Crossmatching Basics
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Direct answer
Donor red cells plus recipient serum — that pairing is the major crossmatch, the tube whose compatibility must be established before any unit is released, because the recipient's antibodies are what will attack the transfused cells. The minor crossmatch (donor serum plus recipient red cells) has largely disappeared from routine practice now that donor plasma is screened so thoroughly. A full serological crossmatch walks through three phases: saline at room temperature to catch ABO incompatibility, incubation at 37 degrees with albumin or low-ionic-strength saline (LISS) to bring IgG antibodies into play, and the anti-human globulin (AHG) phase to reveal clinically significant coating antibodies. A compatible crossmatch is valid for 72 hours, after which a fresh patient sample is mandatory.
What you must remember
- Major crossmatch: donor red cells (about 2-5 per cent suspension) with recipient serum; agglutination or haemolysis at any phase = incompatible.
- Minor crossmatch: donor serum with recipient cells; historically routine, now omitted because donor antibody screening has made it redundant.
- Saline phase at room temperature detects IgM (ABO); the 37 degrees and AHG phases detect IgG (Rh, Kell, Kidd, Duffy) — the antibodies responsible for delayed reactions.
- LISS shortens the 37-degree incubation from 30-60 minutes to about 10-15 minutes without losing sensitivity.
- A negative antibody screen allows an immediate-spin (saline-only) crossmatch or even computer (electronic) crossmatch for rapid issue.
- Compatibility expires at 72 hours because a recently transfused or pregnant patient can mount new antibodies within days.
- In exsanguinating haemorrhage, group O Rh negative red cells are released under an emergency-release protocol before testing completes — with documented accountability.
- An AHG-incompatible, ABO-compatible crossmatch means an antibody is present: proceed to identification with a panel and select antigen-negative units.
A rushed night call, worked through
At 2 a.m. the labour ward calls: postpartum haemorrhage, pressure falling. The sequence that follows is drilled. A clotted or EDTA sample reaches the blood bank with a properly signed requisition, and while grouping proceeds, two units of O negative red cells are issued under the emergency protocol with the physician's acceptance noted — treatment never waits for serology.
In parallel, the ABO and Rh group is established and an antibody screen is set. If the screen is negative, the technologist performs immediate-spin crossmatches with ABO-identical units: no agglutination at room-temperature saline within minutes, and the first group-specific unit follows the O negative units into use. If the screen is positive — an anti-K or anti-c, say — each donor unit is crossmatched through the full LISS-AHG technique and only antigen-negative, AHG-compatible units are labelled for issue; the delay buys safety against a haemolytic reaction far worse than the minutes lost. After roughly ten units transfused, dilution and consumption shift the picture, and many protocols then allow group-compatible units with ongoing monitoring. The paperwork — who released what, when, to whom, and who acknowledged — is as much a part of the procedure as the tubes, because transfusion chains are audited link by link.
How examiners probe crossmatching
"Why is the major crossmatch called major?" — because the recipient's antibody volume attacking donor cells is the dominant, dangerous direction; the donor's small plasma volume against recipient cells is minor by arithmetic and by risk. Expect the validity question ("the patient was transfused on Monday; can Monday's crossmatch cover Thursday's unit?") — no, 72 hours is the limit precisely because new antibodies can appear after exposure. And beware the overconfidence trap: a compatible crossmatch does not promise a reaction-free transfusion — febrile non-haemolytic reactions, TRALI, sepsis from contaminated units and anamnestic responses below the detection threshold all occur through mechanisms crossmatching cannot touch.
Frequently asked questions
What is tested in the major crossmatch?
Donor red cells incubated with recipient serum across saline, 37-degree and AHG phases — it detects antibodies in the patient that would destroy the transfused cells.
Why has the minor crossmatch been discontinued?
Donor plasma is now screened for irregular antibodies during collection, removing the risk the minor crossmatch guarded against.
Which crossmatch phase detects IgG antibodies?
The 37-degree LISS or albumin incubation followed by the anti-human globulin phase; the saline phase alone detects only IgM such as ABO antibodies.
How does LISS speed up crossmatching?
Low ionic strength medium accelerates antibody-antigen binding, cutting 37-degree incubation to about 10-15 minutes with full AHG sensitivity retained.
For how long is a compatible crossmatch valid?
72 hours from collection of the patient sample; beyond that, a fresh sample is required to catch newly formed antibodies.
When is O negative blood released without crossmatching?
In life-threatening haemorrhage before testing can be completed, under a documented emergency-release protocol, switching to group-specific AHG-crossmatched units as soon as possible.