Phlebotomy techniques for exotic companion mammals (Proceedings)

Collection of blood for diagnostic testing can be challenging in exotic pet mammals, especially in smaller species. Modern technologies allow acquisition of useful information from very small samples, but samples still must be of high quality and meet minimum volume requirements.

Volume of sample is limited by patient size. A common guideline is to collect no more than 10% of blood volume. Total blood volumes vary from species to species but can generally be assumed to be 6-8% of body weight. Clinical judgement may necessitate adjustments to recommended volume limits, especially in debilitated patients.

Samples can be run with in-house equipment or sent to reference laboratories familiar with diagnostic testing in these species. The Abaxis in-house blood chemistry analyzer can run a chemistry panel on whole blood samples, and practical minimum volume is 0.13 cc of high quality whole blood. Reference laboratories will often analyze smaller samples, but may use dilution techniques. Individual laboratories should be contacted for instructions on submission of low volume samples for analysis.

When sample volume does not allow clinical chemistry or complete blood count, a manual CBC can be obtained on a single drop of blood collected into a heparinized hematocrit tube. Information that can be obtained from a manual in-house CBC includes hematocrit, differential white blood count and analysis of both red and white blood cell morphology and characteristics, complete blood count utilizing a hemocytometer, and measurement of total serum solids.

Sample collection in small exotic mammals requires practice. Collection site depends on species, patient condition and practitioner preference. The following guidelines are offered based on the author's personal preference and experience, and others may advocate other sites or techniques. The "correct" technique is that which results in consistently good results with optimal patient safety.

Risks of Blood Sample Collection

Venipuncture inherently involves some degree of risk, including risk of injury or death caused by restraint and handling. Risks from blood collection itself include vessel laceration and hemorrhage, infection, inadvertent damage to adjacent soft tissue structures and sudden hypotension due to decreased blood volume. Careful patient evaluation, restraint and technique help minimize risk.

Restraint

Restraint is relatively easy in larger species such as ferrets and rabbits, and increasingly challenging in smaller species. Method often depends upon the preferred collection site. In some cases, sedation or anesthesia is appropriate and necessary, especially when considering collection from the vena cava in species other than the ferret.

In the author's experience, most ferrets can be safely restrained manually for blood collection by scruffing and stretching, or by rolling the patient in a towel with the head, neck and forelimbs exposed. It should be noted some ferrets will not tolerate this form of restraint.

Rabbits are prone to self-injury during restraint and handling, especially luxation and sub-luxation of the vertebrae caused by kicking with powerful rear legs. Restraint, therefore, must minimize this risk. The author prefers using a burrito-style towel wrap with either the head or caudal portion of the rabbit exposed, depending on desired collection site.

Guinea pigs and larger rodents can often be restrained by simply grasping them securely around the shoulders and thorax or with a towel wrap. It becomes exceedingly difficult to safely manually restrain species smaller than the guinea pig for blood collection.

For individuals where manual restraint for sample collection is difficult and risky, or patient struggling is excessive, sedation and/or anesthesia should be considered. The risk of sedation and anesthesia must be balanced against the risk of foregoing diagnostic blood testing. Various sedation and protocols can be considered, and method should be chosen depending on a number of factors, including overall patient condition. Methods include simple sedation, and anesthesia with injectable and/or inhalant agents.

Blood Sample Collection Sites

Sample site is based on clinical experience and practitioner preference. Author preferences are listed in table 1. The greatest limiting factors in regard to site selection are patient size and ability to safely restrain.

Notes on Sample Collection from the Vena Cava

Exotic practitioners have utilized the vena cava as a safe and effective method of sample collection for many years. Careful understanding of the anatomical relationship of the vessel, the heart and external landmarks greatly reduces risk. In the ferret, the vena cava is exceptionally long due to relative caudal placement of the heart in the thoracic cavity. The cava is surrounded and protected by fat, and is accessible just below the notch of the sternal manubrium. Correct needle placement is very shallow, and poses no risk to inadvertent penetration of the heart.

The cava is shorter in other species, and the distance between accessible vessel and the heart is progressively smaller as patient size decreases. Use of small, short needles (1/2 and 5/8th inch) reduces risk. In all but very small guinea pigs, these needles are too short to reach the heart. In other smaller species, however, risk of cardiac penetration is greater, and the practitioner must avoid advancing the needle into the thoracic cavity. It should be noted that in most cases, inadvertent cardiac puncture is fortunately not associated with severe complications or death.

The general procedure for vena cava puncture is as follows:

• Isolate the center of the cranial sternum and manubrium

• Insert a 25-27 g needle to the right or left of the manubrium at a slight angle aiming for the opposing hip

• Advance the needle while applying negative pressure, redirecting slightly until a flash of blood is detected. The vessel is very close to the surface under the cranial sternum, and it is not necessary to advance the needle very far

• Blood should flow readily after detecting the flash. Failure to flow often means the needle was advanced into and through the vessel, or the bevel is obstructed by the vessel wall. Pull out, rotate and/or redirect slightly until blood flow improves.

Table 1

References available upon request.