Whooping cough or Pertussis is difficult to diagnose in the initial phases. This is because the initial signs and symptoms mimic a cold or flu or bronchitis.
Over a week or so the features of characteristic coughing bouts with vomiting appear making diagnosing the disease clinically easier.
Diagnosis begins with taking a detailed history of the symptoms and exposure to the disease. Physical examination follows to check on the signs and symptoms.
Laboratory tests to diagnose the condition (1, 2, 3, 4)
There are several laboratory tests used to diagnose the condition including bacterial culture, blood tests and so forth.
Bacterial culture
A swab is taken from the nose or throat usually during the first two weeks of the infection. Alternatively a suction sample of the mucus is taken from the throat.
The sample is checked under the microscope for the bacteria. This is considered the gold standard for diagnosis because it gives 100% accurate results.
Results however are not much reliable after two weeks when the viable bacteria die off and go undetected on cultures.
Blood tests
Test for routine blood parameters is essential to detect infection.
There is a rise in white blood cell count. These white blood cells are those that help the body fight infections.
A rise in these cells’ count means there is an underlying infection or inflammation.
Chest X rays
These are needed to detect pneumonia and accumulation of fluid in the lungs as a result of the disease.
Polymerase chain reaction (PCR)
This is a more sophisticated test that is used for confirmation and detection of the exact strain or species of the bacteria Bordetella pertussis that is infecting the individual.
PCR results need to be corroborated with clinical findings.
The sample for PCR is taken from the nose and throat (Nasopharyngeal swab or suction sample) usually between 0 to 3 weeks of the onset of cough.
Results may be accurate up to 4 weeks of cough in infants or unvaccinated individuals. Thereafter the chances of accurate diagnosis decreases and the bacteria is eliminated.
Serology assays
The body produces infection fighting chemicals called antibodies in response to infections.
Serological assays are used to diagnose the rise of these antibodies to predict the course of the disease.
The assay here looks at the infection specific antibody titers.
Blood is taken between 2 to 8 weeks of cough onset. At this time the titers of antibodies peak.
Serology tests may yield positive results up to 12 weeks after cough onset.
The assays target proteins released from the bacterium called antigens against which the antibodies are produced. These may be pertussis toxin, filamentous hemagglutinin and pertactin.
Results may be difficult to interpret in vaccinated or adult patients since they may have a baseline antibody level from their shots or earlier infections.
Further Reading
Denmark's pertussis epidemic reveals the power of maternal vaccination