Leukocyte and Platelet Antigen Testing

Poor platelet increments, neonatal cytopenia, and a suspected antibody-mediated transfusion reaction can each involve a different antigen system. The clinical pattern determines which specimen, antibody assay, and antigen typing the investigation needs, and which component-selection question the result has to answer. A reactive result is interpretable only when the report names the target antigen or antibody and the method.¹

Investigation map

The blood-group systems page explains HLA and HPA inheritance and nomenclature.

HLA methods in transfusion testing

Platelets carry HLA class I antigens. Antibody to HLA class I is the most common immune cause of platelet transfusion refractoriness. HLA typing and antibody identification can therefore support platelet selection even when the patient is not being evaluated for a transplant.^1,2

Complement-dependent cytotoxicity

Complement-dependent cytotoxicity (CDC), also called microlymphocytotoxicity, demonstrates complement-mediated injury to viable lymphocytes. Test lymphocytes are incubated with antibody of known specificity for typing, or with patient serum for a crossmatch. Complement is added, and a vital dye distinguishes cells with damaged membranes from cells that exclude the dye. Positive and negative controls establish complement activity, cell viability, and the valid scoring range. The laboratory assigns the HLA type or crossmatch result from the complete reaction pattern rather than from one well.^1,2

CDC depends on viable cells and complement-fixing antibody. A wash step in an Amos-modified method can reduce interference from anticomplementary serum factors, and an AHG-augmented method can increase detection of weak cell-bound antibody. Reagent choice and scoring follow the laboratory’s validated procedure. DNA-based methods now provide the primary HLA type in most settings, while CDC remains useful for selected antibody and crossmatch questions.^1,2

Molecular typing and antibody assays

PCR with sequence-specific primers, sequence-specific oligonucleotide probes, Sanger sequencing, and next-generation sequencing identify HLA alleles at different levels of resolution. The report must state the loci, resolution, ambiguity, and assay coverage. A genotyping result predicts the encoded HLA type; it does not show whether an antibody is present.^2,3

Cell-based flow cytometry and solid-phase assays detect antibody without requiring complement-mediated cell death. In a single-antigen bead assay, each coded bead carries one HLA antigen, and a fluorescent anti-human immunoglobulin detects bound antibody. Mean fluorescence intensity (MFI) is an assay signal rather than a measured antibody concentration. Antigen density, denatured antigen, interference, lot, instrument settings, and analysis rules can change that signal. No universal MFI cutoff defines a clinically important antibody.^2,3

For platelet support, the HLA laboratory may combine the patient’s HLA-A and HLA-B type with the HLA antibody profile. An exact HLA match is one option; antibody-specific prediction expands the search by excluding donors who carry antigens targeted by the patient’s antibodies. A platelet crossmatch tests patient serum against donor platelets and can detect incompatibility caused by HLA or HPA antibody, but the crossmatch alone does not identify the antibody specificity.^4,5

The transplantation immunology page covers detailed transplant typing, donor-specific antibody assessment, and organ crossmatch policy.

Platelet transfusion refractoriness

Platelet refractoriness is a repeated lower-than-expected response to platelet transfusion. A single low posttransfusion count does not establish it. Active bleeding, fever or sepsis, disseminated intravascular coagulation, splenic sequestration, medications, and other nonimmune factors cause most cases. ABO incompatibility, HLA class I antibody, HPA antibody, and platelet-reactive autoantibody can also reduce the increment.^4,5

The corrected count increment (CCI) adjusts the platelet-count change for patient size and the platelet dose:

Corrected count increment

CCI = (posttransfusion count − pretransfusion count) × body surface area in m² ÷ platelet dose in 10¹¹ platelets

Obtain a posttransfusion count 10 to 60 minutes after each of two sequential transfusions when evaluating a suspected refractory state. In an afebrile, nonbleeding patient receiving conventional platelets that are not pathogen reduced, a 10- to 60-minute CCI is typically greater than 7,500. A CCI below 5,000 with supportive serologic results may indicate immune refractoriness. Published and local criteria differ, and immune and nonimmune patterns overlap, so the trend, component, and clinical setting govern the interpretation.^4,5

Worked example. The platelet count rises from 8,000/µL to 28,000/µL after a dose of 3.0 × 10¹¹ platelets in a patient with a body surface area of 1.8 m². The CCI is (20,000 × 1.8) ÷ 3.0 = 12,000. One acceptable increment does not establish a sustained response; interpretation continues with the next timed count and the clinical course.

