Introduction 

Every safe transfusion starts with the same question: does this patient have any antibody that will destroy the donor's red cells? Antibody screening answers it. ABO and Rh typing rule out the most dangerous mismatches, but they cannot detect the dozens of other clinically significant antibodies a patient may have developed after a previous transfusion, pregnancy, or transplant [1,2].

This protocol explains the principle of antibody screening, the practical steps used in the laboratory, and how to read an antigram.

When Is Antibody Screening Ordered?

Antibody screening is requested in three main settings.

The first is pre-transfusion testing. Every patient who may need a red cell transfusion has a "type and screen" performed, which includes ABO/Rh group and antibody screening [1].

The second is pre-operative workup. Surgical patients with a moderate or higher chance of needing blood have a type and screen ordered, sometimes followed by crossmatching units to be held in reserve.

The third is antenatal care. RhD-negative pregnant women, and increasingly all pregnant women, are screened at booking and again around 28 weeks to detect antibodies that could cause hemolytic disease of the fetus and newborn (HDFN) [1].

Principle of Antibody Screening

Antibody screening works by combining patient plasma or serum with reagent red cells whose antigens are already known. If the patient has an antibody against any of those antigens, it binds. The bound antibody is then made visible, either directly as agglutination or indirectly through an antihuman globulin (AHG) reagent.

The reagent red cells used are called screening cells. They are group O, so naturally occurring anti-A and anti-B in the patient's plasma do not interfere [1]. A screening set contains at least two different donor cells. Together they express the clinically significant antigens you need to rule out, including the Rh system (D, C, E, c, e), Kell (K, k), Duffy (Fy^a, Fy^b), Kidd (Jk^a, Jk^b), MNS (M, N, S, s), Lewis (Le^a, Le^b), and P1 [1,4].

The Indirect Antiglobulin Test

The detection step in screening is the indirect antiglobulin test (IAT), also called the indirect Coombs test. IgG antibodies, which are the most clinically dangerous, bind to red cells but rarely cause spontaneous agglutination because they are small. AHG reagent contains antibodies against human IgG. When added to sensitized red cells, it bridges them and produces visible clumping [1,4].

Test Platforms

Modern labs use one of three platforms, though automation has largely replaced manual methods for high-volume testing [3,5].

Column agglutination (gel card) and Solid-phase red cell adherence (SPRCA) are now the standard of care in modern laboratories. Because they can be fully automated via robotic analyzers, they offer superior traceability, standardization, sensitivity, and high-throughput capabilities [3,5]. In column agglutination, plasma and reagent cells are loaded into a microtube containing dextran-acrylamide gel or glass beads pre-impregnated with AHG. After centrifugation, agglutinates are trapped at the top of the column while unagglutinated cells pellet at the bottom.

The tube method is the classic, manual technique. Patient plasma and reagent cells are mixed in a glass tube, incubated at 37°C, washed, and AHG is added. While it remains vital for resolving complex antibody workups and is the best way to teach the principles of agglutination, it is rarely used for routine high-volume screening today due to its labor-intensive nature and subjective visual interpretation [3].

Whichever platform is used, the principle is the same: known antigen plus unknown antibody, then a way to make binding visible.

Materials

• Patient plasma or serum. Use samples collected within exactly 72 hours of the draw time if the patient has been transfused or pregnant within the last 3 months [1,2]. This strict timeline accounts for the rapid secondary immune response; an antibody that was undetectable on Monday could climb to dangerous, hemolytic levels by Thursday.
• Validated screening cell set (minimum two cells, with the antigen profile described above).
• Anti-human globulin (AHG) reagent (anti-IgG or polyspecific anti-IgG plus anti-C3d).
• Normal saline (0.9% NaCl).
• LISS (low ionic strength solution) or PEG (polyethylene glycol), if used as a potentiator.
• Coombs control cells (IgG-coated red cells) for validating negative AHG results.
• Test tubes, gel cards, or microplates depending on platform.
• Calibrated serofuge or column agglutination centrifuge.
• 37°C incubator or heat block.
• Calibrated micropipettes and sterile tips.
• The antigram sheet matched to the lot of screening cells in use.

Reagent Criteria

These specifications follow the BSH Guidelines for Pre-Transfusion Compatibility Procedures (2013), which remain the current reference [1].

