Understanding Chemical Tests

After a driver has been arrested for DUI, the law requires that the driver submit to a chemical test to determine if the driver is under the influence of alcohol, marijuana, drugs or prescription medication. 

If the officer is arresting the driver for an alcohol DUI, the driver has the right to choose a breath test or a blood test. 

If the officer believes that the driver is driving under the influence of marijuana, drugs, or prescription medication, the officer has the right to require the driver to take a blood test.  If for some reason a breath or blood test is not available, a urine test may be used. 

A chemical test, assuming it is administered and analyzed correctly, can only tell what was in the driver’s body at the time of the test.  However, the purpose of the chemical test is an attempt by law enforcement to determine how much alcohol, marijuana, drug, or prescription medication is in a driver’s bloodstream at the time of driving.  It is important to remember that the chemical test is often administered well after the time of driving; the prosecutor will then try to argue that the test result was the same or more at the time the car was being driven by making a number of assumptions.

It is always the prosecution's burden to prove beyond a reasonable doubt:

  1. that the test result itself is accurate and reliable
  2. the amount of the substance in the driver’s blood at the time of driving
  3. that at the time of driving, the driver was under the influence of the impairing substance. 

These conclusions are not always easy to prove, and this is why it is essential to have a lawyer that is an expert in chemical test analysis defending you.

The two most common chemical tests are:

  1. The Breath Test, where the driver actually blows into a machine
  2. The Blood Test, where the driver’s blood is drawn for analysis.  Breath testing and blood testing can be inaccurate if not performed and analyzed correctly.

The Breath Test

The police officer asks the driver to blow into a breath machine and a result is given. For the result to be accurate, the alcohol must be fully absorbed into the driver’s blood system and reach equilibrium in the body.  When fully absorbed, no alcohol remains in the driver’s stomach or small intestine; all of the alcohol is now in the blood system.  When equilibrium is reached, alcohol is distributed evenly throughout the body.  In this state, a blood sample drawn from the ear lobe and another drawn from the driver’s toe would show matching results.  When alcohol has reached equilibrium in the body, breath testing is presumed to be accurate.

When a person drinks alcohol, it first goes into the stomach, and then moves to the small intestine, where it is absorbed into the blood system.  Initially, as the alcohol is absorbed into the blood system, it goes into the artery flow first and is pumped to the lungs from the heart before it gets anywhere else in the body.  Thus if a person now blows into a breath machine, the lungs are saturated with alcohol molecules and the result will be artificially high (by 200-300% according to some studies) compared to a blood sample drawn from the person’s forearm at the same time.  The alcohol has reached the lungs, but it has not yet spread to the rest of the body, not even the brain.        

No one really knows when a person fully absorbs alcohol and reaches equilibrium, so avoid blowing into a breath machine.  If two or three hours have elapsed since the last drink, more likely than not the alcohol has been fully absorbed.  If less than an hour has elapsed since the last drink with food, then it is likely that the alcohol has not yet been fully absorbed.  

Blood Testing

Blood Testing is used to try to determine the amount of alcohol, marijuana, drugs, or prescription medication in a driver’s blood.

Blood Testing for Alcohol:

The process used to analyze a blood sample for alcohol content is usually gas chromatography, a very complicated procedure that can yield inaccurate results if not implemented correctly.   

Contamination of the blood sample before it is even analyzed is another factor that causes inaccurate results.  How a blood sample is handled from the time the blood is drawn from the arm, the transportation of the sample, and the storage of the sample is extremely important.  Using an expired tube or not using the tube correctly can lead to contaminating the blood sample.  Improper handling, transportation, and/or storage of the sample can also result in contamination of the blood sample before it is analyzed.  A contaminated sample may lead to an inaccurate and unreliable test result.

In addition, the amount of time it took from the time the blood sample was drawn to the time of analysis is also an essential factor that can affect the accuracy and reliability of a blood test result.   

Blood Testing for Drugs:

The process used to analyze the blood sample for marijuana, drug, and prescription medication content is usually liquid chromatography along with mass spectrometry, which is a highly sophisticated and complicated process that can give inaccurate and unreliable results if not done correctly.

There are two main issues when it comes to drug cases:  (1) Was the analysis method used to analyze the blood sample accurate and reliable, and (2) is the amounts of the drug, etc., found in a driver’s blood a sufficient amount to cause the driver to be impaired by the drug?  

It is often difficult for the prosecution to prove that any particular amount of a particular substance (drug, etc.) can cause a particular driver to be impaired or “under the influence.”  The main reason is that (1) drugs affect different people differently, and (2) the amount of a drug found in a person’s blood does not necessarily correlate to an amount of drug in a person’s brain that would cause impairment.    

DUI defense attorney Manuel J. Barba is an expert in blood sample analysis and the science upon which it is based. To successfully defend your DUI case, you need an expert DUI defense attorney with this specialized knowledge.


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