# ABO/Rh Typing, Antibody Screening, and Crossmatch

## Introduction

Pre-transfusion testing is the foundation of transfusion medicine safety. ABO and Rh typing, antibody screening, and crossmatch procedures ensure immunologic compatibility between donor and recipient, preventing potentially fatal hemolytic transfusion reactions.

## ABO Blood Group System

### Biochemistry and Genetics

ABO antigens are **carbohydrate structures** on glycoproteins and glycolipids of the red blood cell membrane. The **H antigen** is the precursor structure onto which A and B transferases add specific sugars. **A transferase** adds N-acetylgalactosamine to the H antigen, while **B transferase** adds D-galactose. The **O phenotype** encodes a non-functional transferase, leaving the H antigen unmodified. ABO genes are located on **chromosome 9**, and inheritance follows a codominant pattern.

### ABO Typing: Forward and Reverse

**Forward (cell) typing** tests patient RBCs with known anti-A and anti-B reagents. **Reverse (serum/plasma) typing** tests patient serum with known A1 and B reagent red cells. Forward and reverse results must **agree** before a blood type is assigned, and any ABO discrepancy requires investigation before blood products are issued.

| Blood Type | Forward (Anti-A) | Forward (Anti-B) | Reverse (A1 cells) | Reverse (B cells) | Antibodies Present |
|---|---|---|---|---|---|
| O | 0 | 0 | + | + | Anti-A, Anti-B |
| A | + | 0 | 0 | + | Anti-B |
| B | 0 | + | + | 0 | Anti-A |
| AB | + | + | 0 | 0 | None |

### ABO Discrepancies

**Group I** discrepancies involve weak or missing antibodies and are seen in neonates, elderly individuals, immunosuppressed patients, and those with hypogammaglobulinemia. **Group II** discrepancies feature unexpected antibodies, such as anti-A1 in A2 or A2B subgroups, cold autoantibodies, or passively acquired antibodies. **Group III** discrepancies involve weak or missing antigens and occur with subgroups of A (A2, A3, Ax, Aend), recent transfusion, bone marrow transplant, or leukemia. **Group IV** is a miscellaneous category encompassing polyagglutination, cold autoantibodies, and Wharton jelly contamination.

## Rh Blood Group System

### D Antigen and Rh Typing

The Rh system includes over **50 antigens**, with D being the most immunogenic. Approximately 85% of the Caucasian population is **RhD-positive** (D antigen present), while **RhD-negative** individuals lack the D antigen and are at risk for anti-D immunization. Other clinically significant Rh antigens include **C, c, E, and e**. The Rh genes (RHD and RHCE) are located on **chromosome 1**.

### Weak D and Partial D

**Weak D** (formerly Du) represents a reduced quantity of normal D antigen detected by the indirect antiglobulin test (IAT). **Partial D** is a qualitatively altered D antigen with missing epitopes, meaning the individual can form anti-D against the epitopes they lack. Current guidelines recommend **RHD genotyping** for weak D phenotypes to distinguish weak D types 1, 2, and 3 (who can safely receive RhD-positive blood) from partial D (who should receive RhD-negative blood).

![ABO forward and reverse typing reaction patterns in tube method](images/abo-typing-reactions.jpg)

## Antibody Screening

### Indirect Antiglobulin Test (IAT)

Patient plasma is incubated with **reagent screening cells** (group O RBCs with known antigen profiles), typically a two or three cell panel covering clinically significant antigens. After incubation, **anti-human globulin** (AHG/Coombs reagent) is added to detect IgG-coated cells. A positive screen indicates the presence of **clinically significant alloantibodies**.

### Antibody Identification

Identification is performed using an **extended panel** of 10-16 reagent cells with known antigen profiles. The reaction pattern is matched against the **antigen profile** of each panel cell using a rule-out approach, where antigens present on non-reactive cells can be excluded. **Multiple antibodies** may coexist and require additional panels or selected cells. Common clinically significant antibodies include anti-D, anti-K (Kell), anti-E, anti-c, anti-Fya, and anti-Jka.

### Techniques and Enhancement Media

The **tube method** is traditional and uses LISS (low ionic strength saline) or PEG (polyethylene glycol) enhancement. **Gel/column agglutination** (gel cards) provides standardized, stable reactions and is widely adopted. **Solid-phase red cell adherence** is used on automated platforms such as Immucor NEO and Ortho Vision. PEG enhances detection of weak IgG antibodies but can cause false-positive reactions.

![Antibody identification panel worksheet showing reaction pattern and antigen profile](images/antibody-panel-worksheet.jpg)

## Crossmatch

### Types of Crossmatch

The **immediate spin (IS) crossmatch** detects ABO incompatibility by centrifuging patient plasma with donor RBCs immediately. The **antiglobulin crossmatch (full crossmatch)** includes 37 degrees C incubation and an AHG phase, and is required when the antibody screen is positive or the patient has a history of clinically significant antibodies. The **electronic (computer) crossmatch** involves no serologic testing; ABO/Rh is verified by a computer algorithm and is permitted when the antibody screen is negative and two concordant ABO/Rh typings are on file.

### Requirements for Electronic Crossmatch

The electronic crossmatch requires a validated computer system with logic to prevent release of ABO-incompatible units, a current negative antibody screen, two concordant ABO/Rh determinations (one from the current sample), no history of clinically significant antibodies, and donor unit confirmation testing recorded in the system.

## Special Situations

### Emergency Release and Massive Transfusion

**Uncrossmatched blood** (group O RBCs, with O-negative for females of childbearing potential) is released in life-threatening emergencies. Type-specific blood is issued as soon as the ABO/Rh type is confirmed, typically within 5-10 minutes. Massive transfusion protocols may switch to type-specific blood after the initial O-negative units. All emergency release events must be documented for quality review.

### Neonatal Transfusion

The mother's sample is used for the antibody screen; if negative, the infant can receive crossmatch-compatible or O-negative RBCs. ABO typing on cord or infant blood is performed, but reverse typing is not reliable due to maternal antibody transfer. RBCs for neonates must be **irradiated**, CMV-safe, and hemoglobin S-negative.

![Gel card crossmatch showing compatible (negative) and incompatible (positive) results](images/gel-crossmatch.jpg)

## Clinical Pearls

Forward and reverse ABO typing must agree before a blood type is finalized, and any discrepancy must be resolved before transfusion. Weak D genotyping is recommended to distinguish patients who can safely receive RhD-positive products from those who cannot. Electronic crossmatch eliminates the need for serologic testing when the antibody screen is negative and system validation criteria are met. In emergencies, group O RhD-negative uncrossmatched RBCs should be issued immediately without waiting for compatibility testing.

## References

1. Fung MK, et al., eds. *AABB Technical Manual*. 20th ed. AABB Press; 2020.
2. Sandler SG, et al. It's time to phase in RHD genotyping for patients with a serologic weak D phenotype. *Transfusion*. 2015;55(3):680-689.
3. Tormey CA, Hendrickson JE. Transfusion-related red blood cell alloantibodies: induction and consequences. *Blood*. 2019;133(17):1821-1830.
4. Delaney M, et al. Transfusion reactions: prevention, diagnosis, and treatment. *Lancet*. 2016;388(10061):2825-2836.