RATIONALE FOR THERAPY

Whole blood is a mixture of cellular constituents suspended in a liquid transport medium. The cells have different functions. Erythrocytes carry oxygen and participate in host defense by surface adsorption and absorption of many materials, phagocytes control bacteria, platelets are required for hemostasis, and lymphocytes mediate immunity. The liquid medium also contains an array of dissolved substances: albumin, globulins, coagulation proteins, metabolic intermediates, electrolytes, organic anions, and trace elements. Practical techniques for separation and concentration of some of the cellular constituents of whole blood are within the capabilities of all major veterinary blood donor centers. Modern transfusion therapy should be based upon use of components to treat specific defects with concentrates of the deficient blood constituent. There are a number of rationales for the preferential use of blood components. (Barnes, 1993)

Consideration of the Limited Resource

The most cogent argument supporting component therapy is that blood is a precious resource considering its therapeutic potential and the logistics and costs required in obtaining and delivering blood products. Separation into components permits a single donation to meet the individual needs of more than several patients. Blood donor screening eligibility criteria should be sufficient to obtain a safe donation.

Kinetic Considerations

Following hemorrhage, homeostatic mechanisms restore the various blood constituents at differing rates, depending on the capacity for synthesis, endogenous consumption, degradation, and distribution in various physiologic compartments. The half inactivation time of canine and feline red cells is in terms of months whereas the half-life of albumin is just three to four days. Surgical blood loss may require restoration of red cells. Albumin may not be required as it will be restored within several days. Another consideration is tolerance. Loss of fifty percent of red cell mass is well tolerated in a healthy individual whereas loss of fifty percent of blood volume can be fatal unless rapidly corrected.

Consideration of Adverse Effects

Other rationale for supporting the use of blood components include the myriad of possible adverse effects that can result from transfusion of unnecessary blood constituents. Any transfusion reaction means that the transfusion is not doing the intended job and, importantly, has burdened a patient already burdened by the physiologic state requiring transfusion. Sensitization to blood cells can result in refractory results in subsequent transfusions. Transfusion of multiple units of whole blood sequentially in order to achieve a certain hematocrit may also produce pulmonary edema due to volume overload.

Blood Donor Screening

All blood donors should be given thorough physical examinations at each donation and be annually screened hematologically, biochemically, and serologically. Donors should be healthy, receiving adequate nutrition, and be parasite free. All donors should be blood typed and be current on appropriate vaccinations. Female donors should not have had pups or kittens and preferably not be intact. In addition, all canine donors should be screened for brucellosis, heartworm microfilaria, ehrlichiosis, Rocky Mountain spotted fever, trypanosomiasis, and systemic mycoses. Feline donors should be house cats not allowed to roam. Cats should be screened for retroviruses, heartworm microfilaria, toxoplasmosis, and hemobartonellosis.

Blood Typing (see below)

The feline AB blood group system consists of three blood types: type A, type B, and type AB. All type B cats have strong alloantibodies against type red blood cells. Type A cats have weak but potent antiB alloantibodies in terms of the life expectancy of transfused type B cells. These alloantibodies are responsible for transfusion reactions and neonatal isoerythrolysis in cats and can be detected by crossmatch procedures. Feline patients receiving blood products should receive donor products of the same blood type as the patient and... have had crossmatch testing which indicates compatibility. Cats with the rare AB blood type should receive AB blood (often quite difficult to obtain) or type A blood which is compatible or only slightly incompatible in the minor crossmatch. There are no feline "universal" donors.

A simple "in-house" card test for feline red cell antigens A, B, and AB and canine DEA 1.1 has The dog has eight different blood types identified as dog erythrocyte antigens (DEA) 1.1, 1.2, 3, 4, 5, 6, 7, and 8. The use of DEA 1.1 and 1.2 positive blood products that are crossmatch incompatible may cause hemolysis. Controversy exists as to whether DEA 7 is an important determinant in canine transfusion reactions. Ideally canine blood negative for DEA 1.1, 1.2 and 7 should be used as it conforms with the concept of "universal" donor blood. In random source, first time canine transfusion of non-crossmatched or typed blood the transfusion reaction rate is approximately fifteen percent. Again, transfusion reaction indicates that the materials transfused are not effective and are causing a physiologic burden on an already burdened patient--reasons to blood type and crossmatch. Recently there has been suggestions that the only significant canine antigen is DEA 1.1. Donors blood products negative for DEA 1.1 that are crossmatch compatible have a much reduced chance of transfusion reaction. Recently, card tests to detect feline blood types A, B, and AB and canine blood type 1.1 have become available.^a

The Crossmatch

Crossmatching reveals the presence of naturally occurring isoantibodies or antibodies generated in response to a previous incompatible transfusion. Crossmatching does not prevent sensitization of the patient to future transfusions. In fact, even though a specific blood donor is crossmatch compatible with the patient, if five or more days have elapsed since the first transfusion, another crossmatch must be performed if more blood products are to be administered and especially when the same donor blood products will be used.

Crossmatching is a simple technique that can be performed with standard laboratory equipment. A major, minor, and autocontrol crossmatch should be performed although the minor crossmatch is rarely used in dogs. The major crossmatch should always be compatible at room temperature and at 37 degrees Celsius (°C).

The following is a simple (nonpurist) major crossmatch protocol (donor red cells and patient serum or plasma):

1.  Centrifuge EDTA or citrated DONOR blood at the LOWEST centrifugal rate possible on your centrifuge...for about 10 minutes.

2.  Remove 0.2 mLs of packed and place in 4.8 mLs of normal (0.9%) saline. Mix. (There are now 5.0 mLs in the tube; this essentially replaces washing step.)

3.  Place 0.1 mL of this mixture into three small test tubes.

4.  Place 0.1 mL of PATIENT serum or plasma into each of the three tubes described above. Each tube will now have 0.1 mL of the donor red cell-saline mixture and 0.1 mL of patient serum or plasma, a total of 0.2 mLs each.

5.  Incubate-for 15 minutes-one tube at 37 degrees C, one at room temperature (25 degreesC), and one at refrigerator temperature (4 degrees C).

6.  Centrifuge briskly for one minute.

7.  Examine the supernatant for any hemolysis. Any hemolysis indicates crossmatch incompatibility.

8.  Examine the cell button. Flick or swish the test tube. The fluid in the tube should redden as red cells disperse. If the button is agglutinated or microagglutinated (examine a drop under low microscopic power), this indicates crossmatch incompatibility. To complete the minor crossmatch, use patient red cells & donor serum or plasma. To complete the autocontrol use patent red cells and patient plasma. In dogs the minor crossmatch is useful for a patient receiving multiple plasma product transfusions.

The Decision to Transfuse

All transfusion therapy can produce only transient improvement in the patient's condition. Unless the patient is able to produce the deficit component endogenously, more transfusions will be necessary. Furthermore, transfusions dampen the physiologic response to deficiency of a blood constituent. For example if a patient has a low red cell mass, tissue hypoxia results in increased erythropoietin production and the marrow responds with reticulocytosis. Red cell transfusion, in this patient, will result in diminished and delayed reticulocyte response. Several questions should be considered when considering transfusion.

1.  Is blood transfusion really necessary?

2.  What is the patient's particular clinical need?

3.  Does the prospective benefit justify the risks of transfusion?

4.  What blood component will effectively meet this special need at the lowest cost?

5.  (After transfusion) Did the transfusion result in the anticipated benefit for the patient?