Carstens’s lawyer Milton de la Harpe was granted a postponement until Wednesday to finalise his cross-examination of State witness Tim Lourens, the head of forensic toxicology at the University of Pretoria.
Lourens started testifying in October last year.
Carstens was arrested three years ago for allegedly driving under the influence of alcohol.
He has pleaded not guilty to a charge of drunk driving, alternatively driving with a blood-alcohol level of 0.20 percent. The legal limit is 0.05 percent.
On Tuesday, De la Harpe continued to attack the methodology used by the laboratory which analysed the blood sample.
He also put it to Lourens that blood samples could be deemed unreliable because of the presence of micro-organisms such as bacteria, viruses, and fungi.
He said glucose levels presented another possibility for a faulty reading, because glucose was often elevated by shock or stress.
Lourens replied: “If one can inhibit [ethanol] fermentation, then other things can be controlled and reliable.”
When a blood sample was taken from an accused and put into a sterile tube, it was mixed with a sodium fluoride powder to prevent the formation of such micro-organisms.
De la Harpe asked if the powder would also be effective against Candida albacans, a common fungus that lived in much of the human population without harmful effect.
Lourens said the fungus had an enzyme which changed glucose into fermentation but that this would be inhibited by the powder.
The defence lawyer said he found this strange since Lourens had testified in another case at the same court that it would not inhibit the fungus.
The doctor denied he had said this and based his opinion on numerous studies.
“With all respect, you are not an expert in this field,” De la Harpe put to the doctor.
Lourens acknowledged he was not an expert in this narrow field but that he would leave it up to the court to decide.
“I don’t claim to be an expert,” Lourens said.
“Exactly,” replied the defence lawyer.
Carstens’s team was expected to call its own expert Neels Viljoen, retired head of the forensic laboratory in Pretoria, sometime during the trial.