Microbiologic diagnostic tests when asymptomatic carriers are present. Aspects of the use of conventional throat and nasopharyngeal culture as examples

Abstract

Carriers of potentially pathogenic bacteria simultaneously ill from a viral infection complicatethe diagnostic procedure in respiratory tract infections. The present statistical methodsavailable to evaluate common diagnostic tests either ignore the phenomenon of carriers orprovide test characteristics that are difficult to apply in clinical decision making. In thisdissertation, the influence of carriers on the diagnostic process has been elucidated.· The etiologic predictive value (EPV) is a new statistical method developed to predictdisease caused by the bacteriological findings, taking carriers into consideration. Tocalculate EPV, it is necessary to have the proportion of positive tests among patients, theproportion of positive tests among a healthy control population and the sensitivity of thetest. This enables calculating the positive and negative EPV with a 95% confidenceinterval.· A throat culture was found to be a reliable indicator for illness caused by group A beta-haemolyticstreptococci (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 ofGABHS 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 permitus to conclude that this seasonal variation will be found every year.· Findings of Haemophilus influenzae in a nasopharyngeal culture, taken from patients witha sore throat, may indicate the true etiology of the disease. The prediction in regard todisease caused by H. influenzae (PEPV) was 93% (73-99%) for adults ³16 years of ageand 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 tractinfection, it was found that growth of H. influenzae in a nasopharyngeal culture wouldindicate the etiology for infection with PEPV 90% (30-99%). Growth of Moraxellacatarrhalis in a nasopharyngeal sample, taken from a pre-school child with a long-standingcough 0-6 years of age, will indicate the etiology for infection with a PEPV of90% (66-99%).· A questionnaire sent to different microbiologic laboratories revealed a substantialvariation between different geographical areas propensity to perform a throat ornasopharyngeal culture. There was also a large variation between the different areas in theoutcome of these cultures. It could be shown that the variation in outcome of the culturesmakes it difficult to directly apply predictive values calculated from many scientificstudies.