The state Appeals Court rejected a controversial test used on a sex offender that measures sexual arousal as too error-prone in a ruling that keeps a serial child rapist who terrorized the South Shore decades ago behind bars.

The court ruled that Lucas Ortiz, 43, formerly of Duxbury, should remain civilly committed for the rape or indecent assaults of six boys in the 1990s in Plymouth County. He was a member of the Boy Scouts during four of the attacks.

The court agreed with a lower court that the PPG — penile plethysmograph — exam had a troublesome 35 percent “false positive” error rate making it a risky predictor of an offender’s likelihood to strike again.

The test measures the physical response when a sex offender is shown various photographs.

The Boston psychologist who administered the test to Ortiz told the Herald yesterday the PPG is just one tool examiners use.

“The test is a key part of every offender’s treatment program,” said Joseph J. Plaud. “It measures a person’s pattern of sexual arousal. ... We’re just trying to figure out which (sex offenders) are dangerous. But most don’t reoffend.”

The PPG test he gave Ortiz in 2012 just before his sentence was up showed the rapist “displayed sexual arousal to adult consensual sexual scenarios, and did not display deviant arousal to children,” according to court records.

Two other examiners each diagnosed Ortiz with pedophilia and other “sexual and personality disorders,” records state. Those opinions were used to keep him civilly committed to prison.

Plaud called it “ironic” that the Appeals Court decision comes in the same week he blasted the media’s coverage of serial child rapist Wayne W. Chapman.

On Monday, Plaud joined attorney Eric Tennen to slam what he called “fearmongering” from the press over Chapman’s pending release. Two psychologists — not Plaud — have declared Chapman is too old to likely reoffend.

But Chapman is not going anywhere yet. The 70-year-old is being held at MCI-Shirley as he faces new lewdness charges for allegedly exposing himself and masturbating near prison nurses.

Plaud said independent “qualified examiners,” such as himself, working for the state Department of Correction are paid about $180 an hour on cases that normally take 20 hours to produce a determination if a sex offender is or isn’t a risk.

“We try to do the best we can,” he added. And that includes using the PPG exam.

If two examiners rule a sex offender should remain in prison once the sentence is up, the offender can be civilly committed.

Plymouth District Attorney Timothy J. Cruz — in announcing Ortiz’s continued incarceration yesterday — repeated the list of convictions and how the Level 3 offender attacked his sixth young victim when he was out on parole in 1995.