Investigation and component selection

1. Record the pretransfusion count, platelet dose, completion time, posttransfusion count time, and clinical factors that can consume or sequester platelets.
2. Calculate the 10- to 60-minute CCI for two sequential transfusions under comparable conditions.
3. If poor increments persist after nonimmune factors are assessed, test for HLA class I and HPA antibodies according to the laboratory algorithm. Obtain the patient’s HLA type when HLA-selected support may be needed.
4. Use a limited trial of crossmatch-compatible platelets or select HLA-matched or antibody-compatible platelets. Select HPA antigen-negative platelets when an HPA specificity is present, most platelet crossmatches are incompatible, or HLA-selected products fail.
5. Measure another 10- to 60-minute posttransfusion count. If the increment does not improve despite a compatible laboratory result, reassess nonimmune causes and the selection strategy.

In many services, platelet crossmatching is faster than a donor search and tests HLA and HPA compatibility together. Highly alloimmunized patients may have too few compatible crossmatched donors. HLA matching and antibody-specific avoidance require an HLA-typed donor inventory and an interpretable antibody profile. The laboratory report should name which strategy was used rather than labeling every selected unit “matched.”^5,6

HNA testing

HNA names antigen systems defined through neutrophil alloantibodies. Some carrier proteins occur on other cells, so an HNA designation does not mean that expression is limited to neutrophils.⁷

Granulocytes deteriorate rapidly after collection. Intact-cell testing therefore uses freshly collected specimens and validated handling, commonly within 24 hours, and is usually performed by a specialized laboratory. A granulocyte immunofluorescence test (GIFT) detects antibody bound to intact cells. A granulocyte agglutination test (GAT) detects visible cell agglutination. Current reference-laboratory practice combines GIFT and GAT because either method alone can miss clinically significant antibodies. GAT improves detection of anti-HNA-3a.^7,8

Antigen-specific capture methods, called monoclonal antibody immobilization of neutrophil antigens (MAINA) or monoclonal antibody-specific immobilization of granulocyte antigens (MAIGA), isolate a defined membrane glycoprotein before detecting bound human antibody. Assay design and available capture antibodies limit coverage, particularly for HNA-3. HNA phenotyping can demonstrate antigen expression. Genotyping predicts the antigen from the alleles tested and requires interpretation when copy-number variation, untested variants, or altered expression could affect the result. HLA class I antibody can react with intact granulocytes and obscure an HNA result, so HLA antibody testing is often performed in parallel.^7,8

Neonatal neutropenia and TRALI

An investigation of suspected alloimmune neonatal neutropenia pairs maternal granulocyte antibody testing with antigen testing of the infant and parents when indicated. A maternal antibody and an incompatible paternal antigen inherited by the infant support the diagnosis. Negative testing does not exclude an antibody that is weak, targets an antigen absent from the panel, or whose detection depends on cell quality. Clinically strong but unresolved cases require reference-laboratory consultation.^7,8

TRALI is a clinical diagnosis. Detection of an HLA or HNA antibody is not required to classify the reaction. In an immune-mechanism investigation, testing may identify HLA class I, HLA class II, or HNA antibody in an implicated donor and the corresponding recipient antigen. These results support the mechanism and donor evaluation, while the clinical and hemovigilance investigation determines the reaction classification.⁹

HPA testing

Most HPA antigens arise from a nucleotide change that produces one amino-acid difference on a platelet glycoprotein. The glycoprotein assignment matters because antigen-capture assays target a specific protein complex.¹⁰

Panel composition follows the population served and the reference laboratory’s validation. Low-frequency HPA variants, private antigens, and CD36 deficiency can require specialized testing.

Whole-platelet solid-phase or immunofluorescence assays detect platelet-reactive antibody. A positive reaction can reflect HLA, HPA, ABO, autoantibody, or another platelet-reactive antibody. Glycoprotein-specific assays narrow that result. In the monoclonal antibody immobilization of platelet antigens (MAIPA) method, a monoclonal antibody captures a selected platelet glycoprotein after exposure to test serum, and an anti-human reagent detects human antibody bound to the same complex. Other validated antigen-capture and bead-based assays use related principles. No single method detects every HPA antibody.^6,10

HPA genotyping identifies the alleles covered by the assay and predicts the antigen profile. It can confirm an antigen incompatibility, select an antigen-negative donor, and resolve serology when platelets are unavailable or recently transfused. The report must distinguish a genotype-predicted antigen from a serologically demonstrated phenotype and state any alleles the assay does not cover.^1,10

Fetal and neonatal alloimmune thrombocytopenia

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) results when maternal antibody targets a paternally inherited fetal platelet antigen that the mother lacks. It can occur during a first pregnancy. Laboratory confirmation evaluates the incompatibility and the antibody together:

