kurloff cells is currently unknown. These cells are more numerous in pregnant sows, less common in males and rare in prepubescent animals. Kurloff cells have the appearance of lymphocytes and contain round or oval inclusions called Kurloff bodies.