When is a human platelet antigen (HPA) antibody test indicated and how is it performed?

HPA Antibody Testing: Indications and Methodology

HPA antibody testing is primarily indicated for suspected fetal and neonatal alloimmune thrombocytopenia (FNAIT), particularly when a neonate presents with thrombocytopenia (platelet count <100 × 10⁹/L) or intracranial hemorrhage without alternative cause, and should be performed using two different serological methods plus a crossmatch with paternal platelets in a specialized reference laboratory 1.

Clinical Indications for HPA Antibody Testing

Primary Indication: Suspected FNAIT

• Neonatal thrombocytopenia with platelet count below 100 × 10⁹/L in the absence of other causes (hypoxia, infection, anatomical defects, prematurity) 1
• Fetal or neonatal intracranial hemorrhage occurring without apparent cause 1
• History of previously affected infant with FNAIT in subsequent pregnancies

Other Clinical Scenarios

• Post-transfusion purpura 2, 3
• Platelet transfusion refractoriness 2, 3
• Other platelet immune conditions requiring antibody identification 3

Important Caveat

FNAIT should be considered in fetal ICH without apparent cause, though the frequency of positive serology in infants with isolated ICH is actually low 1. There is no confirmed association between FNAIT and isolated fetal brain anomalies such as ventriculomegaly or brain cysts 1.

Complete Testing Algorithm

The comprehensive workup requires three components 1:

1. HPA Genotyping

• Test maternal, neonatal (preferred), or paternal DNA to identify platelet antigen incompatibility
• Standard panel should include: HPA-1, -2, -3, -4, -5, -6, -9, and -15 alleles 1
• Add HPA-6b and anti-Naka (CD36/GPIV) for Asian and African populations 1
• Validated PCR techniques: RFLP, sequence-specific primer (SSP), real-time PCR, next-generation sequencing, or bead-chip methods 1

2. Maternal Antibody Testing (Two Methods Required)

Testing must use TWO different serological methods to ensure concordance 1:

Available Methods:

• Bead-based multiplex assays 1
• MAIPA (Monoclonal Antibody-Specific Immobilization of Platelet Antigen) - glycoprotein-specific assay 1
• Antigen capture assays (MACE) 1
• Flow-based assays 1
• Platelet adhesion immunofluorescence test using whole (intact) platelets 1
• Mixed passive hemagglutination specifically for anti-HPA-3 antibodies that only react with native platelets 1

Critical Technical Requirements:

• Use maternal SERUM, not EDTA plasma - EDTA significantly reduces MAIPA assay sensitivity for HPA-1a antibodies 1
• Target platelets: Fresh from donors (normal count, no illness, no antiplatelet medication) or frozen at -80°C for up to 6 months 1
• Warning: Freezing/thawing can disrupt certain antigens (e.g., cryptic antigens on GPIIbIIIa) 1

3. Crossmatch with Paternal Platelets

• Mandatory crossmatch of maternal serum with fresh paternal platelets 1
• Purpose: Detects alloantibodies to low-frequency (private) antigens that may be missed by standard panels 1

Critical Pitfalls and False-Negative Results

Delayed Antibody Appearance

Anti-HPA-1, -3, and -5 antibodies may appear several weeks after birth 1. When these antigens are implicated by incompatibility:

• Determine infant genotype immediately
• Repeat maternal antibody testing 2-8 weeks post-delivery 1

Low-Affinity Antibodies

Different antibodies require different detection methods 1:

• Anti-HPA-3: Not readily detected by MAIPA but detectable using fresh whole platelets in immunofluorescence tests 1
• Anti-HPA-15 (CD109): Can be missed in paternal crossmatch; requires fresh HPA-15aa or HPA-15bb platelets from high CD109 expressers 1
• Anti-HPA-9b: Missed in paternal crossmatch due to inherent antigen characteristics 1
• Other low-affinity antibodies: Lost during MAIPA washing steps 1

This is why two different methods are mandatory - combining glycoprotein-specific assays (MAIPA/MACE) with whole-platelet assays (immunofluorescence) reduces false-negative risk 1.

Neonatal Testing Limitations

Serology testing in neonates is less reliable than maternal antibody testing due to antibody binding to fetal tissues expressing HPA antigens (endothelium, placenta) 1.

Specialized Laboratory Requirement

All HPA serological testing for FNAIT must be performed in a reference laboratory due to the specialized techniques and expertise required 1. These are not routine hospital laboratory tests.

Interpretation Considerations

• Anti-HLA antibodies detected in up to 30% of multiparous women can interfere with platelet-specific antibody detection 1
• Whole platelet (non-lysed) assays will identify anti-HLA antibodies, which must be interpreted in clinical context 1
• HPA-1a incompatibility doesn't always cause immunization - depends on other factors including HLA-DRB3*0101 genotype 1

Emergency Management Note

In severe thrombocytopenia or active bleeding, administer random donor platelets immediately without waiting for test results 1. Rapid genotyping can be performed within hours post-delivery to select antigen-negative platelets for subsequent transfusions 1.