By Michael Kahn

LONDON (Reuters) - Testing whether a person has swine flu is not easy and it can take days to confirm that suspected cases are actually caused by the virus, a health expert said on Tuesday.

The World Health Organization has confirmed 79 cases of a virus it said has spread from Mexico - where it has killed up to 149 people - into the United States, Canada, New Zealand and Europe, raising fears of a pandemic.

But officials suspect more than 1,600 people may have the infection - a disparity explained in part because there is no quick test, said Andrew Easton, a virologist at the University of Warwick in Britain.

"Many countries now will have several labs to carry this out," he told Reuters in a telephone interview. "You can't do this in the field."

WHO officials only report cases that have been confirmed by their own staffers, in their own labs, at the U.S. Centers for Disease Control and Prevention in Atlanta and a few other places.

Mexico, for instance, ships samples to the U.S. Centers for Disease Control and Prevention in Atlanta or Winnipeg in Canada for testing.

While testing for seasonal flu is relatively simple, doing the same for H1N1 is far more complicated and none of the three techniques are widely available.

One way to confirm swine flu is with a test that involves growing a virus culture in a lab and then sequencing its genetic material. Testing increases of antibodies and a technique called real-time PCR are the other approaches.

The PCR technique - which researcher Easton called the most efficient - uses a device to amplify copies of genes so researchers can easily compare a sample taken from a sick person to the genetic material of the potentially pandemic infection.

"You start with a small number of copies of the virus genes and the technique multiplies them up in a test tube to give them many hundreds of thousands of copies of the virus genes which are easy to measure," Easton said.

The first step for doctors and health officials is often collecting a sample at a local clinic or hospital and then sending it on to a more well-equipped lab for analysis. This can produce some of the biggest delays.

Next researchers must purify the sample to get rid of natural compounds that may interfere with a reading - something that also takes time, Easton said.

"Once you know the genetic makeup of the virus, the machine can tell whether the (genetic) sequence is that of the suspected infection," Easton said.

"If everything was optimized and there were plenty of people around to handle the result you could easily get a result that day."

The PCR device itself can within a few hours compare the genetic makeup of the sample with that of a pandemic virus, Easton said.

But a challenge for researchers is designing a test that health officials can conduct in the field, he added.

"It is technology that is stopping us from doing that at the moment," Easton said.

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