More than 80 per cent of patients with tuberculosis who are potentially contagious do not have persistent cough, a study has suggested, rekindling questions about the reliability of using only cough as a key diagnostic criterion.

The study that used data from 6,02,800 patients in 12 countries across Africa and Asia found that up to 83 per cent of adults with confirmed TB disease reported no persistent cough for two weeks or longer. And 63 per cent of such patients reported no cough of any duration.

The results indicate that patients with subclinical TB — infection without persistent cough or other TB symptoms — may experience delayed diagnosis and remain a potential source of transmission to close contacts, health researchers who conducted the study have said.

The study used TB surveys’ data from Cambodia, Laos, Pakistan and Vietnam in Asia but did not include data from India which has the world’s largest burden of TB. The World Health Organisation’s 2023 global TB report said India had accounted for 27 per cent of the 10.6 million cases worldwide in 2022.

A public health researcher who led the study said the data from India was not available.

“We tried to include data from the national TB prevalence survey in India in 2019, but despite several requests to the Indian TB control programme, we did not receive any response,” said Frank Cobelens, a professor of global health at the Amsterdam University Medical Centre in the Netherlands.

A query sent by this newspaper to the Union health ministry on Tuesday asking why the TB control programme had been unable to share the data with the researchers has not evoked a reply.

The study by Cobelens and his colleagues also found that up to 28 per cent of patients with TB did not report any of the classic TB symptoms such as cough, fever, night sweats, or weight loss. The findings were published on Tuesday in The Lancet Infectious Diseases, a peer-reviewed medical journal.

Nearly one in four (23.1 per cent) of the TB patients without cough had high bacterial loads in their sputum, indicating that they were likely contagious, according to the study.

“When we take all these findings into account, it is clear we need to rethink aspects of how we identify people with TB,” Cobelens, who is also a senior fellow at the Amsterdam Institute for Global Health and Development, said in a media release.