Compatibility Testing And Transfusion Practice

Pretransfusion testing and the crossmatch

Compatibility testing has three legs: ABO/Rh typing, an antibody screen, and a crossmatch of donor cells against recipient serum. By AABB standard, the specimen must be drawn within 3 days of the planned transfusion if the patient was transfused or pregnant within the preceding 3 months (or if history is uncertain), because a new antibody can appear quickly. Patient and specimen identification — two independent identifiers — is the single most important error-prevention step; clerical/identification errors cause most fatal ABO-incompatible transfusions.

The crossmatch comes in three forms, and the screen result dictates which is acceptable:

The electronic crossmatch is permitted only when the patient has no history of clinically significant antibodies, the current antibody screen is negative, and there are two determinations of ABO group (one on the current sample). It cannot be used if any of those conditions fail.

Component selection rules

• RBCs must be ABO-compatible and Rh-matched (give D-negative to D-negative females of childbearing potential). In emergencies, group O RBCs are issued (O-negative for women of childbearing age, O-positive acceptable for males/older women to conserve O-negative inventory).
• Plasma/FFP must be ABO-compatible with the recipient's red cells in reverse: group AB plasma is the universal plasma donor.
• Platelets are best ABO-matched but may cross groups; large volumes of incompatible plasma can cause minor hemolysis.

Recognizing transfusion reactions

Reaction recognition is heavily tested. The first action for almost any acute reaction is the same: stop the transfusion, keep the line open with saline, and notify the blood bank and physician.

Massive transfusion and citrate

Massive transfusion (replacing one blood volume in 24 hours, or about 10 units in an adult) is managed with a balanced 1:1:1 ratio of RBCs to plasma to platelets to avoid dilutional coagulopathy. Several metabolic complications recur on the exam:

• Citrate toxicity: anticoagulant citrate chelates calcium, causing hypocalcemia (perioral tingling, tetany, prolonged QT) — monitor ionized calcium and give calcium if symptomatic.
• Hyperkalemia: aged/irradiated units leak potassium, a risk in neonates and renal failure.
• Hypothermia: rapid cold infusion warrants a blood warmer.

The transfusion-reaction workup

After stopping the transfusion, the standard clerical and laboratory workup is the same opening sequence and is heavily tested. First, perform a clerical check comparing the unit tag, the patient wristband, and the records — most fatal ABO errors are caught here. Then collect a post-transfusion specimen and:

1. Inspect the plasma for pink/red discoloration (free hemoglobin = intravascular hemolysis).
2. Perform a DAT on the post-transfusion sample; a positive DAT with mixed-field reactivity suggests an incompatible transfusion.
3. Repeat the ABO/Rh on pre- and post-transfusion samples and on the unit.
4. Check a urine for hemoglobinuria when AHTR is suspected.

If all are negative and the patient had only a temperature rise, a febrile nonhemolytic reaction is the likely diagnosis. The decision tree separating AHTR (positive DAT, hemoglobinuria, clerical error) from FNHTR (negative workup, isolated fever) from TRALI versus TACO (respiratory distress, distinguished by BNP, fluid balance, and donor antibody testing) is a classic one-best-answer item.

The MLS exam expects you to recognize each reaction by its hallmark presentation, choose the correct workup, and apply the overriding rule that ABO incompatibility from patient or specimen misidentification is the deadliest and most preventable transfusion event — which is why two independent identifiers and the immediate-spin or electronic crossmatch ABO check exist. Practice by drilling the reaction table until naming the reaction and the first action becomes reflexive within the 90-second-per-item pace of the computer adaptive exam.

Emergency release and documentation

When a patient is exsanguinating before testing finishes, the lab issues emergency-release uncrossmatched blood under a physician signature: group O red cells (O-negative for women of childbearing potential) and AB plasma if plasma is needed. The crossmatch and full type are completed in parallel and the record documents the urgency. The exam expects you to know that emergency release trades a small compatibility risk for survival, that the signed physician request is required, and that any antibody detected afterward must be reported so subsequent units are antigen-negative.

Equally testable is the rule that the antibody screen, not the crossmatch, detects most clinically significant antibodies, so a negative screen with a compatible immediate-spin crossmatch is acceptable while a positive screen always escalates to the antiglobulin crossmatch and antibody identification before nonemergent release.