Amid growing international demand for rapid Zika testing, the Food and Drug Administration wants to know more about a Minnesota company’s plans to sell a service to test for the virus in human lab samples.

MD Biosciences in St. Paul received a letter from the FDA on Wednesday night saying the company’s new rapid assay for Zika virus appears to be a “high risk test” that has not been approved by the agency. But the question of whether the agency regulates such tests falls into a legal gray area in which the FDA has recently been trying to assert more control.

The letter to MD Biosciences asks the company to submit data on its test design, validation and performance characteristics, “in light of the current public health emergency.” Company CEO Eddie Moradian pledged to cooperate with the request.

“It is a federal agency that is asking to have this conversation, and of course we will be open and transparent and I think it’s a good conversation to have,” Moradian said. “If they determine that we do need premarket approval, then we will apply for it. It is not an issue. … Based on our understanding, a laboratory-developed test does not require that, but they may have a different opinion.”

Companies around the United States are quickly developing Zika diagnostic tests. Researchers at Houston Methodist Hospital and nearby Texas Children’s Hospital told Reuters they have developed such a test, and a spokeswoman with New Jersey-based Quest Diagnostics confirmed Thursday that the massive lab-services company is working on two tests of its own. Several companies are also developing Zika vaccines.

The World Health Organization declared Zika an international public health emergency in February and called for the development of diagnostic tests. As many as 4 million cases of Zika infection are projected in the Americas this year.

Zika belongs to the same family of microorganisms as West Nile virus and Dengue Fever. It is spread by mosquitoes in South and Central America and the Caribbean, including a cluster of cases in Puerto Rico. Most of the 193 confirmed U.S. cases as of Wednesday, including six in Minnesota, happened during travel to those places. The virus may also be spread through sexual contact with infected men.

About 80 percent of people infected show none of the symptoms, like sudden onset of fever, rash, joint pain and reddened eyes. However, pregnant women are being strongly urged to avoid exposure to Zika because the infection has been linked to a serious birth defect called microcephaly, in which a baby’s head is much smaller than expected, the Centers for Disease Control and Prevention says.

So far the FDA has approved one test to diagnose Zika infection. Distributed by the Centers for Disease Control and Prevention, that test is designed to detect whether a person’s immune system has developed antibodies to Zika. Waiting for an immune response can introduce delays of at least four days and add uncertainty about some test results because of the timing.

The Star Tribune reported Tuesday that St. Paul’s MD Biosciences has developed a rapid assay that can detect a unique snippet of Zika DNA, eliminating the need to wait for an immune response. The nucleic-acid test can be used on human blood, urine, serum and plasma. The $190 test is being offered to all interested parties, including large health care clinics and international organizations, the company says.

The test is not available as a kit. Rather, MD Biosciences is a type of lab that has certification from the Centers for Medicare and Medicaid Services to run specialized tests of its own design — but only if the samples are tested on-site in St. Paul, using approved machines and methods.

The FDA has been seeking broader authority over the hundreds of laboratory-developed tests (LDTs) that look for “biomarkers” for everything from rare diseases to cancer. Last November, the FDA published a report highlighting 20 lab-developed tests that it called problematic because they could provide false positives or false negatives.

“FDA has proposed to step up our oversight of LDTs,” FDA Associate Commissioner Dr. Peter Lurie wrote in a blog post in November. “FDA oversight would help ensure that tests are supported by rigorous evidence, that patients and health care providers can have confidence in the test results, and that LDTs have more scientifically accurate product labeling.”

Minneapolis health care regulatory attorney Mark DuVal said whether the FDA has the power to regulate tests like MD Biosciences’ Zika assay depends on unique circumstances, including the specific marketing claims made by the lab.

FDA officials sometimes encroach into new areas of regulation, DuVal said, by claiming to have the authority and then declaring it will exercise “enforcement discretion,” as it has over lab-developed tests.

“It’s a smart strategy because they assume newfound jurisdiction over new product areas historically not regulated and then placate Congress and industry by saying they will exercise ‘enforcement discretion,’ ” DuVal said via e-mail. “With the Zika virus, the political dynamics help the agency when it wants to assert its jurisdiction, but the legal/regulatory framework has not changed.”

An FDA spokesman said in an e-mail Tuesday that the agency is encouraging the development of Zika diagnostics, but that such tests should not be used for clinical diagnoses without FDA approval. The agency requested any company developing such a test to apply for emergency use authorization.

“This is a discussion that is coming,” MD Biosciences’ Moradian said Thursday. “I think this one was highlighted simply because of the topic of what it is. It’s very hot in the press.”