A patient has been infected with the Hepatitis B virus from a blood transfusion, in the first known case of its kind in Ireland.
The Irish Blood Transfusion Service Board has said the case is not due to a test failure.
It says the blood donation was made in the 'window period' when the presence of the virus cannot be consistently detected, even by the most sensitive tests available.
Speaking on RTÉ's News At One, Dr Stephen Field, IBTS Medical and Scientific Director, said there is a period of a "about 15" days, where infected blood can pass as negative.
He said there have been cases of this happening in other countries, including the UK.
Dr Field said it is a rare, one-in-two million event. The blood service said that it "does not have any implications for blood that will be transfused to patients in the future".
The patient is being managed by a medical team and is said to be clearing the virus.
Dr Field has said the blood service had a responsibility to preserve the identity of the patient and the blood donor.
The service says it has tested 1.2 million donations to date and there has been no other confirmed transfusion transmitted infection of Hepatitis B.
Hepatitis B is a common infection of the liver, transmitted through exposure to infected blood and body fluids.
Transmission by transfusion is now rarely encountered due to highly sensitive laboratory testing of each blood donation.
In this case, at the time the blood donation was taken for molecular testing, it tested negative for Hepatitis B and other viruses.
The red cells were later transfused to a patient.
Subsequently the donor was found to have acute Hepatitis B and the blood service was informed by the public health authorities.
An archived sample of the blood donation was tested again and it tested negative.
An extra test for Hepatitis B virus DNA was positive when tested.
An investigation made by Grifols/Hologic, the makers of the test used to check for the virus, found that the test performed to the expected standard of sensitivity for detecting Hepatitis B.
The blood service said that most people with normal immune systems will clear the virus.
Those infected early in life or with immune deficits are more likely to develop chronic Hepatitis B infection.
While treatment is available for those who become chronically infected, vaccination and screening is key in preventing spread to others.
Short-term (acute) hepatitis B does not usually need specific treatment, but may require treatment to relieve the symptoms.
Long-term (chronic) hepatitis B is often treated with medication, to keep the virus under control.
Minister for Health Simon Harris has said there is no ongoing patient safety incident.
He said this is an extraordinarily rare incident.