Microbiologic diagnostic tests when asymptomatic carriers are present: aspects of the use of conventional throat and nasopharyngeal cultures as examples
Gunnarsson, Ronny (2001) Microbiologic diagnostic tests when asymptomatic carriers are present: aspects of the use of conventional throat and nasopharyngeal cultures as examples. PhD thesis, Göteborg University.
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Abstract
Carriers of potentially pathogenic bacteria simultaneously ill from a viral infection complicate the diagnostic procedure in respiratory tract infections. The present statistical methods available to evaluate common diagnostic tests either ignore the phenomenon of carriers or provide test characteristics that are difficult to apply in clinical decision making. In this dissertation, the influence of carriers on the diagnostic process has been elucidated.
* The etiologic predictive value (EPV) is a new statistical method developed to predict disease caused by the bacteriological findings, taking carriers into consideration. To calculate EPV, it is necessary to have the proportion of positive tests among patients, the proportion of positive tests among a healthy control population and the sensitivity of the test. This enables calculating the positive and negative EPV with a 95% confidence interval.
* A throat culture was found to be a reliable indicator for illness caused by group A beta-haemolytic streptococci (GABHS) in adult patients with a sore throat. Positive EPV (PEPV) was 99% (95% confidence interval is 94-100%). A seasonal variation, however, was found in pre-school children (0-6 years of age). A throat culture with growth of GABHS was found to be reliable only in the winter season, with a PEPV of 94% (75-100%) as opposed to only 61% (0-91%) in the summer. However, our data did not permit us to conclude that this seasonal variation will be found every year.
* Findings of Haemophilus influenzae in a nasopharyngeal culture, taken from patients with a sore throat, may indicate the true etiology of the disease. The prediction in regard to disease caused by H. influenzae (PEPV) was 93% (73- 99%) for adults ³16 years of age and 86% (28-99%) for pre-school children 0-6 years of age.
* In adults with a long-standing cough combined with other symptoms of a respiratory tract infection, it was found that growth of H. influenzae in a nasopharyngeal culture would indicate the etiology for infection with PEPV 90% (30-99%). Growth of Moraxella catarrhalis in a nasopharyngeal sample, taken from a pre-school child with a long-standing cough 0-6 years of age, will indicate the etiology for infection with a PEPV of 90% (66-99%).
* A questionnaire sent to different microbiologic laboratories revealed a substantial variation between different geographical areas' propensity to perform a throat or nasopharyngeal culture. There was also a large variation between the different areas in the outcome of these cultures. It could be shown that the variation in outcome of the cultures makes it difficult to directly apply predictive values calculated from many scientific studies.
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