Diphtheria can be diagnosed usually by proper clinical examination, throat culture from the infected area and blood tests:

Physical examination may reveal the characteristic gray membrane (pseudomembrane) in the throat, enlarged lymph glands and swelling of the neck or larynx.

Presumptive rapid diagnosis in which methylene blue and Gram stain or immunofluorescent staining is done on the throat cultures to detect the bacteria.

Definitive identification and diagnosis: a sample of the pseudomembrane is taken and allowed to grow on Loeffler or tellurite selective media, and metachromic granules can be observed with Loeffler stain.

Toxin A subunit production can be demonstrated in vitro by Elek plate test, polymerase chain reaction probes and in vivo by guinea pig inoculation test.

In case of blood count there is moderate leukocytosis and possible thrombocytopenia.

Cardiac enzyme, serum aspartate transaminase (AST) helps to detect myocarditis.