More than 500 nursing assistants in Minnesota will be required to retake a certification test due to “suspicious” patterns in test results, the state’s health department announced Tuesday.

The patterns sparked an investigation into testing held at two Inver Hills Community College sites between May 1, 2014, and October 16, 2015. A press release issued by the Minnesota Department of Health did not give details of the results in question, but said 569 people will need to retake the tests as a result of the “potential improprieties.”

Nursing assistants affected by the testing recall — many of which have already gone to work in nursing facilities — will have until March 31, 2016, to retake the test, but may continue to work until then. Those who do not pass the retesting will be ineligible to work in federally certified nursing homes, MDH noted.

“Since nursing assistants must be supervised and receive specialized training after hiring and prior to working with patients, there is a relatively low risk to patients,” said MDH Assistant Commissioner Gil Acevedo. “Regardless, it is important to re-establish the credentials of these nursing assistants as quickly as possible without causing an undue disruption for workers, employers, or patients.”

MDH will contact the affected nursing assistants and their employers, the department said.