Guthrie tests involve blood samples being taken from the baby's heel.

The mother of murdered teenager Jane Furlong was stunned to learn blood samples taken from her daughter at birth were later used by the police to identify her body.

Police are able to seek access to the blood samples of more than two million New Zealanders under "exceptional circumstances".

Officers have used the database to prosecute two homicide cases, to test blood spots found in the Scott Watson murder investigation, and to identify the remains of missing persons, and victims of natural disasters.

JOHN KIRK-ANDERSON/STUFF Police accessed DNA samples for Ben Smart and Olivia Hope during the Scott Watson murder investigation.

The data was accessed in the Watson murder trial, when Olivia Hope and Ben Smart's DNA was retrieved but did not match with spots of blood found at the crime scene.

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Judith Furlong learned only in the past week that police had used DNA taken from a heel test, also known as a Guthrie test, to identify her daughter Jane, whose body was found buried in sand dunes at Port Waikato in 2012.

STUFF Judith Furlong has only recently been informed police used bloodspot data to identify her daughter Jane's remains.

Other parents said they were not informed of the possibility that their children's DNA could be used in criminal investigations.

National screening unit clinical director Dr Jane O'Hallahan said police accessed the Guthrie cards "as a last resort".

She said every request from police was carefully reviewed and the Health Ministry considered the "individual concerned and the wider public interest in law enforcement and public safety".

SUPPLIED Jane Furlong went missing from Karangahape Rd in 1993. Her body was found at Port Waikato in 2012.

Figures provided by police showed the blood spot samples were accessed once in 2010 to identify human remains, 13 times in 2011 to identify victims of the Christchurch earthquake, and in two homicide inquiries – both resulting in convictions – after a judge issued a search warrant.

Parents can apply to have the cards containing the blood returned to them. Otherwise, the information is kept and can be used for research.

For Judith Furlong, it felt like another betrayal to find out that a sample from her only daughter had been used by police without her consent or knowledge.

"Forty two years ago they certainly didn't say they were keeping it. I had no idea.

"It's invasive because they didn't inform you of anything.

"I'm gobsmacked by it all, 42 years on and suddenly they're using it."

Scott Watson's father Chris said he considered the cards "a DNA database by stealth".

Watson said he commissioned a report by a forensic investigator looking at the police investigation, which found police had obtained four Guthrie cards for the case.

"They've released the ones that mattered, the Hope and Smart ones, but they've also released somebody else's as well, as a control sample."

Watson said the use of Guthrie Cards "didn't really have an effect on the case as such".

"These were for identifying victims rather than anything to do with identifying a perpetrator."

A police spokeswoman said access to samples was rare and strictly controlled.

The spokeswoman said information was tightly protected by the Privacy Act, the Official Information Act, the Health Information Privacy Code, the Code of Health and Disability Services Consumers' Rights, and the Human Tissue Act.

Civil rights lawyer Michael Bott said parents had every right to feel aggrieved if they gave consent for the sample to be used for one purpose, only to find out later it had been used for another.

"The police will come back and argue that this is for a very important purpose, we're identifying a deceased person and something like that.

"The trouble with that is that once you go down that path as a reason for basically over-riding informed-consent provisions, where does it stop?"

In Western Australia, the obtaining by police of Guthrie card DNA for investigative purposes led to public outcry and resulted in the samples being destroyed after two years.

BLOOD SPOT TESTS

Blood spot testing began in 1969 to identify and diagnose more than 20 metabolic diseases, which have high rates or mortality and can sometimes be treated by early intervention.

The sample is placed on a collection card and sent to LabPlus at the Auckland District Health Board for analysis, and stored there unless parents request it's return.

The blood spot card contains information about the baby including its name, date and place of birth, gender, birth weight, National Health Index number, lead maternity carer and contact details.

Requests from police to access the data must be carried out by the manager of the National Criminal Investigations Group.