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Laboratory diagnosis

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Page updated December 2018

If whooping cough is suggested as a diagnosis it is natural to ask how it can be proved or disproved.

There are now several ways, but which is best depends on where you live, because different laboratories are likely to do different tests.

The best, but most difficult way is to try to culture the causative organism (Bordetella pertussis) from the back of the nose. This usually 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 usually taken as 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. Most blood tests look for pertussis toxin antibodies so will not detect Bordetella parapertussis.

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, the patient can still have whooping cough.

A better and more modern way of detecting the organism is by detecting its unique DNA pattern by means of polymerase chain reaction (PCR) 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.

The 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.

Antibody tests are now becoming the most common test as laboratories can now readily obtain the test reagents, although they are expensive. A blood sample 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 whooping cough with an accuracy of more than 90% provided there has been no pertussis immunization in the previous 12 months. This antibody is measured as International Units (IU), and a level over 60 IU can be taken as very strong evidence of recent infection.
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. Note that it is not 100% accurate, so a level below 60 IU does not rule out recent pertussis. A similar test can also be done on oral fluid, useful in children when blood testing can be a problem. In the UK laboratories will accept oral fluid sample from patients under the age of 17.

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 cough of more than 2 weeks duration that is paroxysmal. 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!)

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.

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