Blood transfusion is a life-saving treatment for severe anaemia and coagulopathies. Because blood transfusions can induce severe harmful reactions, particularly in cats, it is necessary to consider pre-transfusion testing and transfusion administration monitoring.
Pre-transfusion testing
Blood type systems
Today, we have identified several blood systems in cats, with the AB system being the major one. Blood types are determined by the presence of antigens on the surface of the red blood cells (RBCs). We can describe feline blood groups as A, B and AB types. While there are well-recognised breed and geographical variations, type A is the most common blood type found in cats, followed by type B and the rare type, AB (Nectoux et al., 2019).
One specificity of cats is that they have naturally occurring alloantibodies. Type B cats have strong anti-A alloantibodies that can lead to acute and life-threatening haemolytic reactions. Type A cats have few weak anti-B antibodies that can decrease the lifespan of transfused RBCs (Knottenbelt et al., 1999; Goy-Thollot et al., 2019).
Since 2007, we have discovered other feline erythrocyte antigens that are associated with the presence of naturally occurring alloantibodies (Weinstein et al.,2007; McClosky et al., 2018; Binvel et al., 2021). These antibodies motivated the Association of Veterinary Hematology and Transfusion Medicine (AVHTM) to strongly recommend pre-transfusion tests (Davidow et al., 2021).
Donor selection
Blood donor selection is crucial to the transfusion process as it decreases the risk of complications during donation and the transmission of disease during transfusion. As followed, the criteria for donor selection usually used are (Robyn and Humm, 2016; Wardrop et al., 2016):
- Body weight of at least 3kg
- Age between 1 and 10 years
- Up to date with vaccines, including rabies, if possible
- Correctly dewormed
- Healthy condition based on history, physical examination, biochemistry, complete blood count, blood smear, and FIV and FeLV tests. A PCR for Mycoplasma spp. is also recommended depending on the geographic region
- Good temperament
Blood donor selection is crucial to the transfusion process as it decreases the risk of complications during donation and the transmission of disease during transfusion
Blood typing
There are several blood typing methods for the AB system, which have been compared with each other over the years (Stieger et al., 2005; Seth et al., 2011; Spada et al., 2016). Some of these blood typing methods are dedicated to laboratory use (plate or tube tests, flow cytometry and gel agglutination), while others are available at the patient’s bedside (card agglutination and immuno-chromatography typing). In current practice, the most reliable results arise from the immuno-chromatography strip technique (Spada et al., 2016).
To date, there is no blood typing test available for non-AB systems. Moreover, due to the other blood systems, a major crossmatch should be performed prior to any transfusion, including the first one, to reduce the risk of transfusion reaction. Crossmatching tests (major and minor) must be performed if blood typing is not available (Davidow et al., 2021). Because new antibodies are synthesised as early as two days following a transfusion, it is strongly recommended that you perform a crossmatch for any cats that have previously undergone a blood transfusion.
Crossmatching
A crossmatch test is an in vitro simulator of a transfusion reaction that could appear due to antibody-antigen binding. A major crossmatch will detect blood recipient antibodies that could be specific against donor RBCs. A minor crossmatch will detect blood donor antibodies that could be specific against recipient RBCs.
A crossmatch test is an in vitro simulator of a transfusion reaction that could appear due to antibody-antigen binding
In veterinary medicine, several crossmatch blood compatibility tests exist. The first methods developed for cats were card and tube methods. These, however, are time-consuming and technically difficult, and the reading can be false due to rouleau formation, lack of standardisation or operator-dependent interpretation. Gel techniques containing antiglobulin were developed to increase test sensitivity and method standardisation (Goy-Thollot et al., 2019; Humm and Chan, 2020).
Blood donation
For cats, blood collection should not exceed 12ml/kg and usually ranges between 40 and 50ml. Closed, semi-closed and open systems are available. None of these systems appear superior to the others if good aseptic conditions and careful storage are respected.
There are multiple feline blood collection systems available, but you can prepare a collection system with syringes, a three-way stopcock and a butterfly needle. In this situation, anticoagulant citrate dextrose (ACD) or citrate phosphate dextrose adenine (CPDA) with 7ml of anticoagulant solution per 1ml of blood should be used to collect for a transfusion.
