13.3 Compatibility Testing and Crossmatch

13.3 Compatibility Testing and Crossmatch

Quick Answer: Compatibility testing answers two questions: is this unit ABO compatible, and does the recipient have unexpected antibody that will react with it? The major crossmatch is donor red cells + recipient plasma. Immediate-spin XM detects ABO incompatibility. It will not detect most IgG alloantibodies. AHG XM is required if there is a current or historical clinically significant antibody — issue antigen-negative units and carry them through AHG. Electronic / computer XM needs two determinations of ABO, a negative current screen, no significant antibody history, and a validated system. Emergency release is uncrossmatched group O — O-neg for women of childbearing potential; many hospitals use O-pos for adult males. Massive transfusion has a switch policy.

IV.A.2 on the June 9, 2026 outline is detection plus this section. Identification of the antibody is Chapter 14. Here you decide which serologic crossmatch, or no serologic crossmatch, a given patient is allowed.

What the crossmatch is for

The major crossmatch incubates donor red cells (usually from the segment) with recipient plasma. It is a final check that this specific unit is compatible with this specific patient. It is not a second antibody screen against random group O cells, and it is not a typing of the donor. The donor was already ABO/D typed at the collection facility; you are asking whether this recipient will destroy these cells.

The minor crossmatch (donor plasma + recipient cells) is largely obsolete for packed red cells. There is almost no donor plasma left in an RBC unit, and donor isoagglutinins are not the routine risk. Do not add a minor XM to a packed-RBC stem unless the question is testing that you know it is historical.

Clerical check is part of compatibility: right patient, right specimen, right unit, right interpretation. More ABO disasters are wrong unit to the bedside than failed serology.

If the patient was transfused or pregnant in the last 3 months, the type-and-screen specimen must be collected within 3 days of transfusion. Many laboratories apply the 3-day rule to all patients so they never have to reconstruct the history at 2 a.m.

Immediate-spin XM versus AHG XM

Immediate-spin crossmatch mixes donor cells and recipient plasma, spins, and reads. It is an ABO incompatibility detector. Group A plasma versus a group B unit, or group O plasma versus a group A unit, agglutinates now. That is the job.

IS XM will not detect most IgG alloantibodies. Anti-D, anti-K, anti-Jka, and anti-Fya sit on IgG molecules that do not span the zeta potential in saline at immediate spin. If your only compatibility test is IS, you are trusting the antibody screen (or the computer, or the history) to have found those antibodies. An item that asks what IS XM misses wants clinically significant IgG, not “nothing — IS is a full XM.”

AHG (IAT) crossmatch incubates at 37 °C, washes (tube) or uses a column/solid-phase AHG card, and reads after antiglobulin. It detects IgG that the IS spin cannot see. AHG XM is required when the patient has a current clinically significant antibody or a history of one, even if today’s screen is negative. Memory outlives titer. The unit must also be antigen-negative for that specificity. A K− unit that is then AHG-crossmatched is the package; a random K+ unit that “happens to be IS compatible” is not.

A positive AHG XM on a patient with a negative screen still needs an explanation: low-prevalence antigen on the donor, donor unit DAT-positive, ABO that IS somehow missed, rouleaux, or an antibody the screen cells lacked. Do not force the unit out because the screen was negative.

Electronic / computer crossmatch

A computer (electronic) XM is a validated LIS release of ABO-compatible units without a serologic XM. It is not a waiver of thinking. AABB-style requirements you must recite:

1. Two determinations of recipient ABO. One must be on the current specimen. The second may be a historical type or a second current type (another method or another technologist, per procedure).
2. Current antibody screen negative.
3. No history of a clinically significant unexpected antibody.
4. The computer system is validated on-site and contains logic that blocks release of ABO-incompatible units and alerts on mismatches.

If any of those four is missing, you do not computer-crossmatch. A patient with historical anti-E and a negative screen today gets E− units and an AHG XM, not a computer release. A patient with only one ABO type on file gets a second type, not a hopeful click. A system that will release a group A unit to a group O recipient is not a computer XM — it is an unvalidated spreadsheet.

Electronic XM does not detect unexpected IgG. It is allowed only because the screen plus history already asked that question. It does detect ABO incompatibility if and only if the stored types are correct and the logic is sound. Garbage ABO in, garbage unit out.

Emergency release and massive-transfusion switches

When blood is needed before testing is finished, you emergency-release uncrossmatched group O red cells.

• Women of childbearing potential get O-negative so you do not put D+ cells into a D-neg woman who can still make anti-D.
• Many hospitals issue O-positive to adult males (and often to females beyond childbearing potential) to protect the O-neg inventory. That is a written policy, not a freelance decision at the issue window.
• The physician signs for uncrossmatched blood. The unit is tagged as uncrossmatched. Keep the segments. Start the type and screen immediately and finish a serologic or electronic XM as soon as you can. If the XM later comes up incompatible, call the floor — do not wait for them to notice the hemoglobin is not rising.

Once you have a confirmed ABO (two determinations when the policy requires it), switch to type-specific so you stop burning group O. That switch is part of massive-transfusion sense, not a courtesy.

Massive transfusion (about one blood volume, classically ~10 RBC units in an adult, or replacement within 24 hours) has additional switch rules:

• O to type-specific as soon as the ABO is solid.
• D-neg to D-pos is commonly allowed for adult males and for females beyond childbearing when O-neg / D-neg stock is collapsing. Women of childbearing potential stay D-neg as long as inventory allows.
• If the patient has a known alloantibody, keep issuing antigen-negative units as long as you can; massive bleeding is not a license to ignore anti-Jka if antigen-neg units still exist.
• Plasma and platelets follow the hospital’s ratio protocol; this chapter’s exam point is the RBC ABO/D switch, not the 1:1:1 debate.

Exam traps

• Calling IS XM a full compatibility test for IgG antibodies.
• Computer-crossmatching a patient with historical anti-c because today’s screen is negative.
• Releasing electronic XM with only one ABO determination.
• Issuing type-specific uncrossmatched blood before the ABO is known.
• Giving O-pos emergency units to a 26-year-old D-unknown woman.
• Hoarding O-neg for adult males while the obstetric fridge is empty.
• Forgetting to switch from O to type-specific after the type is confirmed in a massive transfusion.