What are false positives for lettuce from FDA Taylor Farms

The Food and Drug Administration’s reversal of a key part of its investigation into cyclosporiasis has caused confusion in the United States, but the agency said Monday it has not changed its key conclusions about the outbreak.

A test that indicated a sample of iceberg lettuce provided by Taylor Farms de Mexico carried the Cyclospora parasite was a false positive, the agency said on Sunday. However, this only applies to one specific shipment of lettuce made by the company, which the agency does not believe was the vehicle that sickened more than 1,600 people and possibly thousands more.

In a clarification issued Monday, the agency said its false positive results do not change its previous conclusion that the outbreak was linked to shredded iceberg lettuce from central Mexico supplied by Taylor Farms, which was also served at some Taco Bell locations. The agency still advises Americans against eating iceberg lettuce.

“The FDA investigation and outbreak data continue to converge on shredded iceberg lettuce from Taylor Farms locations in central Mexico,” she said in a statement. “The FDA will continue to work with federal and state partners to investigate this multi-state outbreak and ensure that the products implicated in this outbreak are removed from the market.”

Investigators are continuing to examine exactly what is causing the outbreak, which can lead to symptoms similar to those of a stomach bug for days or even weeks. CNBC has reached out to the FDA for details on the next phase of the investigation, including whether additional ingredients or suppliers are under scrutiny and whether the agency expects to issue further guidance. The agency has not yet responded.

The false positive test comes as the evolving investigation creates uncertainty for both consumers and the food industry. While the U.S. Food and Drug Administration (FDA) has said not to eat lettuce from Taylor Farms, some diners have turned away from salads altogether as the number of cases rises.

“Unfortunately, this latest development may add further confusion to an already complex situation for consumers,” said Frank Yiannas, former deputy commissioner for food policy and response at the FDA.

Although industry analysts do not expect the outbreak to have a long-term impact on Taco Bell or other restaurant chains, it could affect sales at least temporarily, based on traffic data, and cause one to two affected businesses associated with it. Traffic at Taco Bell was down nearly 30% on Friday compared to the weekday average so far this year, according to data from research firm Placer.ai.

In foodborne disease outbreak investigations, a false positive result can occur when the initial screening signal is not reproduced during confirmation testing.

Donald Schaffner, chair of the department of food science at Rutgers University, said Cyclospora tests are vulnerable to false positives because the process of administering them can sometimes make it appear as if there is a parasite in the sample when there isn’t. He said the FDA’s own protocol recommends another test to confirm the result.

“I’ve never seen this before,” Schaffner said of the reversal. “It’s a very big mistake.”

But it has not caused a shift in the way public officials view the roots of the outbreak.

The Michigan Department of Health and Human Services, where the outbreak has seen explosive growth, says it continues to recommend people buy whole heads of lettuce instead of pre-washed, bagged or pre-mixed bunches of salad.

The agency added that based on interviews with more than 2,000 infected patients, many of them did not report eating lettuce in a restaurant, although they often said they ate leafy vegetables in some places.

In fact, the cause behind most foodborne illness outbreaks is identified this way, Schaffner said. He said it’s unusual for a positive test result or “smoking gun” to show up.

Taylor Farms said Monday morning that it would continue its voluntary recall of the implicated iceberg lettuce from central Mexico, and Taco Bell has already begun removing the suspected lettuce from restaurants in the affected states.

The ripple effects extend beyond those companies.

Walmart It said Monday it had proactively removed four packaged salad kit products, which fall under its Marketside brand and are supplied by Taylor Farms, from stores in 27 states even though it said there was no indication its products were part of the outbreak.

“There have been no confirmed illnesses associated with these products at this time,” the company said in a statement. “We are working closely with our suppliers and have taken immediate steps to remove the products from sale.”

Former FDA Commissioner Dr. Scott Gottlieb suggested to CNBC that the test result does not exonerate Taylor Farms from being linked to the outbreak. He said Friday’s recall was for produce that was imported several weeks ago and was separate from the lettuce that tested positive with the Food and Drug Administration.

He also noted that because Cyclospora has an incubation period of up to two weeks and lettuce has a short shelf life, it is important to test multiple shipments, even if they are coming from the same farm.

While the source of the outbreak may be a single vendor at the moment, it is possible that the contamination has already spread more widely, according to Dr. Norman Beatty, an assistant professor of medicine at the University of Florida College of Medicine.

“We may find that this current outbreak may have led to more disease outbreaks because of the nature of this parasite,” Beatty told CNBC. “The reason this is important for this current outbreak is because there are over 30 states where it has been isolated, and there could be other, smaller outbreaks in other areas.”

For restaurants, the uncertainty is as challenging as the outbreak itself. Salad-focused chains and other operators that rely heavily on fresh produce are left without knowing whether any other ingredients still pose a potential risk.

Taco Bell told CNBC in a statement on Saturday that it voluntarily removed all suspected lettuce from its locations within 72 hours of starting the operation and adjusted its supply chain accordingly. The company added that it is “confident” that consumers can eat safely at its locations.

“This is clearly an industry-wide, nationwide issue, not a Taco Bell-specific issue,” the company said. “We hope that other members of the industry will take the same degree of immediate action and accountability to the consumer as we have.”

Taco Bell isn’t the only restaurant seeing its business slow, according to Placer.ai. Other restaurant chains that sell lettuce, including Chopt, Panera Bread and Chipotle, also saw a drop in traffic on Friday, the company said.

As headlines spread about the outbreak last week, the industry’s largest advocacy group stressed that it takes food safety seriously.

“The Food Code requires every restaurant to have a certified food safety manager on every shift,” Michelle Korsmo, CEO of the National Restaurant Association, said in a LinkedIn post.

“We will continue to support public health officials as they investigate the outbreak and ensure our members have the information they need to evaluate their food safety decisions,” she added.

Despite the false positive result, Taylor Farms and parent Taco Bell Yum brands They are still navigating the reputational fallout from the outbreak that led to widespread removal of products and headlines. However, restaurant analysts previously told CNBC they do not expect a long-term financial hit to businesses linked to the outbreak.

Yum Brands shares are down nearly 9% in the past five days.

Correction: An earlier version of this story included traffic data from Placer.ai which the research firm later corrected. Traffic at Taco Bell was down nearly 30% on Friday, July 17.