Cell composition. A minimum of two donor cells per screening set. Cells must collectively express K, k, Fy^a, Fy^b, Jk^a, Jk^b, S, s, M, N, P1, Le^a, Le^b. One cell should be R2R2 (cDE/cDE) and another R1R1 (CDe/CDe) or R1^wR1 [1].

Homozygous expression. At least one cell must be homozygous for Fy^a, Fy^b, Jk^a, Jk^b, S, and s. This protects against missing weak antibodies that show "dosage," meaning they react strongly with homozygous cells but weakly or not at all with heterozygous cells [1,6].

Individual cell use. Pooled donor cells are not acceptable for screening. Each cell is tested separately so a reaction can be traced to a specific antigen profile [1].

Storage and expiry. Screening cells must be stored in a validated diluent at controlled temperature, used within the manufacturer's expiry, and locally validated for stability on the bench or analyzer [1].

Protocol

The following outlines the tube method because it is the easiest to learn conceptually. For gel card or solid-phase methods, follow the manufacturer's instructions. Incubation times and volumes differ.

Preparation

1. Collect and process the sample. Centrifuge the blood tube to separate plasma or serum. Check the sample is within the 3-day window if the patient has been recently transfused or pregnant [1].
2. Prepare reagents. Bring screening cells to the working concentration specified by the manufacturer. Check expiry dates on AHG, LISS, and Coombs control cells.

Antibody Screen, Tube Method

1. Label one tube per screening cell and one negative control (saline plus patient plasma, no screening cell).
2. Add 1 drop of each screening cell to its tube.
3. Add 2 drops of patient plasma or serum to every tube except the negative control.
4. Optional: add 2 drops of LISS to enhance antibody uptake and shorten incubation.
5. Incubate at 37°C for 15 to 30 minutes (or as the manufacturer specifies).
6. Wash each tube three times with saline to remove unbound proteins. Decant fully after each wash.
7. Add 1 drop of AHG reagent to every tube except the negative control.
8. Centrifuge briefly per manufacturer's settings.
9. Gently resuspend the cell button and inspect for agglutination. Grade and record the result.
10. To any tube that shows no agglutination, add Coombs control cells and centrifuge again. They must agglutinate. If they don't, the test is invalid and must be repeated.

Interpretation

The results pattern is straightforward.

A negative screen does not guarantee that the patient has no antibody. It means no antibody was detected against the antigens on the screening panel under the conditions used. Antibodies to low-prevalence antigens not on the panel will be missed [1].

How to Read an Antigram

The antigram is the worksheet that comes with each lot of screening cells. It lists each reagent cell and the antigens it carries. Read it using the rule-out method.

1. In the example of the antigram provided above, choose those with negative (0) results in the AHG column. In our example, it will be cell 1 and 2.
2. Cross (X) out any positive (+) antigens in cell 1 and 2. For example in cell 1, for Rh antibodies, cross out D, E and c. These means that the patient does not have antibodies against these antigens.
3. Circle (0) all remaining antigens which have not been crossed out.
4. These findings indicate the possible antibodies present and would require further investigation to identify the specific antibodies present in the patient's blood.
5. Move on to antibody identification using an extended panel to confirm which of the remaining specificities the patient actually has [1,2].

Strengths and Limitations of Antibody Screening

What Antibody Screening Does Well

Screening cells carry uniform, well-characterized antigen profiles, often homozygous for key antigens. This means weak antibodies with dosage effects, like anti-Jk^a, are reliably detected [1,6]. Standardized reagents reduce error compared with crossmatching against random donor cells. Gel and solid-phase platforms allow automation, which improves reproducibility and audit trail [3,5].

What Antibody Screening Cannot Do

Screening will not detect antibodies to antigens absent from the screening panel, such as low-prevalence antigens [1]. Weak or newly forming antibodies may fall below the detection limit. Antibodies that have evanesced over time can also be missed, which is why patient transfusion history is essential. The screen does not identify the antibody. It only flags that one is present. Crossmatching is still required to confirm compatibility with the specific donor unit, unless local protocols permit electronic crossmatch with a fully validated negative screen [1].

Common Interferences

A few real-world situations cause confusing screens.

