Your article (3 November) highlights a recent study by a network of researchers which claims that multidrug-resistant TB (MDR-TB) rates in west Africa are actually higher than estimates published by the World Health Organisation. These claims are misleading and unhelpful. The study is based on data collected from selected referral centres located in the capital cities of eight west African countries which typically have a concentrated level of the most difficult TB cases, including MDR-TB. To extrapolate nationwide resistance rates from such focused data is misleading and does not present an accurate picture of the problem.

The collection of data is vitally important for all diseases, for all countries, and west Africa remains a part of the world where MDR-TB surveillance data are most lacking. WHO estimates on MDR-TB are based exclusively on population-based surveys, such as those recently conducted in Nigeria and Senegal. They are designed to generate information representative of all TB cases over the entire country, not only in the hotspots. Evidence has shown that population-based surveys enable a better understanding of the overall epidemiology of MDR-TB.

Dr Matteo Zignol

World Health Organisation

• Join the debate – email guardian.letters@theguardian.com