1. Test maternal serum for platelet alloantibody with two validated serologic methods when available. EDTA plasma can substantially reduce anti-HPA-1a detection by MAIPA and should not be used when serum is available.
2. Genotype the mother and affected neonate when neonatal DNA is available. The father’s genotype can define the incompatibility and inheritance risk when the neonate is unavailable.
3. Crossmatch maternal serum with paternal platelets. This can detect antibody against a private or low-frequency paternal antigen that is absent from a routine panel.
4. Compare serology, genotype, platelet counts, and the clinical findings. A negative routine panel does not exclude FNAIT when the antibody is weak, develops late, or targets an antigen outside the panel. Repeat maternal testing 2 to 8 weeks later when clinical suspicion remains high.¹¹

Severe neonatal thrombocytopenia or bleeding requires transfusion without waiting for test results. Give the immediately available platelet component under the emergency or local protocol, including random-donor platelets when necessary. Use HPA antigen-negative or otherwise compatible platelets when they can be provided without delay, and continue the laboratory investigation. The laboratory report should state the implicated specificity and family genotypes because those findings also affect future pregnancies.¹¹

Posttransfusion purpura

Posttransfusion purpura typically presents as abrupt, profound thrombocytopenia about 5 to 10 days after transfusion in a previously sensitized patient, often after pregnancy. Anti-HPA-1a is the classic specificity, although other HPA antibodies can occur. Both transfused and autologous platelets are cleared. The exact mechanism of autologous platelet destruction remains incompletely defined.¹²

The laboratory tests patient serum for HPA antibody and compares the specificity with the patient’s HPA genotype. The antibody should target an antigen absent from the patient’s predicted profile. The transfusion history, timing, platelet trend, and exclusion of other causes remain necessary because antibody alone does not establish the clinical diagnosis. Documented specificity informs future antigen-negative support.¹²

Result limits and reporting

The final interpretation should name the specimen, method, antibody class or specificity, antigen panel, genotype coverage, crossmatch result, and component-selection consequence. It should also state the limitation that matters for the case.

Platelet autoantibody testing has limited sensitivity and does not establish immune thrombocytopenia. A negative result does not exclude it.¹

References

1. Bloch EM, Campbell-Lee S, McKenna DH Jr, Montemayor-Garcia C, Schwartz J, Shaz B, Storry J, eds. Technical Manual. 22nd ed. AABB; 2026.
2. American Society for Histocompatibility and Immunogenetics. 2025 ASHI Standards for Accredited Laboratories. Approved by the Centers for Medicare & Medicaid Services January 27, 2026. Accessed August 30, 2026.
3. Joint United Kingdom (UK) Blood Transfusion and Tissue Transplantation Services Professional Advisory Committee. Chapter 16: HLA typing and HLA serology. Guidelines for the Blood Transfusion Services in the United Kingdom. Accessed August 30, 2026.
4. Association for the Advancement of Blood & Biotherapies, American Red Cross, America's Blood Centers, Armed Services Blood Program. Circular of Information for the Use of Human Blood and Blood Components. June 2024.
5. Cohn CS. Platelet transfusion refractoriness: how do I diagnose and manage? Hematology Am Soc Hematol Educ Program. 2020;2020(1):527-532. doi:10.1182/hematology.2020000137.
6. American Red Cross. National Reference Laboratory for Specialized Testing. Accessed August 30, 2026.
7. Joint United Kingdom (UK) Blood Transfusion and Tissue Transplantation Services Professional Advisory Committee. Chapter 17: granulocyte immunology. Guidelines for the Blood Transfusion Services in the United Kingdom. Accessed August 30, 2026.
8. American Red Cross. National Neutrophil Laboratory. Accessed August 30, 2026.
9. Vlaar APJ, Toy P, Fung M, et al. A consensus redefinition of transfusion-related acute lung injury. Transfusion. 2019;59(7):2465-2476. doi:10.1111/trf.15311.
10. Versiti. Human Platelet Antigen (HPA) Database. Accessed August 30, 2026.
11. Petermann R, Bakchoul T, Curtis BR, Mullier F, Miyata S, Arnold DM; Subcommittee on Platelet Immunology. Investigations for fetal and neonatal alloimmune thrombocytopenia: communication from the SSC of the ISTH. J Thromb Haemost. 2018;16(12):2526-2529. doi:10.1111/jth.14294.
12. Hawkins J, Aster RH, Curtis BR. Post-transfusion purpura: current perspectives. J Blood Med. 2019;10:405-415. doi:10.2147/JBM.S189176.