More than a million tuberculosis cases may be missing from official statistics in India, the country worst affected by the deadly bacterial infection, a study published in The Lancet journal Thursday found.
Many patients seeking treatment for TB turn to unregulated private doctors who often do not report cases, say researchers from Imperial College London, India's national TB programme, and the World Health Organisation (WHO).
Official estimates put the number of TB cases treated in the Indian private sector in 2014 at around 800,000, but researchers' analysis of drugs sales suggests the figure could exceed two million.
"TB is the top infectious disease killer worldwide, yet we have had little idea of the true scale of the problem in India, the worst-affected country," said Dr Nimalan Arinaminpathy of the School of Public Health at Imperial College London.
Under-reporting of the disease hampers health officials seeking to understand its prevalence, but tracking cases across the vast country of 1.2 billion people is extremely difficult.
"It's a tough nut to crack. There has been a lot of progress made, with a new reporting system and greater engaging with the private sector," Dr Arinaminpathy told AFP.
Despite the efforts of India's national TB programme, hundreds of thousands of cases also go undiagnosed each year, while the spread of drug-resistant strains of TB is a key concern.
India has the highest number of TB cases in the world, accounting for around one in four cases globally, according to the WHO.