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Investigations

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Typically, pertussis is diagnosed clinically. Several laboratory techniques exist for the identification of B. pertussis:

culture
- diagnosis can be confirmed through isolation of B. pertussis organism from nasopharyngeal secretions (aspirated or obtained by a swab) and has been the diagnostic gold standard (1,2)
- sensitivity is low and is affected by patient age (decreasing with increasing age), vaccination status and length of illness
- timing of specimen collection is also important since sensitivity decreases substantially (55% to <10%) from week 1 to week 4
- negative culture does not exclude pertussis (4)
PCR
- for all age groups presenting <21 days after symptom onset
- rapid test that has been shown to have improved sensitivity over culture (3)
serology
- detection of anti-pertussis toxin (PT) IgG antibody levels in serum can be done by using an enzyme linked immunosorbent-assay (ELISA)
- may confirm the diagnosis of pertussis in patients who have been symptomatic for some weeks when culture and PCR are unlikely to yield positive results (4)
- recommended for those who have not received a dose of pertussis-vaccine in the preceding year
oral fluid testing
- offered to children aged 2 to <17 years who have not received a pertussis-containing vaccine in the preceding year
- less sensitive than serological assay (4)

An absolute lymphocyte count of more than 20 000 per mm3 is suggestive of pertussis - lymphocytosis has been referred to as a hallmark of the disease.

Recommendations for testing:

infants and children under the age of two years:
- PCR (offered by the Bordetella Reference Laboratory at RVPBRU) is recommended for infants and children with suspected pertussis in the early stages of the illness and <21 days post cough onset
- culture should be performed if local facility permits
- serology can be undertaken in those who present late (>14 days post cough onset); not recommended for infants under 12 months or children who have received a pertussis containing vaccination in the previous year
children aged from two years of age and adults:
- PCR recommended in the early stages (<21 days post cough onset and within 48 hours of antibiotics therapy)
- culture should be performed if local facility permits
- for children aged 2 to <17 years:
  - oral fluid testing and serology is recommended for individuals whose onset of cough is greater than fourteen days AND who have not been immunised against pertussis in the previous year (4)
- for children aged 17 or older and adults:
  - serology is recommended for individuals whose onset of cough is greater than fourteen days AND who have not been immunised against pertussis in the previous year (4)

References:

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