It is recommended that you add an additive solution in packed RBC units to improve storage and limit haemolysis. Storage time and temperature depend on the blood product and vary from 30 days at 4°C for whole blood and packed RBCs to one year at −20°C for plasma. Platelet-rich plasma is specific and must only be stored for less than five days at 22°C with continuous agitation.
Transfusion administration and monitoring
No pre-transfusion treatment (ie antihistamine or antipyretic) was shown to decrease the risk of transfusion reaction and is therefore not currently recommended. Before administration, the unit must be inspected for haemolysis. This can be done by measuring free haemoglobin and calculating the percentage of haemolysis or by visual inspection at least. Any blood unit with more than 1 percent of haemolysis should be discarded.
When having a blood transfusion, cats must receive AB-type-compatible blood; however, type AB cats can receive type A packed RBCs if the AB type is not available. Whole blood is mainly used for active bleeding, while packed RBCs are used for anaemia in euvolaemic cats. Plasma is used for coagulopathies with signs of bleeding. Platelet-rich plasma is reserved for cats with severe thrombocytopenia or when thrombocytopenia is associated with active bleeding.
One unit is routinely transfused for each blood product, but the formulas in Table 1 can help the clinician to accurately predict the increase in haematocrit. There is no current recommendation on the ideal blood infusion rate; however, the first 15 minutes should be at a slow rate (1ml/kg/h or less) for euvolaemic cats. Blood products should be administered over a maximum of four hours to decrease the risk of bacterial proliferation in the transfused unit.
| Type of anaemia | Volume to transfuse (ml) |
| Regenerative anaemia | Required PCV increase (%) x body weight (kg) |
| Non-regenerative anaemia | (desired PCV-patient PCV)/(donor PCV) x 100 x body weight (kg) |
Method
For cats, blood units that contain RBCs should be administered via a syringe pump with an 18-micron microaggregate (Davidow et al., 2021). Haemolysis has been shown using certain types of infusion pumps in human literature, but no study has been performed in cats. The best way to detect any transfusion reaction early is to perform effective clinical monitoring. A monitoring form was proposed in the recent consensus statement on transfusion reaction (Figure 1).
Transfusion reactions
The prevalence of blood transfusion reactions varies from 0 to 38 percent. The new consensus statement published by the AVHTM describes the different transfusion reactions that can occur (Davidow et al., 2021).
The most common reaction in feline blood transfusions is the febrile non-haemolytic transfusion reaction, characterised by a temperature above 39°C and an increased temperature (more than one degree above that at pre-administration) within four hours of the end of the transfusion. This reaction must be distinguished from haemolytic reactions, transfusion-transmitted infections and respiratory reactions such as transfusion-associated circulatory overload (TACO) and transfusion-related acute lung injury (TRALI).
The most common reaction in feline blood transfusions is the febrile non-haemolytic transfusion reaction, characterised by a temperature above 39°C and an increased temperature […] within four hours of the end of the transfusion
If a reaction is suspected, the transfusion must be stopped; the unit should be visually inspected, with the identification and expiration date checked, and a physical examination of the patient should be performed. The patient’s plasma colour then needs to be checked for signs of haemolysis. If no signs of haemolysis or haemodynamic instability are found and there is no unit error, the transfusion can be restarted at the same or a slower rate without specific treatment.
For cases of respiratory distress, the most common reaction is TACO, diagnosed as fluid overload, which must be treated with oxygen therapy and diuretics.
Xenotransfusion
If feline type-compatible blood is not available and the patient has severe life-threatening anaemia or is actively bleeding, canine blood can be used for transfusion. However, delayed haemolytic reactions were reported in recent studies as early as four days after xenotransfusion (Le Gal et al., 2019). Moreover, strong antibodies are synthesised, and the transfused cat will be at high risk of acute haemolysis for future xenotransfusions with canine blood. Due to these risks, xenotransfusion should only be performed in severe emergency conditions, only once, with owner consent.
If feline type-compatible blood is not available and the patient has severe life-threatening anaemia or is actively bleeding, canine blood can be used for transfusion
