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Diphtheria is a contagious disease transmitted by Corynebacterium diphtheriae bacteria. The bacteria produce a toxin that attacks the upper respiratory tract (sore throat, difficulty breathing) and then the heart and peripheral nervous system. The bacteria is transmitted by inhaling respiratory droplets or, rarely, by coming in contact with articles soiled by infected people. Diphtheria has been virtually eradicated in Canada as a result of mandatory immunization programs for children. However, it is still very prevalent in several countries and poses a risk of infection for inadequately protected travellers. Following contact with the bacteria or a diphtheria vaccine, the body produces bacteria-specific antibodies that generally last for life. The anti-diphtheria test measures the level of these antibodies to ensure that an individual is adequately protected against diphtheria. Results are expressed in international units of anti-diphtheria antibodies per millilitre of blood (IU/mL), and levels equal to or greater than 0.01 IU/mL are considered sufficient for adequate protection.

Anti-diphtheria levels below 0.01 IU/mL are insufficient to protect a person from diphtheria and may indicate who should be vaccinated, if applicable. It usually takes at least one month after vaccination to measure whether the body’s response to the vaccine was adequate. The absence of anti-diphtheria antibodies following vaccination may be associated with a deficient immune system or may be caused by the use of immunosuppressive drugs. Diphtheria is a reportable disease.

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