Improving on 'n-1'
A user-friendly tool for estimating the proportion of TB incidence due to recent transmission

Authors: Parastu Kasaie1, Barun Mathema2, Andrew Azman1, Jeff Pennington1, David W. Dowdy1

1 - Department of Epidemiology, Bloomberg School of Public Health, The Johns Hopkins University, Baltimore, MD
2 - Public Health Research Institute Tuberculosis Center, International Center for Public Health, Newark, New Jersey

Input your values for the six variables below to get an estimation of TB in your area that reflects recent transmission

In order to use the model, please provide the following information (in the requested scale), and hit "Go!"
DescriptionInput BoxSymbolAcceptable range


Estimated proportion of active tuberculosis due to recent transmission: %

Linear Regression Model

c = C/SS, n = N/SS



In developing public health responses to tuberculosis (TB) epidemics, it is often important to estimate the proportion of active TB cases that result from recent infection versus reactivation. This is traditionally done with molecular epidemiological data (e.g., DNA fingerprinting), but the traditional ('n-1') method for converting these data into estimates of recent transmission is known to carry substantial bias.


We develop a stochastic, individual-based simulation model of TB epidemic to model the long term dynamics of transmission and strain-clustering. Simulations are carried out across a variety of epidemiological settings and study conditions (defined by population coverage of fingerprint data and duration of data collection). In each experiment, we compare the clustering estimates from 'n-1' method with the true level of recent transmission in the model, and compute the estimation bias. Using these simulations, we developed a simple regression-based tool to better estimate the recent transmission proportion, as a function of four inputs: TB incidence, proportion of observed cases that are clustered, population coverage of DNA fingerprint data, and duration of time over which fingerprint data were collected.