Diphtheria is a contagious and potentially fatal disease caused by a bacterium called Corynebacterium diphtheria. The symptoms of the disease include the following:

- Fever        

- Weakness

- Swollen neck

- Sore throat

- Inflammation of the upper respiratory tract

- A thick gray membrane forms over the tonsils, pharynx larynx and back of the nose

- Difficulty swallowing and/or breathing

The primary problem with a diphtheria infection is that the bacteria that causes it creates a toxin that is distributed throughout the body. The toxin can cause heart failure and even paralyze the diaphragm so breathing stops. It may also damage the kidneys and nerves. 

The Diphtheria Antibodies test measures how many antibodies you have against the diphtheria bacteria that causes the disease. You may have antibodies present in your blood for one of two reasons:  1) You have diphtheria and your immune system is trying to fight the infection or you had it previously, or 2) You were vaccinated against the disease. 

The test may be used to see if you still have immunity against the disease.

Normal Ranges for Diphtheria Antibodies in IU/mL:

Vaccinated:  Positive (> or =0.01 IU/mL)

Unvaccinated: Negative (<0.01 IU/mL)

Sources:

1. https://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/36670
2. https://www.cdc.gov/diphtheria/about/symptoms.html 
3. https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=167&contentid=diphtheria_antitoxoid_antibody