SINGAPORE - In an unusual move, prosecutors had to engage an expert to estimate the alcohol content a motorist had in his breath when his car ploughed into an elderly rag-and-bone collector, killing her.

Although Tibrewal Sunil Kumar, 48, had initially failed a breathalyser test when Traffic Police tested him at the scene of the fatal accident on Feb 20, 2016, a second analysis found that he had 32 microgrammes of alcohol in every 100 millilitres of breath - three microgrammes under the limit.

But it emerged in court that it was only 2 hours and 40 minutes following the accident that the second test was administered at the Traffic Police headquarters in Ubi Avenue 3, after the Indian national was arrested. Court documents did not state the reason for the delay.

About five months after he was charged in Nov 2016, the prosecution team engaged Dr Yao Yi Ju, divisional director of the analytical toxicology division of the Health Sciences Authority (HSA).

She estimated Tibrewal, a Singapore permanent resident, had between 42 and 58 microgrammes of alcohol in every 100 millilitres of breath at the time of the accident.

The Straits Times understands it is the first time a "back calculation" has been performed in a drink driving-related matter.

Tibrewal was on Wednesday (Nov 14) sentenced to 10 months' jail and disqualified from driving all classes of vehicles for eight years for causing the death of Madam Tan Powi Kim, 61, by committing a rash act.

One count each of drink driving and causing grievous hurt to her husband, 69-year-old Cheng Kiat Yan, by committing a rash act, were considered during sentencing.

Deputy Public Prosecutor Li Yihong said the accident happened after a drinking session that lasted about nine hours.

Tibrewal had left Orchard Towers with a colleague at about 6.20am and was driving his black BMW along Collyer Quay towards Esplanade Drive about 40 minutes later when he started feeling sleepy.

Tibrewal, who was then the vice-president of a firm here dealing with shipping vessels, was negotiating a bend when he took his eyes off the road and removed one hand from the steering wheel to wake his colleague.

As a result of this rash act, Tibrewal's car swerved towards the extreme left lane of the four-lane road and hit Madam Tan who was standing at the rear of her stationary lorry.

She was flung onto the roof of the car before landing on its bonnet, while her husband fell onto the road. Madam Tan was pronounced dead in hospital about an hour later.

Mr Cheng sustained several injuries, including a fracture in his left knee.

At the time of the accident, the couple were performing their Saturday morning routine of collecting discarded cardboard at the pavement below OUE Link.

Defence lawyers Yusfiyanto Yatiman and Josephine Chee said in mitigation that their client, who had never committed any traffic offences prior to this incident, has accepted full responsibility for his actions.

Tibrewal is appealing against the sentence and has been offered bail of $15,000.