Laboratory diagnosis of whooping cough
Tests for whooping cough.
What laboratory diagnostic tests will tell you if you have or have had whooping cough (pertussis)?
It may depend where you live.
In many developed countries a blood or oral fluid test (usually just 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.
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 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. IgA is sometimes measured instead, or sometimes both. IgA only rises after natural infection. IgG rises after either natural infection or immunization.
The test will be negative in 10% of pertussis infections. It will also be negative in Bordetella parapertussis and Bordetella holmesii infections, (which cause similar symptoms). That is because they do not produce pertussis toxin, so test negative.
Oral fluid obtained by using a special sponge kit can be tested for pertussis toxin antibodies in the same way. It it not quite as accurate as blood testing in the sense of having more false negatives. Oral fluid testing is usually reserved for children.
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.
UK guidelines for doctors 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that a doctor will refer to CDC guidelines as state health practices may predominate, and they are sometimes a bit out of date. There is a CDC website page you might find useful.
PCR (polymerase chain reaction)
This is a more successful way of detecting the organism. It detects its unique DNA pattern. This involves getting secretions from the back of the nose or throat by swab or aspiration and testing in a specialist laboratory. A result can be obtained in 24 to 48 hours.
A negative PCR does not rule out pertussis especially if taken in the later stages. It should be positive right from the first day of the illness and may remain positive for 4 weeks or more.
The PCR test depends on traces of 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.
PCR has the advantage that it can be successful on a throat swab, unlike culture that has to be taken from an area of ciliated epithelium where the bacteria are living which is at the back of the nose. A throat swab for PCR should be sent to the lab dry, not in transport medium, although that does not usually stop it being tested.
One thing that can happen with PCR testing that can be confusing is that it detects infections that may not be associated with illness from whooping cough. Some people get the infection and get no symptoms at all but they will be PCR positive.
PCR can be too sensitive
This can be a problem for statistics. For example, if a parent takes a child with whooping cough to a doctor and a sample is taken for PCR, the parent and doctor may arrange for other children in contact to be tested too, even if they have no symptoms. Some may show PCR positive but not go on to develop whooping cough or be given antibiotics to prevent it.
A positive PCR from such cases will show up in the pertussis statistics and make the incidence look greater. Because previous to PCR availability only clinical whooping cough was counted for statistical purposes.
If comparisons are have any validity, clinical whooping cough needs to be recorded and notified separately for PCR positives.
This may explain some of the resurgence described in Australia.
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. 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 100 cases. It may be less severe because it does not produce pertussis toxin. Culture by perusal swab only identifies about a third of cases, even in the best hands.
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. 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 or oral fluid antibody tests or PCR can be done.