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19-05-2010 | Infectious disease | Article

DFA unreliable for swine flu testing in critically ill patients


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MedWire News: Direct fluorescent antibody (DFA) testing for H1N1 influenza (swine flu) is unreliable in severely ill patients, US research shows.

"Our findings suggest that in patients with severe H1N1 influenza, in whom rapid and precise diagnosis would be most important, DFA unfortunately does not perform well. This is in contrast to less severely ill patients, where DFA appears to be quite reliable." said lead researcher Chanu Rhee, from Stanford University School of Medicine in California.

Polymerase chain reaction (PCR) testing has emerged as the most sensitive and specific test for diagnosing H1N1 influenza, but the availability of the test and the relatively slow turn-around time often limit its clinical usefulness. DFA testing is therefore used at many institutions as an accurate and rapid means of diagnosing influenza.

However, several months after the start of the H1N1 pandemic, Rhee and colleagues noticed that critically ill patients with H1N1 influenza had negative DFA test results more often than those who were less severely ill.

To investigate further, the team reviewed the records of 29 patients with PCR- and/or viral culture-confirmed H1N1 influenza who were admitted to Stanford University Hospital between May 2009 and January 2010. All the patients also underwent DFA testing.

In total, 10 patients were admitted to the intensive care unit (ICU) to undergo mechanical or non-invasive positive pressure ventilation.

The researchers found that just two (20%) ICU patients had a positive DFA result for H1N1 influenza compared with 17 (89%) non-ICU patients.

One possible explanation for the poor performance of DFA in ICU patients is that severe H1N1 influenza may be due to an “over-exuberant” inflammatory response, rather than high viral load, said Rhee.

He concluded: “This study reinforces the fact that patients with suspected H1N1 influenza who are severely ill should be placed in respiratory isolation and receive antiviral treatment without delay, even if DFA testing is negative.

“Furthermore, all critically ill patients with suspected H1N1 should have PCR testing done to confirm the diagnosis, as PCR is significantly more sensitive than DFA, though not perfect either.”

Rhee added: “The next logical step would be analyzing data from a much larger pool of patients from different institutions to confirm these findings."

The research was presented at the American Thoracic Society International Conference in New Orleans, Louisiana, USA.

MedWire ( is an independent clinical news service provided by Current Medicine Group, a trading division of Springer Healthcare Limited. © Springer Healthcare Ltd; 2010

By Mark Cowen

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