Anthrax Tests Are Far From Foolproof

Pornographic material mailed to a Microsoft office in Reno tested positive for anthrax on an initial screening. A second, more sophisticated test came out negative, but a third test--all at the Nevada State Lab--showed positive results.

The national Centers for Disease Control and Prevention is now looking at the material, and a spokesman says preliminary indications are that the sample is negative.

The flip-flopping results highlight how unreliable tests for the deadly bacterium can be--both for suspect substances and the people who may have been exposed to it.

Experts say field tests are particularly error prone and can produce false positive or false negative results. More extensive laboratory procedures have their own limitations. And accurate tests for bacteria in people are even more problematic.

The inconclusive results have heightened the agony for the people involved and the nation watching their plight so carefully.

But public health officials involved in the anthrax scare and experts around the country say certainty is not possible given the fallibility of the tests.

“It’s confusing to the public but it’s not confusing to us, because we know there can be equivocal testing that needs to be verified,” said Bob Salcido, a Nevada investigator who worked on the case.

Even when the stakes are high, conclusive findings take time. Senate Majority Leader Tom Daschle’s office discovered a suspicious letter about 10:30 a.m. Monday, but Capitol police did not get final confirmation that the substance was anthrax until late that evening.

“This certainly warranted rapid response; Tom Daschle’s office would get the fastest analysis available,” said Calvin Chue, a research scientist for Johns Hopkins University’s Center for Civilian Biodefense Studies.

Investigators on the scene of an anthrax scare can perform preliminary field tests with a simple device that resembles a home pregnancy test or with a high-tech, suitcase-sized DNA scanner--the gold standard for field tests. Both have limitations.

“None of the rapid tests can be considered a final confirmation,” Chue said. “The trade-off for being very rapid is it’s not 100% accurate. There is no single test that is 100%.”

A false positive can result if the test is fooled by another member of the bacillus family, which includes anthrax and a large number of more benign bugs.

“These tests are very fast but they do have a tendency for false positives,” said Gary Andersen, a senior scientist at Lawrence Livermore National Laboratory.

False negatives are possible if there is simply not enough bacteria in the particular test sample.

“This is why a sample may test negative a couple times, but a more sensitive third test would pick it up and you can say the test is positive,” Chue said.

In the lab, scientists spread a specimen in a petri dish and place it in an incubator for 18 to 24 hours at body temperature to see if the bacteria grow. Once the bacterial colonies appear, several tests are used to try to confirm the presence of anthrax.

In the Nevada case, two of these tests came out positive and another was negative, according to Salcido, who works at the state emergency operations center.

The final verdict lies with the CDC, which can perform sophisticated DNA tests as well as more advanced bacterial culture analyses.

“The one we have is looking like it’s going to be negative,” said Lisa Swenarski of the CDC.

State officials--surprised by the early indications of the center’s findings--worried that the sample sent over the weekend may have been damaged during shipment, so they sent an additional sample Monday.

Exposure Tests Are Undependable

Tests to determine if people have been exposed to anthrax present even greater challenges. In fact, they are so undependable that doctors do not rely on them when deciding whether to prescribe treatment.

That’s why antibiotics were prescribed for all employees and visitors who spent up to one hour at the Boca Raton, Fla., publishing house where the anthrax scare originated two weeks ago.

Because of their limitations, cultures grown from nasal swabs--which have been used broadly in Florida, Washington and New York--are primarily an investigative tool for public health professionals to judge the breadth of the exposure, according to Swenarski.

False negatives are possible with these tests because the anthrax spore may be missed when the test is taken.

“The bottom line is: That’s not going to determine whether someone needs treatment or not,” Swenarski said. “Even if you test negative you need to stay on the antibiotics.”

The five employees of American Media Inc. in Boca Raton who tested positive for anthrax antibodies in blood tests had tested negative in a nasal swab test, Swenarski said.

Swenarski was quick to add that although the company announced Saturday that these people tested positive for exposure, the CDC considers the tests preliminary. This underlines the problems with the other main test for people.

They showed only borderline positive results, she added, and could have had anthrax antibodies in their blood because of exposure to anthrax-infected farm animals decades ago.

“It doesn’t test if you’ve got anthrax right now, but if you have had it any time in life,” said Steven M. Block, professor of biological sciences at Stanford University. “Blood carries the memory--like with vaccines.”

A second blood test is needed to be sure of recent exposure, Block added.

The incubation period may be as long as 50 days for anthrax spores that have been inhaled. That is why people are given 60-day regimens of antibiotics.

It is not surprising that the general public misunderstands the testing procedures for anthrax. In response, the CDC is trying to put together a two-page explanation of testing to post at its Web site. But so far, the task has proved too difficult, Swenarski said.

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Times Staff Writer Marlene Cimons contributed to this story.