Monday, May 09, 2005

Faulty bird flu tests

It will not be news to readers of this blog there have almost certainly been many "false negatives" in the diagnosis of H5N1 cases in Vietnam. Now, thanks to excellent reporting by Canadian journalist Helen Branswell we have some indication of the cause.

The diagnostic tests used to screen cases in Vietnam were designed in early 2004 by Canadian researchers using genetic sequencing information from viruses isolated at that time. The tests detect small stretches of three different genes and if those particular areas have changed, the tests will not be accurate. It now appears this is what has happened. The Canadians are trying to "update" the genetic information to produce a more sensitive test, but are having difficulty isolating and growing the currently circulating virus.

The Canadian test is a screening test, so positive results are then confirmed at CDC labs in Atlanta. But negative tests are not retested. This has led to the likely under-ascertainment of cases.

It is time to put pressure on the Vietnamese to use the most accurate and up-to-date diagnostic methods available and to provide it to them. Vietnam is a country with limited resources and is a sovereign nation, so it might be necessary to give them incentives to accept help. This is clearly an urgent matter. We could have widespread human-to-human transmission without detection.