Methods for estimating the duration of bacterial carriage.

In two cohort studies on asymptomatic pharyngeal carriage of meningococci and of streptococci among schoolchildren, four methods are used to estimate the duration of bacterial carriage. A first estimate of the mean duration is given by the quotient of prevalence by incidence rate. A second method is based on the estimation of the actual length of truncated observations. A third estimate is given by the analysis of carriage survivorship according to a life table. A fourth method is the fitting of a negative exponential model to the carriage survival curve. For meningococci, the estimates of the median duration of carriage range from 7.7 up to 10.2 months, and from 11.1 up to 14.5 months for the mean. For streptococci, the median values vary from 1.9 up to 2.1 months, and from 2.3 up to 4.4 months for the mean duration. The validity and the consistency of the different estimates are discussed.