After being called in for the second time tonight to crossmatch blood for a c-section, I got to wondering why it is really necessary in the first place. When you crossmatch blood, you are essentially trying to match your patient’s RBC antigens to donor antigens. If you just feed 'em O negs, the antigens that can cause the biggest problems aren’t even a factor anymore.
Assuming that 95% (I’m pulling this out of my ass) of the time a transfusion is not needed for a c-section, and that a woman who has never been pregnant before or had any previous blood transfusions can’t have produced any alloantibodies, why is crossmatching units necessary? Is the only reason the scarcity of O neg units?
One time a doctor called me at home after I had crossmatched some A neg units to an A pos patient (that is all I had at the time) to ask me if this was ‘standard procedure’. What is up with that? To me it seemed like she was trying to ask “are you SURE that these match?”, but she should know better than that!
I don’t know if that’s the only reason, but it sure seems like reason enough to me. In the emergency room, when someone’s spurting blood as you’re working, and you don’t even know their name yet, much less their type, there you need to use precious O neg. But with a planned procedure where the patient is conscious and in your care beforehand and you can work at your leisure, why is blood matching a problem? If you just got lazy and used O neg for every situation, whether you needed it or not, there’d be no point for any of the rest of us to donate in the first place.
That’s exactly it - and you need to remember that O- is pretty rare. IIRC from the last time I gave, about 40% each is A+ and O+, their respective negatives are about an eighth as common, AB and B are rare in this country (England).
You should note that plasma is separated and used in instances where blood volume has to be kept up at all costs, and they can worry about cross-matching later. It’s a stop-gap treatment but a lifesaver, and plasma doesn’t have to be matched up.
While the scarcity of O- is certainly a factor, another is that undiagnosed early miscarriages are common. A significant number of pregnancies (googling yields anything from 20-60%) end in miscarriages. The majority of these occur very early, oftentimes in the first weeks, before a woman is aware that she’s pregnant. Even though the woman was not aware of the pregnancy, there’s a very real possibility that at some point she did have a miscarriage and as a result has developed alloantibodies.
From most common to least common:
O+ 38%
A+ 34%
B+ 9%
O- 7%
A- 6%
AB+ 3%
B- 2%
AB- 1%
I’m A- but can’t donate 
Crossmatching blood before transfusion is always necessary (severe, even fatal, reactions can occur even when there’s no ABO incompatibility).
What is not necessary, though, and what leads to blood shortages and unjustified expense, is to crossmatch in case the blood will be transfused. What should be done is an “antibody screen”. Indeed, so long as the recipient’s serum has been screened for reactive antibodies, and none is present, there will be no problem or risk in getting and relying on a quick (15 min) crossmatch. Very, very few, if any, people can’t wait 15 minutes for a RBC transfusion (even in major trauma, the issue is virtually never lack of hemoglobin; it’s lack of blood volume - something that can be done temporarily with crystalloid and other measures).
We almost never crossmatch blood, rather we depend on antibody screening well before the blood might be needed.
Even being the same blood group won’t prevent some reactions from occurring.
For example, close relatives, contrary to what you might think ,are not ideal donors, as receiving whole blood from a genetically very similar individual may provoke an auto-immune reaction.
In recent years western blood transfusion services have lessened the chances of this reaction happening by removing the white cells and exposing the blood to radioactivity. However, it still happens in parts of the world where close relatives are often the only source of available blood, and it is less extensively treated.
If you only need to replace blood volume, what are the pros and cons of saline versus plasma?
Plasma is not used as a volume expander. It is used to replace coagulation factors. (certain ones. Cryoprecipitate is better for replacing factors) Pentaspan is the volume expander that I am aware of, it is a 10% Pentastarch solution in 0.9% saline. The colloidal properties of the starch results in a blood volume expansion in excess of the volume infused.
Back to the OP: I was mostly just irritated at having gotten called in twice for the same thing. I know that pretransfusion testing is necessary. KarlGauss I think it is a good idea for this lab in particular to just do a crossmatch and be done with it in the middle of the night. If I just did a screen and then went home, then had to come back later to match units, they would have to pay me for 2 callbacks instead of one. Y’know?
Terrific question.
Here’s a nice critique of the key trial studying the issue.
And here’s the abstract from that important article.