The random tests Lohan is subjected to twice a week are part of her ongoing punishment following a probation violation issue earlier this year (10).
Lohan has been ordered to appear in court on Friday, when Judge Elden Fox will address the actress' latest probation problem - and he's taken steps to ensure she obeys his ruling by signing a bench warrant for her arrest.
The warrant has been placed on hold until 8.30am local time on Friday - when it will be activated if Lohan does not appear before Judge Fox.
The probation report detailing her two 'dirty' drug tests was filed with court representatives on Monday (20Sep10), reports TMZ.com.
Lohan, who is undergoing intense outpatient treatment following her release from court-ordered rehab last month (Aug10), confirmed the failed drug tests in a post on her Twitter.com page, in which she vowed to take responsibility for her actions.
The 24 year old wrote, "Regrettably, I did in fact fail my most recent drug test and if I am asked... I am prepared to appear before judge Fox next week as a result.
"Substance abuse is a disease, which unfortunately doesn't go away over night. I am working hard to overcome it and am taking positive steps forward every day. I am testing every single day and doing what I must do to prevent any mishaps in the future.
"This was certainly a setback for me but I am taking responsibility for my actions and I'm prepared to face the consequences."