Police said the tests have yet to establish the cause of four-year-old Olivia Clarence's death or those of her twin brothers Ben and Max, three.
Further tests will now be carried out.
The youngsters had the life-limiting genetic disorder Spinal Muscular Atrophy.
Tania Clarence, 42, was arrested shortly after their bodies wereound at the family home in New Malden, Surrey, last week. Mrs Clarence, whose husband Gary is a stockbroker appeared in court on Friday and has been remanded in custody to appear at the Old Bailey in London tomorrow.
Mr Clarence, a director of investment firm Investec, was on holiday in South Africa at the time of the deaths.