Two tests sit behind every feline transfusion, and they answer different questions. Blood typing tells you which antigens the red cells carry. A crossmatch tells you whether this particular donor's blood will survive in this particular recipient. A cat can be correctly typed and still react, which is why most protocols run both and why the crossmatch is not optional before a second transfusion.
What typing and crossmatching each cover
Typing looks at the cat's own red cell antigens and reports A, B or AB. It takes 5 to 10 minutes on an in-clinic card or strip, or 1 to 2 days at a reference laboratory. Crossmatching mixes recipient serum with donor red cells (the major crossmatch) and donor serum with recipient red cells (the minor crossmatch), incubates at 37 C for 15 to 30 minutes, spins at roughly 1000 g for 5 minutes, and scores agglutination from 0 to 4+. Typing needs 1 sample; a crossmatch needs 2 and compares them.
Neither test is a guarantee on its own. Typing screens for the 2 antigens a card can see; a crossmatch screens for antibodies against those and any others. Both are read by eye at 10x, so technique changes the result.
Type A, B and AB: what the distribution looks like
Most domestic shorthairs in North America and northern Europe are type A, with B falling between 1 and 5 percent and AB under 1 percent. Those figures move sharply by breed and region: some British Shorthair and Devon Rex lines run 20 to 40 percent B, while certain Asian and Australian populations show much higher B frequencies. A card reading B in a Devon Rex is unremarkable; the same result in a random street cat is worth repeating.
| Type | Share of domestic shorthairs | Natural antibody | Can receive |
|---|---|---|---|
| A | About 90 to 99 percent | Weak anti-B | A, crossmatch advised |
| B | About 1 to 5 percent | Strong anti-A | B only |
| AB | Under 1 percent | None detected | A or B, still crossmatch |
| B in some breed lines | 20 to 40 percent | Strong anti-A | B only, type the donor too |
Why cats react on the first transfusion
Dogs need a prior exposure to make antibodies against foreign red cells. Cats do not: type B cats carry naturally occurring anti-A strong enough to destroy transfused A cells within minutes to hours, and type A cats carry weaker anti-B that can still shorten red cell survival. There is no safe first transfusion by guesswork. The classic picture is a type B cat given 10 to 20 mL of type A blood and showing hemoglobinemia, hemoglobinuria, fever or collapse within 1 to 2 hours.
This is also why a cat transfused 3 years ago needs a fresh crossmatch, not just a type card. Prior transfusion sensitizes the recipient, and the second exposure is where delayed reactions in the 3 to 14 day window appear as a falling hematocrit with no visible bleeding.
Reading the crossmatch score
Results are graded 0, 1+, 2+, 3+ and 4+, where 0 is a smooth suspension of free cells and 4+ is one solid clump with clear supernatant. A 0 at both major and minor steps is compatible. A 1+ is the awkward middle: some laboratories accept it if the cells resuspend gently, others repeat the test or pick a different donor. Anything at 2+ or above in the major crossmatch is incompatible, and the donor is swapped, not diluted.
Ask for the numbers rather than the word compatible. A report that says major negative without a score, a control tube and an incubation time has told you less than it appears to.
Strong agglutination: what a high score means
A 3+ or 4+ major crossmatch means recipient antibodies are binding donor cells hard. In a type B cat this is expected against A cells and the transfusion should not proceed. In a type A cat with a 3+ reaction against a type A donor, the cause is usually a previous transfusion, a pregnancy, or an unmeasured antigen, and the answer is another donor plus a repeat crossmatch. A high score after the line is already running is a stop signal: pull the line, record the reaction, and keep the bag for culture rather than discarding it.
Weak or negative results: what a low score means
A 0 or 1+ major crossmatch is compatible enough to transfuse, but not proof of safety. False negatives come from a sample that sat in the fridge for 3 days, from a hematocrit so low that few donor cells reached the tube, or from a reader working too fast. False positives are more common and come from autoagglutination, rouleaux with high plasma protein, lipemia after a fatty meal, or a dirty tube. Saline washing plus a saline control tube separates most of these.
Timing matters both ways: crossmatch within 24 hours of collection, and finish the transfusion within 4 hours of starting, because a bag sitting at room temperature for 6 hours is a bacterial culture.
Neonatal isoerythrolysis in 3 numbers
A type B queen bred to a type A tom can produce type A kittens. Those kittens are fine in the uterus and fine for the first 12 to 24 hours, until they absorb anti-A from colostrum through the gut. Hemolysis then shows up between 24 and 72 hours as dark urine, weakness, jaundice, or sudden death in a litter where 1 or 2 kittens fade. The practical step is to type the queen before breeding, or remove kittens from a B queen for 24 hours and hand-rear them.
What to ask your vet before blood is ordered
Ask 4 questions. Was the recipient typed in-clinic or sent out, and how long did the result take. Was a crossmatch done, and what were the major and minor scores. Was the donor typed and crossmatched too, since a type B donor given to a type A recipient is a mismatch in the other direction. And is a second transfusion likely, because a cat that receives blood once has a 3 to 14 day window where a delayed reaction can mimic ongoing blood loss.
The 5 item checklist for transfusion day
- Type card result written in the record with date and method.
- Major and minor crossmatch scores, plus a saline control.
- Baseline hematocrit, total protein, temperature and heart rate before the line goes in.
- Vital signs every 15 minutes for the first hour, then hourly.
- Hematocrit recheck at 12 to 24 hours to confirm the cells survived.
Reference intervals, typing reagents and crossmatch protocols differ between laboratories and hospitals, so use the values printed on your own report and read more on feline blood volume and renal blood flow if the anemia is chronic.
Frequently asked
Do cats really need a crossmatch if both are type A?
Yes, because typing only covers the 2 antigens the card can see. A type A recipient that was transfused or pregnant before can carry antibodies against other antigens, and a major crossmatch score of 2+ or higher means that donor is incompatible even though both cards read A.
How long does a blood type card take, and can it be wrong?
An in-clinic card or strip takes 5 to 10 minutes, while a reference laboratory needs 1 to 2 days. Weak A antigen in an AB cat is the classic misread, where a card reports B and a laboratory method finds AB, so an unexpected B result in a random cat deserves a repeat.
What does a 1+ crossmatch mean for my cat?
It sits in the grey zone. Some laboratories accept 1+ if the cells resuspend smoothly and a saline control is clean, others repeat the test or choose a different donor. Ask for the major and minor scores plus the control result rather than the single word compatible.