Sen. Bob Casey Jr. (D-Pa.) said on Friday that he had tested positive for coronavirus antibodies, becoming the second senator in as many days to disclose that they had likely contracted the virus.

Casey, in a statement, said he had a "low-grade fever and some mild flu-like symptoms" earlier in the spring and received an antibodies test last week to try to determine if he could donate blood plasma, which is being studied as a potential treatment for COVID-19. 

"The results of this test revealed substantial levels of COVID-19 antibody in my blood, significantly more than the amount required to qualify me as a plasma donor. In an effort to help others fighting this virus, I will be making my first donation today in Taylor, Pennsylvania," he said.

The disclosure comes after Sen. Tim Kaine (D-Va.) said on Thursday that he had recently tested positive for coronavirus antibodies.

Sen. Rand Paul (R-Ky.) is the only senator known to have tested positive for the coronavirus, though several others have had to be quarantined due to exposure to an infected individual. 

Casey, similar to Kaine, said he did not get tested at the time that he felt sick earlier this year, though he did self-isolate at his home in Pennsylvania. 

"In consultation with my doctor, I chose not to seek medical care because my symptoms were relatively mild and manageable. My fever went away on its own by mid-April, and it was never recommended that I be tested for the virus. I was able to work during my illness, remotely engaging with constituents and staff and keeping a full schedule," Casey said. 

Antibody tests, also known as serologic tests, determine whether someone has been infected and has built up some immune response to the virus. They are different from diagnostic tests that determine whether someone currently has the disease.

But public health officials have warned that antibody tests can be unreliable and should not be used for making determinations such as whether or not an individual should return to work.

The Centers for Disease Control and Prevention (CDC) issued new guidance this week raising concerns about the accuracy of the tests and noting that while someone might have antibodies indicating that they've had the virus, it is not clear how long immunity from catching the disease for a second time would last. 

"Until the durability and duration of immunity is established, it cannot be assumed that individuals with truly positive antibody test results are protected from future infection," the CDC wrote.