Anti-CD38 monoclonal therapy. Daratumumab and similar agents bind weak CD38 expressed on reagent red cells, producing pan-reactivity on every screening and panel cell. Treating screening cells with dithiothreitol (DTT) removes CD38 and resolves the interference [7]. Always check whether the patient is on any monoclonal antibody therapy.

Anti-CD47 monoclonal therapy. Drugs like magrolimab (an anti-CD47 IgG4 monoclonal antibody) cause massive pan-reactivity across all screening and panel cells because CD47 is universally expressed on red blood cells [10]. Unlike anti-CD38 interference, anti-CD47 cannot be mitigated by treating reagent cells with DTT. Interference is often bypassed by using an anti-IgG reagent that specifically lacks anti-IgG4, or through multiple allogeneic adsorptions [10]. As with all such therapies, baseline blood typing and antibody screening must be performed before the patient begins treatment.

Warm autoantibodies. Patients with warm autoimmune hemolytic anemia produce IgG antibodies reactive against most cells. Adsorption techniques are needed to look for underlying alloantibodies.

Cold antibodies. Cold-reactive antibodies of no clinical significance can react at room temperature. This is one reason many modern protocols omit the immediate spin phase and go directly to a 37°C IAT [1].

Recent transfusion. Donor red cells in the patient's circulation can produce mixed reactions. This is why a fresh sample is needed within 3 days of transfusion [1].

Quality Control and Safety Considerations

Every batch of testing should include positive and negative controls. Coombs control cells validate every negative AHG result. Reagents must be used within manufacturer's expiry and within locally validated open-vial limits [1]. Sample identification at the bedside is non-negotiable, since wrong-blood-in-tube events are a major preventable cause of transfusion harm reported annually by SHOT [8]. Modern laboratory information systems should enforce identification, sample timing, and historical antibody rules automatically [9].

Frequently Asked Questions (FAQs)

What is antibody screening used for?

Antibody screening detects unexpected red blood cell antibodies in a patient's plasma before transfusion or during pregnancy. It is most often ordered as part of a type and screen before surgery, before any blood transfusion, and at antenatal booking. A negative screen allows compatible red cells to be issued, often by electronic crossmatch. A positive screen triggers antibody identification before any transfusion is given.

What is the difference between antibody screening and crossmatching?

Antibody screening looks for antibodies in the patient's plasma using a standardized panel of reagent red cells with known antigens. Crossmatching tests the patient's plasma against the specific donor unit chosen for transfusion. Screening predicts compatibility broadly. Crossmatch confirms it for a particular unit. In many modern labs, a negative antibody screen coupled with a validated historical ABO/Rh type allows for an electronic crossmatch (EXM), which safely replaces the physical crossmatch entirely [1,9].

What is the rule of 3 in antibody identification?

The rule of 3, sometimes called the 3×3 rule, is a statistical confidence check used during antibody identification. To confirm an antibody specificity, the patient's plasma must react with at least three reagent cells carrying the suspected antigen and must not react with at least three cells lacking it. This combination provides a 95 percent confidence level using Fisher's exact test. It is a confirmation rule applied during identification, not a feature of screening itself.

Why are screening cells always group O?

Group O cells lack A and B antigens. Using them prevents the patient's naturally occurring anti-A and anti-B from causing reactions during screening. Any agglutination observed therefore reflects an unexpected antibody, not the patient's ABO group.

Can an antibody screen change over time?

Yes. New antibodies form after transfusion, pregnancy, or transplantation. Existing antibodies, especially weaker IgG ones, can drop below the detection threshold over months or years, a process called evanescence. This is why pre-transfusion screens are strictly valid for only 72 hours from the exact time of the blood draw in patients recently transfused or pregnant, and why complete transfusion histories matter [1,2].

What happens if antibody screening is positive?

A positive screen means agglutination was seen in at least one tube or well. It does not identify the antibody. The lab then runs an extended antibody identification panel, usually 10 to 20 cells with detailed antigen profiles. Once the antibody is identified, the lab selects antigen-negative donor units and performs an extended crossmatch before issuing blood. Transfusion is delayed until truly compatible blood is found.

