Compatibility

Two absolutely critical steps in ensuring safe transfusions: the Antibody Screen (Detection) and the Crossmatch. Think of them as partners in crime prevention – the crime being a hemolytic transfusion reaction! They work together but have distinct jobs

Think of it like this: The antibody screen checks if the patient has any known “allergies” (antibodies) to common blood ingredients (antigens). The crossmatch checks if this specific bag of blood contains an ingredient the patient is allergic to. Both are essential safety checks!

Compatibility Testing: The Dynamic Duo

The overall goal is to ensure the blood component we give the patient won’t cause harm. Antibody detection and crossmatching are the key serologic tests we use for red blood cell transfusions after establishing the correct ABO/Rh type

Antibody Detection (The Antibody Screen)

• The Goal: The Reconnaissance Mission
  • This test is our initial search party, sent out to scout the patient’s plasma or serum for any “unexpected” red blood cell antibodies
  • We’re looking for clinically significant antibodies (usually IgG, reactive at 37°C) against common red cell antigens other than the expected anti-A or anti-B. Examples include anti-D, anti-K, anti-Fya, anti-Jka, etc
  • Why?: These antibodies, if present, could destroy transfused red cells possessing the corresponding antigen, causing a transfusion reaction, or could cause Hemolytic Disease of the Fetus and Newborn (HDFN) in pregnancy
• How It Works (Generally)
  • We use the Indirect Antiglobulin Test (IAT) principle
  • Patient’s Plasma/Serum + Commercial Screening Cells (Group O cells with a known, wide range of antigens) –> Incubate at 37°C –> Wash –> Add Anti-Human Globulin (AHG) –> Centrifuge & Read
  • A positive result (agglutination/hemolysis) means we’ve detected something
• Significance
  • Negative Screen: Great! It suggests no detectable common, clinically significant antibodies are present. This often allows for a more streamlined crossmatch (like IS or electronic)
  • Positive Screen: Alert! An unexpected antibody is likely present. We MUST perform Antibody Identification (using a panel of reagent cells) to figure out exactly which antibody(ies) it is before we can select appropriate (antigen-negative) donor blood

The Crossmatch

• The Goal: The Final Compatibility Checkpoint
  • This is the direct test between the patient and the potential donor unit. It’s our last chance to catch an incompatibility before the blood goes into the patient
  • It serves two main purposes:
    1. Final ABO verification Catches potentially catastrophic ABO errors (e.g., wrong unit selected)
    2. Detects patient antibodies against antigens on the specific donor cells This is crucial if the patient has an antibody (even one missed by the screen, like one against a low-frequency antigen) that reacts with the chosen donor unit
• How It Works (Types)
  • We mix Patient’s Plasma/Serum + Red Cells from the specific Donor Unit segment.
  • Immediate Spin (IS) Crossmatch: Quick check primarily for ABO incompatibility. Often sufficient if the antibody screen is negative and there’s no history of antibodies
  • Antiglobulin (AHG) Crossmatch: Full IAT procedure (Incubate 37°C, Wash, AHG). Required if the patient has a clinically significant antibody (current or historical) to ensure the selected antigen-negative unit is truly compatible, or if the IS crossmatch is incompatible
  • Electronic (Computer) Crossmatch: Uses a validated computer system to confirm ABO/Rh compatibility instead of serologic testing. Strict criteria must be met (negative screen, two concordant ABO/Rh types on file, no history of antibodies, etc.). It’s fast and efficient but relies heavily on accurate data and system integrity
• Significance
  • Compatible Crossmatch: No reaction observed (serologically) or authorized (electronically). The unit is deemed safe to transfuse from an immunologic perspective. Green light!
  • Incompatible Crossmatch: Reaction observed (agglutination/hemolysis). STOP! Do Not Transfuse. The unit is not safe for this patient. Investigate the cause (ABO error? Antibody reacting with donor antigen?). Red light!

The Relationship: Screen First, then Crossmatch

• The Antibody Screen is a general search using representative cells
• The Crossmatch is a specific test against the actual unit intended for transfusion
• The results of the Antibody Screen heavily influence the type of Crossmatch performed. A negative screen often allows for IS or electronic crossmatch, while a positive screen mandates finding the antibody’s identity and performing an AHG crossmatch with antigen-negative units