Pertussis is a respiratory tract infection caused by the gram-negative coccobacillus Bordetella pertussis. It is characterized clinically by a severe and prolonged cough. Coughing fits may be paroxysmal and, usually in infants, followed by an inspirational "whoop. " � A clinical diagnosis will form the basis of most pertussis diagnosis and treatment decisions. The CDC provides the following clinical case definition for pertussis: A cough illness lasting at least 2 weeks with one of the following: paroxysms of coughing, inspiratory "whoop, " � or post-tussive vomiting, without other apparent cause. This test is used to detect acute infection caused by the slow-growing, fastidious pathogen B. pertussis, the cause of pertussis or whooping cough. Because of the contagiousness of the infection, specific laboratory testing may be needed to confirm the diagnosis when pertussis is suspected clinically. Laboratory diagnosis of pertussis is complicated by the limitation of available tests. Options for diagnostic and confirmatory testing, when required, depend on the age of the patient and the phase of illness.
Expected result: Negative.
Use
Aids in the detection of B. pertussis infection. Serologic testing for B. pertussis infection involves the detection of antibodies to pertussis antigens using standardized assays. Pertussis toxin (PT) and filamentous hemagglutinin (FHA) are the most widely used antigens and to a lesser extent pertactin (PRN) and fimbriae (FIM). Only PT is specific for B. pertussis; FHA and pertactin antigens cross-react with antibodies arising from infection by other Bordetella species and possibly by other bacteria. Serum antibodies have been measured by ELISA, complement fixation, agglutination, and toxin neutralization; ELISA is the detection method of choice due to its wide availability and ease of performance.
Although B. pertussis serology is most useful in epidemiologic investigations or vaccine trials, it does have some utility in the diagnosis of pertussis in some patients, particularly in adolescents, adults, and previously vaccinated individuals. Serologic testing may also be useful for patients with cough >2 " �3 weeks in duration. Antibodies can be detected against B. pertussis antigens 1 " �2 weeks after the onset of the symptoms of pertussis in nonvaccinated individuals. Both IgG and IgA isotypes are produced in response to infection, whereas IgG is the predominant isotype detected after vaccination. However, no single antigen or isotype can be used to distinguish between infection and a response to vaccination with certainty. IgM responses are usually not measured for pertussis and have questionable diagnostic significance.
The most reliable serologic approach to diagnosis of pertussis is with simultaneous testing of paired acute and convalescent sera. A significant increase (fourfold or greater) in IgG or IgA antibody titers to PT or FHA, comparing convalescent to acute sample, suggests recent B. pertussis infection in patients with a clinical illness compatible with pertussis. Paired sera, however, are not practical in most clinical settings. Single-sample serology tests for antipertussis toxin IgG must be collected at least 2 weeks after symptom onset. A high antibody titer >2 years following vaccination supports the diagnosis of pertussis.
Interpretation
Positive: IgG antibody to B. pertussis detected, which may indicate a current or past exposure/immunization to B. pertussis.
Limitations
The CDC does not currently accept serology as laboratory verification of pertussis; cases that meet the clinical case definition with a positive serology but a negative culture or PCR are considered probable cases. Single serology tests are used for the diagnosis of pertussis by the state laboratory in Massachusetts and in selected countries in the European Union.
The IgG serology test results are not interpretable in children younger than 11 years of age because of interference due to persistent antibody formed by childhood vaccination. The test also cannot be interpreted in older patients who have received the Tdap vaccination in the previous 3 years.