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Laboratory diagnosis

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Page updated March 2019

Blood test for pertussis toxin antibody (IgG).

What lab tests will tell you if you have had pertussis?

It may depend where you live.
In most developed countries a blood or oral fluid test (for children) is standard. Other tests are culture and PCR (see below for all the details)

Antibody tests are now becoming the most common test as laboratories can now readily obtain the test reagents. A blood sample or oral fluid taken after a minimum of two weeks of illness is used. By measuring IgG antibodies to pertussis toxin it is possible to say whether it is likely the patient has had pertussis infection with 90% accuracy, provided there has been no pertussis immunization in the previous 12 months. This antibody is usually measured as International Units (IU), and a level over 70 IU can be taken as very strong evidence of recent infection.
The test will be negative in 10% of pertussis infections, in all Bordetella parapertussis and Bordetella holmesii infections, (which cause similar symptoms) because they do not produce pertussis toxin, so test negative.
Here is a reference to a relevant European document on single sample diagnosis
There are variations on this antibody theme that are done in different parts of the world although most are now conforming to this standardised measure. Antibody tests can be done late in the illness and still show positive which is a big advantage.

In the United Kingdom a blood specimen from suspected cases should be sent to the local NHS laboratory requesting 'pertussis antibodies'. Results are obtained in 1-2 weeks. It can be difficult to persuade doctors to do the test. In the UK there are clear guidelines that include testing any patient with a paroxysmal cough of more than 2 weeks duration. There are other circumstances described and the actions to be taken. The document can be seen here.
Drawing these guidelines to the attention of your doctor may sometimes be necessary as very few will be familiar them (nobody can possible remember them all!).
Australia and New Zealand are similar to the UK with testing procedures. 

In the USA there is less likelihood that a doctor will refer to CDC guidelines as state health practices may predominate, and they are often conservative (euphemism for outdated). There is a CDC website page you might find useful.

PCR is  a better and more modern way of detecting the organism is by detecting its unique DNA pattern by means of polymerase chain reaction This also involves getting secretions from the back of the nose or throat by swab or aspiration and specialist laboratory testing.  A result can be obtained in 24 to 48 hours. A negative PCR does not rule out pertussis.

The PCR test depends on traces the organism being present, alive or dead. since it detects minute quantities of genetic material it is more likely to be positive than culture, and for a longer period of time.

The oldest and most difficult way is to try to culture the causative organism (Bordetella pertussis) from the back of the nose. This involves passing a swab on a wire through a nostril to the back of the throat and sending it to a medical lab to culture the material. This may take 5 to 7 days.  If Bordetella pertussis or parapertussis grows this is proof that it is whooping cough. Parapertussis also causes whooping cough. It is much less common, possibly 1 in 10 cases. It may be less severe because it does not produce pertussis toxin. Culture often fails even in true pertussis infection.

Unfortunately the organisms is delicate, killed easily by many antibiotics and has often been eliminated from the body by natural defences by the time the diagnosis is suspected. It is easiest to find it in the first 2 weeks but very unlikely after 3 weeks, but the patient has often had it for 3 weeks before whooping cough is suspected.  So it is unusual to get a positive culture in whooping cough. In other words, if a swab is negative, you can still have whooping cough.

The bottom line is that in practice the diagnosis has often to be made on symptoms and course of the illness alone, unless blood antibody tests can be done

Todar's online bacteriology chapter on pertussis