Glossary of Related Medical Terms

• Alloantibody: An antibody made by the immune system against red cell antigens from another person, usually after transfusion or pregnancy.
• Antigen: A molecule on the red blood cell surface that the immune system can recognize as foreign.
• Agglutination: Visible clumping of red cells when antibodies bridge antigens between them. The endpoint of most blood bank tests.
• Antihuman globulin (AHG): A reagent containing antibodies against human IgG and complement. It bridges sensitized red cells so that bound antibody becomes visible as agglutination. Also called Coombs reagent.
• Antiglobulin test (IAT): The indirect antiglobulin test. Detects in vitro binding of patient antibodies to reagent red cells. Antibody screening uses IAT.
• Antigram: A grid showing which antigens each reagent screening or panel cell carries. Used to interpret which antibody is present.
• Coombs control cells (CCC): IgG-coated red cells added to negative AHG tests. They must agglutinate to confirm the AHG reagent was active and washing was adequate.
• Crossmatch: A compatibility test between donor red cells and recipient plasma. Performed after screening, not instead of it.
• Hemolysis: Destruction of red blood cells.
• Hemolytic transfusion reaction: A potentially fatal reaction caused by antibody-mediated destruction of transfused red cells.
• Heterozygous expression: A red cell carrying one copy of an antigen gene. Expression is often weaker than in homozygous cells.
• Homozygous expression: A red cell carrying two copies of an antigen gene. Expression is usually stronger and more reliable for detecting weak antibodies. This is called "dosage."
• IgG: The main class of clinically significant red cell antibodies. Reacts best at 37°C and is detected by AHG.
• IgM: A pentameric antibody that reacts at room temperature. Most clinically important IgM antibodies are anti-A and anti-B.
• LISS (low ionic strength solution): A potentiator that shortens incubation time and enhances antibody uptake.
• Panel cell: A reagent red cell with a known, defined antigen profile used for identifying the specific antibody once screening is positive.
• Screening cell: A reagent red cell, usually group O, with known antigens. Used to detect the presence of unexpected antibodies.
• Sensitization: The binding of antibody to red cell antigen without immediate visible agglutination. Made visible by AHG.

Disclaimer: This protocol is for educational purposes only. Local laboratory standard operating procedures take precedence. It is not intended to be a substitute for informed professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional for clinical decision-making. While the information presented here is derived from credible medical sources and is believed to be accurate and up-to-date, it is not guaranteed to be complete or error-free. See additional information.

References

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2. American Association of Blood Banks. (2023). Technical manual (21st ed.). AABB Press.
3. Bain BJ, Bates I, Laffan MA. Dacie and Lewis Practical Haematology: Expert Consult: Online and Print 12th Edition (Elsevier). 2016.
4. Dean L. Blood Groups and Red Cell Antigens [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2005. Available from: https://www.ncbi.nlm.nih.gov/books/NBK2261/
5. Bhagwat, S. N., Sharma, J. H., Jose, J., & Modi, C. J. (2015). Comparison Between Conventional and Automated Techniques for Blood Grouping and Crossmatching: Experience from a Tertiary Care Centre. Journal of laboratory physicians, 7(2), 96–102. https://doi.org/10.4103/0974-2727.163130
6. Knowles, S. M., Milkins, C. E., Chapman, J. F., & Scott, M. (2002). The United Kingdom National External Quality Assessment Scheme (blood transfusion laboratory practice): trends in proficiency and practice between 1985 and 2000. Transfusion medicine (Oxford, England), 12(1), 11–23. https://doi.org/10.1046/j.1365-3148.2002.00353.x
7. Chapuy, C. I., Nicholson, R. T., Aguad, M. D., Chapuy, B., Laubach, J. P., Richardson, P. G., Doshi, P., & Kaufman, R. M. (2015). Resolving the daratumumab interference with blood compatibility testing. Transfusion, 55(6 Pt 2), 1545–1554. https://doi.org/10.1111/trf.13069
8. Serious Hazards of Transfusion (SHOT). Annual SHOT reports. https://www.shotuk.org/shot-reports/ (Accessed 15th September 2026)
9. British Society for Haematology. (2024). Guidelines for the specification, implementation and management of IT systems in hospital transfusion laboratories.
10. Velliquette, R. W., Aeschlimann, J., Kirkegaard, J., Shakarian, G., Lomas-Francis, C., & Westhoff, C. M. (2019). Monoclonal anti-CD47 interference in red cell and platelet testing. Transfusion, 59(2), 730–737. https://doi.org/10.1111/trf.15033