What is HI titer and how is it interpreted?

The HI test, or Hemagglutination Inhibition, is one of the common methods for assessing the level of antibodies against certain viruses such as Newcastle disease and avian influenza. The number obtained from this test should not, on its own, be considered the sole criterion for the health or immunity of the flock; rather, it should be interpreted alongside age, vaccination history, sampling time, clinical status, and uniformity of the response.

The most important point in interpreting HI is that the mean titer, the coefficient of variation (CV), and the trend of changes across successive samplings should be examined together. A high titer without adequate uniformity may indicate poor vaccine distribution or a heterogeneous response within the flock, whereas a moderate titer with a low CV can be a sign of better uniformity.

What does the HI titer show?

The HI test shows to what extent the antibodies present in the serum can prevent the virus or reference antigen from binding to red blood cells. The result is usually reported logarithmically or as a geometric mean.

It should be noted that HI is mainly a representation of humoral immunity and does not provide a complete picture of cellular immunity, mucosal immunity, or the actual quality of protection against all strains.

Important factors in interpreting the results

• The age of the flock and the type of vaccination program
• The time interval between vaccination and blood sampling
• The type of vaccine and its method of administration
• The number of samples and how representative they are of the whole flock
• The mean titer and CV
• The clinical status and the presence of a field challenge

What does a high or low CV mean?

A low CV is usually a sign of greater uniformity of the immune response among the birds. A high CV can result from unequal vaccine uptake, differences in maternal antibody levels, stress, an underlying disease, or poor management.

In a flock with a low mean titer and a high CV, both the level of immunity and the uniformity of the response are usually a cause for concern, and the quality of the vaccination program's implementation and the health status of the flock need to be reviewed.

Limitations of the HI test

HI is not suitable for all diseases, and even for target diseases it is not, on its own, considered a definitive basis for decision-making.

Titers are sometimes affected by recent vaccination, a field challenge, or strain differences; therefore, interpretation should be carried out by someone familiar with the flock's history.

FAQ

Does a high HI titer always mean complete protection? No. The type of strain, the timing of the challenge, and non-humoral immunity are also important.

Is a low CV more important than the mean titer? Neither is sufficient on its own, and they must be interpreted together.

When is the best time for sampling? Depending on the purpose of the test, the time of blood sampling should be determined based on the vaccination date and the status of the flock.

Call to action

To learn about supportive approaches related to the immune system and to receive advice on evaluating the vaccination response, please visit the Thymosil product page and the scientific consultation form.

Suggested internal links

• Poultry immune system
• Infectious bronchitis in poultry
• Thymosil product page
• Scientific consultation form