Contrary to early police reports about Britney Spears being "under the influence of an unknown substance" before she was rushed to the Cedars-Sinai Medical Center Thursday night, the singer’s blood tests came back negative, Life & Style magazine reported.

According to a source close to the Spears family, Britney’s blood test came back clean, "There are no traces of drugs or alcohol of any kind," the source was quoted by Life & Style as saying.

The fact that Britney’s breakdown was not caused by any drugs fuels speculation that the singer might be indeed mentally unstable.

An LAPD spokesperson could not confirm nor deny to the mag that earlier reports indicated that cops believed Britney appeared to be under the influence of a substance when they arrived on the scene.

Early Friday, paramedics swarmed the pop singer’s home and rushed her to the hospital on a gurney for a mental evaluation after she refused to hand over one of her two sons, Sean Preston, 2, and Jayden James, 1, to her ex-husband’s bodyguard. According to reports, Britney locked herself and Jayden inside her house and refused to let anyone in, before Britney’s court appointed monitor called the police.

"She had already put Preston in the car when Britney locked herself in a room with Jayden," a source said. "The cops came and got through the door and tied her down to a gurney."

Designated as a "special needs" patient, a term given to those who "have either overdosed or tried to commit suicide," the troubled pop star remained on "medical hold" at the hospital, undergoing physical and psychiatric tests, while her children are now safe in the hands of Kevin Federline, who was also spotted at the hospital among a crowd of paparazzi and Britney’s dad, Jamie Spears.

According to celebrity gossip website TMZ.com, Britney is on a "5150 Hold" which means she is a danger to herself and her kids. While the hospital declined to comment on Britney’s situation, she was placed under a minimum 72-hour lockdown.

Earlier Thursday Britney arrived late for a court-ordered deposition in the custody case after missing several appointments to do so, but appeared for questioning by Federline's lawyer.

The day before, the singer’s lawyers filed a request to leave her case, citing "a breakdown in communications" with their client that made "further representation of her interests impossible."

Following Thursday’s incident, Federline is likely to get full custody of the couple’s children.

Britney started making headlines for her bizarre behavior after she filed for divorce in November 2006, culminating with her first major breakdown in February last year, when she shaved her head bald as a crowd of paparazzi watched her from outside a hair salon.