Hemagglutination Inhibition Assay — HI / HAI Test
Hemagglutination Inhibition Assay · Also known as: HI assay, HAI test, haemagglutination inhibition test, HI test
The Hemagglutination Inhibition (HI) Assay is a classical serological test used to detect and quantify antibodies against hemagglutinating viruses — most notably influenza and Newcastle disease virus — in animal and human serum. Widely employed in veterinary diagnostics, vaccine efficacy evaluation, and epidemiological surveillance, it relies on the principle that specific antibodies in serum will block a known quantity of virus from agglutinating red blood cells.
Read the full method
Sign in with a free account to read this section.
Method map
The neighbourhood of related methods — select a node to explore.
When to use it
The HI assay is appropriate when the goal is to detect virus-specific antibodies in serum and to quantify their titer against a hemagglutinating virus. It is the reference method for measuring antibody responses to influenza (H1–H16 subtypes), Newcastle disease virus, and several other paramyxoviruses and orthobunyaviruses in veterinary and public-health settings. Use it for post-vaccination monitoring, seroprevalence surveys, and strain-specific immunity assessments. The assay requires a known, safe viral antigen; it is not suitable for non-hemagglutinating pathogens. For highly pathogenic strains (e.g., HPAI H5N1) the antigen must be prepared in a BSL-3 facility, which limits routine use.
Strengths & limitations
- Gold-standard serological method for influenza and Newcastle disease: internationally standardised by WOAH and widely accepted by regulatory bodies.
- Inherently subtype-specific — distinct hemagglutinin antigens distinguish antibodies against H1, H3, H5, and other subtypes that cross-reactive ELISAs cannot separate.
- Low equipment requirement — a microplate, a multichannel pipette, and a light box are sufficient; no sophisticated instrumentation needed.
- Quantitative output — the titer provides a continuous measure of immune status that correlates with protection in many species.
- High throughput — a single trained technician can process hundreds of samples per day on standard 96-well plates.
- Requires handling of live or inactivated viral antigen, which demands appropriate biosafety infrastructure, particularly for high-pathogenicity strains.
- Non-specific inhibitors in some animal sera (e.g., from horses, pigs) can cause false-positive results unless sera are pre-treated with receptor-destroying enzyme (RDE) or kaolin.
- The assay detects only antibodies directed against the hemagglutinin surface protein; neuraminidase or nucleoprotein antibodies are invisible to it.
- Inter-laboratory variability in antigen titration and RBC preparation can shift titer values by one to two two-fold dilution steps, complicating cross-study comparisons.
Frequently asked
What is the difference between HI titer and virus neutralization titer?
Both measure antibody-mediated virus inhibition, but through different mechanisms. The HI assay detects antibodies that block the hemagglutinin from binding red blood cells — it does not require live-cell infection, so it can be performed in standard laboratory settings with inactivated antigen. Virus neutralization tests (VNT) measure antibodies that prevent the virus from infecting live cells in culture, which requires cell culture infrastructure and, for dangerous strains, BSL-3 containment. HI titers and VNT titers are generally correlated but not numerically equivalent.
Why must the antigen be titrated to exactly 4 HAU?
The 4 HAU standard is a deliberate excess over the 1 HAU endpoint: it provides a reproducible, consistent challenge dose so that only antibody — not random variation in antigen concentration — determines whether agglutination is inhibited. Using more antigen makes the assay falsely insensitive (high titers needed to inhibit excess virus); using less risks incomplete agglutination in the antigen-only control and ambiguous results.
What does an HI titer of 1:40 mean in practice?
A titer of 1:40 means the serum still inhibited hemagglutination when diluted 40-fold. In influenza serology this value — equivalent to log2 titer of about 5.3 — is the widely used seroprotection threshold, adopted by WHO and regulatory agencies as a correlate of protection in humans. In poultry, WOAH guidelines use similar thresholds but the protective titer can vary by species and vaccine strain.
Can the HI assay distinguish between vaccinated and naturally infected animals?
Not by itself. The HI assay measures antibodies to hemagglutinin only. DIVA (Differentiating Infected from Vaccinated Animals) strategies pair HI testing with a nucleoprotein-based ELISA: vaccinated birds with inactivated whole-virus vaccines develop both HI and NP antibodies, whereas DIVA vaccines express only one antigen, allowing the companion ELISA to distinguish the two groups.
Sources
- Hirst, G. K. (1942). The quantitative determination of influenza virus and antibodies by means of red cell agglutination. Journal of Experimental Medicine, 75(1), 49–64. DOI: 10.1084/jem.75.1.49 ↗
- World Organisation for Animal Health (WOAH/OIE). (2021). Manual of Diagnostic Tests and Vaccines for Terrestrial Animals, Chapter 3.3.4 (Avian Influenza). WOAH. link ↗
How to cite this page
ScholarGate. (2026, June 3). Hemagglutination Inhibition Assay. ScholarGate. https://scholargate.app/en/veterinary-science/hemagglutination-inhibition
Which method?
Set this method beside its closest kin and read them side by side — the library lays the books on the table; the choice is yours.
- Enzyme-Linked Immunosorbent Assay (ELISA)Veterinary Science↔ compare