Unlike dogs, cats have naturally occurring alloantibodies against foreign blood types without any prior transfusion exposure. A Type B cat receiving Type A blood can die from an acute hemolytic transfusion reaction within minutes, even if it has never been transfused before. This makes blood typing mandatory before any feline transfusion. Use the Blood Transfusion Calculator to determine safe volumes and consult the Internal Medicine Specialist for complex transfusion cases.
The feline AB blood group system is the primary clinically relevant blood group in cats. It consists of three types: Type A, Type B, and the rare Type AB. Unlike the canine DEA system with its multiple independent antigens, the feline system involves a single gene locus with three alleles, where A is dominant over B, and AB appears to be a separate allele.
Type A is by far the most common blood type in domestic cats worldwide. In domestic shorthair and domestic longhair populations, Type A prevalence exceeds 95% in most geographic regions. Type A cats have weak anti-B alloantibodies (low titer, primarily IgM). A Type A cat receiving Type B blood will develop a mild to moderate reaction with shortened red cell survival (red cells survive only 2 days rather than 30+ days), but acute fatal reactions are uncommon.
Type B is less common overall but has significantly higher prevalence in certain purebred populations. Type B cats possess strong, high-titer anti-A alloantibodies (both IgM and IgG). This is the critical clinical concern: a Type B cat receiving even a small volume of Type A blood can develop a fatal acute hemolytic transfusion reaction within minutes, characterized by hypotension, bradycardia, apnea, seizures, and DIC.
Type AB is rare (<1% in most populations) and represents a distinct phenotype, not a heterozygous state. Type AB cats have no alloantibodies and are therefore "universal recipients." They can safely receive either Type A or Type B blood, though Type A is preferred due to greater availability.
Breed significantly influences blood type distribution. Knowledge of breed predispositions helps prioritize blood typing but should never replace actual testing.
| Breed | Type B Prevalence | Clinical Significance |
|---|---|---|
| British Shorthair | 30-50% | Very high risk; always type before transfusion |
| Devon Rex | 20-45% | High risk; type all patients |
| Cornish Rex | 25-35% | High risk; type all patients |
| Exotic Shorthair | 20-30% | High risk; type all patients |
| Persian / Himalayan | 10-20% | Moderate risk |
| Abyssinian / Somali | 10-20% | Moderate risk |
| Birman | 15-25% | Moderate to high risk |
| Scottish Fold | 15-25% | Moderate to high risk |
| Ragdoll | 10-18% | Moderate risk |
| Domestic Shorthair (DSH) | 1-5% | Low risk but still must type |
| Siamese / Oriental | <1% | Very low risk; nearly all Type A |
Warning: Even in breeds with low Type B prevalence, blood typing must be performed before transfusion. A 3% Type B prevalence in DSH cats means that roughly 1 in 33 cats is Type B. The consequences of mismatched transfusion are so severe that skipping blood typing is never acceptable.
Several methods are available for feline blood typing in clinical practice. Point-of-care card typing kits (e.g., RapidVet-H, Alvedia) use monoclonal antibodies to detect Type A and Type B antigens on the patient's red blood cells. These kits provide results in 2-5 minutes and have good accuracy for Type A and Type B identification, though they may have reduced sensitivity for the rare Type AB.
Tube agglutination using anti-A and anti-B reagents is the reference laboratory method. It is more sensitive than card-based typing but requires laboratory processing time. Gel column technology is increasingly available and offers excellent accuracy with standardized interpretation.
Regardless of the typing method used, a major crossmatch should also be performed before transfusion, particularly if the cat has been previously transfused (even once). The crossmatch detects incompatibility due to antibodies beyond the AB system, including the recently described Mik antigen.
Neonatal isoerythrolysis is the feline equivalent of hemolytic disease of the newborn. It occurs when Type A (or Type AB) kittens are born to a Type B queen. The queen's strong anti-A alloantibodies are present in her colostrum and are absorbed through the kitten's intestinal mucosa during the first 16-24 hours of life, before gut closure occurs.
These maternally absorbed anti-A IgG antibodies then bind to and destroy the kitten's Type A red blood cells, causing immune-mediated hemolytic anemia. Clinical signs develop within the first 1-3 days of life and include:
- Failure to nurse, lethargy, fading
- Icterus (jaundice) by 24-48 hours of age
- Dark red-brown urine (hemoglobinuria)
- Tail tip necrosis (due to peripheral red cell agglutination)
- Acute death in severe cases
NI can be prevented by blood typing all breeding queens and toms. If a Type B queen is bred to a Type A tom, the kittens should be removed from the queen at birth and fostered on a Type A queen or hand-raised for the first 16-24 hours until intestinal closure prevents antibody absorption. The kittens can then be returned to the Type B queen for normal nursing.
In 2007, a novel feline red cell antigen called Mik was identified. Mik-negative cats can develop anti-Mik antibodies naturally, similar to the AB system. Mik incompatibility can cause hemolytic transfusion reactions even when AB type is matched. The prevalence of Mik-negative cats is estimated at 5-10% in some populations.
This discovery reinforced the importance of crossmatching in addition to blood typing before every feline transfusion, especially repeat transfusions. A major crossmatch tests the recipient's plasma against the donor's red cells, while a minor crossmatch tests the donor's plasma against the recipient's red cells. Both should be performed.
The Triage/Emergency Specialist can guide you through emergency transfusion protocols when time is critical but blood typing must still be completed. Card typing takes only 2-5 minutes and should never be skipped.
Given the risks, every feline transfusion should follow a structured safety protocol:
- Step 1: Blood type the recipient (card or tube method)
- Step 2: Select a type-matched donor or type-matched blood product
- Step 3: Perform major and minor crossmatch
- Step 4: Calculate transfusion volume using the Blood Transfusion Calculator
- Step 5: Begin at a slow test rate (0.5 mL/kg/hr for the first 15-30 minutes)
- Step 6: Monitor vitals every 5 minutes during the test dose, then every 15-30 minutes
- Step 7: If no reaction, increase to therapeutic rate (typically 5-10 mL/kg/hr, not exceeding 20 mL/kg/hr)
- Step 8: Complete transfusion within 4 hours of spiking the blood bag
- Type B cats have strong anti-A alloantibodies that cause fatal hemolytic reactions; always blood type before transfusion.
- British Shorthair, Devon Rex, and Exotic Shorthair breeds have 20-50% Type B prevalence.
- Neonatal isoerythrolysis occurs when Type A kittens nurse from Type B queens; prevent by typing breeding cats.
- Crossmatch in addition to blood typing is essential due to the Mik antigen and other potential incompatibilities.
- Type AB cats (<1%) are universal recipients; they have no alloantibodies.