Dutch forensic scientist Leatitia Sijen from the Netherlands Forensic Institute is being questioned in the High Court at Wellington over tests that found central nervous system tissue on the shirt.
The double murder jury trial has focused on complex scientific evidence as it moves through the fourth week of evidence.
The Crown case was that brain matter likely to have come from Lundy's wife Christine, 38, was found on his shirt.
The 56-year-old has denied murdering his wife and 7-year-old daughter Amber in their Palmerston North home on August 30, 2000.
Dr Sijen told the court yesterday some of the samples she was given had very little cell material present so a new testing process had to be developed.
She worked with RNA, which told her which part of the body cells came from - different to DNA which would indicate who the cells belonged to.
She conducted the tests of the stained shirt in front of a defence expert.
Addressing the court today, Defence lawyer David Hislop said one of the criticisms regarding one of the tests was that unequal amounts of RNA was used in the test compared to the control sample.
"I do not fully understand that because in my view there are good reasons to use less," Dr Sijen said.
Mr Hislop said by "playing around" with the quantities on either side of the scientific experiment she was comparing apples with lemons.
"I don't think you are comparing apples with lemons - maybe you take a smaller lemon and a bigger lemon," she said.
The trial in front of Justice Simon France continues.
By Rebecca Quilliam of NZME. News Service
