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Taylor Farms continues to be the subject of a Food and Drug Administration investigation into the growing cyclosporiasis outbreak, despite a false positive test result, the agency said Monday.

Late Sunday, the FDA said that a lettuce sample from the company had falsely tested positive for cyclospora, the parasite that causes the illness. That sample, however, did not come from lettuce already implicated in the outbreak and recall. Instead, it came from a separate shipment the agency collected during routine import inspections.

“To clarify, this false-positive lab sample DOES NOT change the basis for FDA’s ongoing outbreak investigation or the overwhelming epidemiological data supporting the current voluntary recall by Taylor Farms,” the FDA said Monday on X.

The company’s voluntary recall of shredded iceberg lettuce to 27 states is still in effect.

On Sunday, Taylor Farms said in a statement that the FDA had “apologized” about the false positive test results. On a call Monday, an FDA official disputed Taylor Farms’ claim that the agency had issued an official apology.

Taylor Farms did not immediately respond to requests for comment.

Determining the source of an outbreak goes beyond finding a positive sample.

Public health officials don’t rely solely on laboratory tests to identify the source of a foodborne outbreak, said Donald Schaffner, a food microbiology professor and chair of the food science department at Rutgers University.

Instead, investigators look at patient samples and interviews, food receipts and information from the food supply chain to determine where the contaminated food likely came from.

In the ongoing outbreak, investigators found that many people who got sick had eaten at Taco Bell restaurants supplied with Taylor Farms lettuce. As of Saturday, the Centers for Disease Control and Prevention had confirmed more than 1,600 cases in people in Indiana, Kentucky, Michigan, Ohio and West Virginia.

“Taylor Farms is still implicated because of the epidemiology,” Schaffner wrote in an email. “This means that people that ate at Taco Bell, and ate Taylor Farm lettuce, are sick with the parasite. It’s important to realize that this is only a statistical correlation. Correlation is not causation, but it was strong enough evidence that FDA and CDC believe that this is the cause.”

On Saturday, Taco Bell said it had removed all Taylor Farms lettuce from its U.S. restaurants.

Schaffner said it’s unusual for investigators to be able to detect the cyclospora parasite in a food sample.

“Almost always it’s too late because the food has already spoiled or has been eaten,” he said.

A positive result for the parasite would have been a strong piece of confirmatory data, said Don Stoeckel, an environmental microbiologist and associate at the Produce Safety Alliance, a food safety training group.

The lack of a positive result, however, “does not reduce the importance of following the CDC’s recommendation to avoid consumption of the product named on the CDC investigations page,” Stoeckel said.