The blood test showed that a toxicologist detected 14 nanograms per milliliter of zolpidem, the sleep aid found in Ambien.
Doctors say that amount is low, typical of someone who would have just taken a pill or taken one several hours before. A urine sample was negative for alcohol, the toxicologist wrote.
Still, the news contradicts her own doctors, who had announced that she had a seizure that day, and that she had no alcohol or drugs in her system.
She acknowledged, however, that she told a police officer she might have taken an Ambien pill by accident instead of her daily thyroid medication.
"It now appears that my first instinct was correct," Kennedy said in a statement issued late Wednesday.
The former wife of New York Gov. Andrew Cuomo was arrested after state police said her Lexus struck a tractor-trailer on Interstate 684. Police said she then took an exit off the highway before her car became disabled.
Kennedy, 52, pleaded not guilty to a charge of driving while drug-impaired. Police said Kennedy failed several sobriety tests and was swaying and slurring her speech.
The Associated Press contributed to this report.