The presence of drugs, at any level, doesn't mean anything necessarily other than drugs were present in the person's system. It would be up to the prosecution to prove that the drugs - at whatever level - were the cause of the bad driving to prove DUI.
Mr. Dane is absolutely correct; just because you have detectable amounts of a particular sedative in your bloodstream, doesn't mean your intoxicated by that drug. However, the State may argue that the presence of that drug and your performance on the standardized field sobriety tests indicates that you had lost the normal use of your physical or mental faculties because of that drug (which is the legal standard in the State of Texas). Typically, they'll call an scientific "expert" to testify about the effects of that medication on the human body and how it could of impaired you.
It's up to your lawyer to effectively cross examine this witness to get him/her to testify what effect (if any) that the amount detected in your blood would have on your physical/mental faculties. You may also benefit by hiring your own expert to testify that this amount would indicate ingestion 4 days in the past and that the amount in your blood would have no effect.
It's important you move to preserve the blood evidence in order to protect your rights. Speak with a criminal defense attorney ASAP.
Disclaimer: This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.
Actually, Mr. Walcutt's answer is more correct. You can be a bad driver and the officer just got quote lucky in pulling you over for bad driving facts. The State need only prove that you have lost the normal use of your mental and/or physical faculties due to the introduction of a specified substance into your body. (For example, if it were alcohol, you would hear the officer talk about the smell of alcohol, slurred speech, etc. For other substances, it is actually harder for the State to meet their burder because they must use an expert to show what the substance does to the mental and/or physical faculties at the level at which they think they can prove you had it. Usually a DRE - drug recognition officer - will give tests that are somewhat similar to field sobriety tests used to test for alcohol. But, he cannot, without more, show the effect on you. Most often, the State prefers to allege a combination of alcohol and drugs because the general public understand the alcohol use and effect better, without a specialized expert.)
Finally, the phrase DUI applies to the offense a minor commits - driving under the influence (which means any at all because there is zero tolerance even for alcohol for under 21. DWI is driving while intoxicated which applies to people 21 & over. A person 21 or over can have alcohol on his/her breath but not be intoxicated and still be legal to drive.