Blood vs. Breath: Which San Bernardino Chemical Test is More Vulnerable to Defense Challenges?

When you are facing DUI charges in San Bernardino, the chemical test result is usually the centerpiece of the prosecution’s case. Whether law enforcement used a breath test or a blood draw, the number that comes back is treated like gospel by prosecutors. But we know the truth: these tests are far from infallible. At Patrick Silva, Attorneys at Law, we have spent years dismantling chemical test evidence, and we can tell you that both blood and breath tests carry distinct vulnerabilities that a knowledgeable defense team can exploit. The question is which type of test is more susceptible to challenge, and the answer depends on the facts of your case.

How Breath Tests Work and Where They Fail

Breath testing devices, often referred to by the brand name Breathalyzer, measure the amount of alcohol in a sample of your exhaled breath. The machine uses that measurement to estimate your blood alcohol concentration (BAC). Notice the word “estimate.” A breath test does not directly measure the alcohol in your blood. It measures alcohol vapor in your lungs and then applies a mathematical ratio, called the partition ratio, to calculate an approximate BAC.

California law enforcement typically uses the Draeger Alcotest or similar devices for evidentiary breath testing at the station. Preliminary alcohol screening (PAS) devices used in the field are even less reliable and are generally considered screening tools rather than definitive evidence.

The partition ratio used by breath testing machines assumes a fixed relationship between alcohol in your breath and alcohol in your blood. The standard ratio is 2,100 to 1, meaning the machine assumes that 2,100 milliliters of breath air contains the same amount of alcohol as 1 milliliter of blood. The problem is that this ratio varies significantly from person to person and even within the same person at different times. Studies have shown individual partition ratios ranging from approximately 1,300:1 to over 3,000:1. If your actual ratio is lower than the assumed standard, the machine will overestimate your BAC.

Common Defense Challenges to Breath Test Results

We attack breath test evidence on multiple fronts. Some of the most effective challenges include:

Mouth alcohol contamination. If you burped, belched, or experienced acid reflux shortly before the test, residual alcohol from your stomach can enter your mouth and artificially inflate the reading. California’s Title 17 regulations require a continuous 15-minute observation period before administering a breath test specifically to guard against this problem. If the officer failed to observe you properly, or if a medical condition like gastroesophageal reflux disease (GERD) caused alcohol vapor to rise from your stomach during the observation period, the result is compromised.

Calibration and maintenance failures. Breath testing machines must be calibrated at regular intervals and maintained according to manufacturer specifications. If the device was overdue for calibration, if calibration records show inconsistent results, or if maintenance logs are incomplete, the accuracy of any reading produced by that machine is questionable. We pull these records on every case.

Breathing pattern effects. How you breathe into the machine matters. Hyperventilating before the test can lower the reading, while holding your breath can raise it. Officers sometimes instruct subjects to blow harder or longer, which can artificially increase the measured BAC. The testing protocol exists for a reason, and deviations from it undermine the result.

Radio frequency interference (RFI). Electronic devices in the vicinity of the breath testing machine can interfere with its readings. Police stations are filled with radios, computers, and other electronics that can produce RFI. If proper shielding was not in place, the result may be unreliable.

How Blood Tests Work and Where They Fail

Blood testing is often considered the more accurate of the two methods because it directly measures the alcohol concentration in a blood sample rather than relying on estimates. However, accuracy depends entirely on how the sample is collected, handled, stored, and analyzed. Every link in that chain is a potential point of failure.

When law enforcement orders a blood draw, a trained phlebotomist or medical professional draws a sample, which is placed into a vial containing preservatives and anticoagulants. The vial is sealed, labeled, and sent to a crime lab for analysis using gas chromatography.

Common Defense Challenges to Blood Test Results

Blood test challenges often require more technical investigation, but the payoff can be substantial. Here are the primary vulnerabilities:

Fermentation. If the blood sample is not properly preserved, microorganisms naturally present in the blood can consume glucose and produce alcohol through a process called fermentation. This means alcohol can be created inside the vial after the draw, resulting in a BAC reading that is higher than your actual blood alcohol level at the time of the stop. Fermentation is one of the most powerful defenses available in blood test cases, and we investigate it aggressively.

Improper anticoagulant or preservative levels. The vial used to collect your blood must contain the correct amounts of sodium fluoride (preservative) and potassium oxalate (anticoagulant). If the preservative amount is insufficient, it may not prevent fermentation. If the anticoagulant is inadequate, the blood can clot, making accurate analysis difficult or impossible.

Chain of custody failures. From the moment your blood is drawn until it is tested at the lab, every person who handles the sample must be documented. If there are gaps in the chain of custody, unexplained delays, or signs of tampering, the integrity of the evidence is compromised. We demand complete chain of custody documentation and scrutinize every transfer.

Contamination at the draw site. If the phlebotomist used an alcohol-based swab to clean the draw site, residual alcohol on the skin can contaminate the sample. Proper protocol requires a non-alcohol antiseptic. We review the draw procedures for compliance.

Lab errors and analyst qualifications. Crime labs process thousands of samples, and mistakes happen. Equipment malfunctions, contaminated reagents, mislabeled vials, and human error can all produce inaccurate results. We examine the qualifications of the analyst who processed your sample and review internal lab quality assurance records.

Which Test Is More Vulnerable Overall?

Both tests have significant vulnerabilities, but they are different in nature. Breath tests are vulnerable to real-time operational errors: calibration failures, observation period violations, mouth alcohol contamination, and physiological variables like GERD or lung capacity differences. These challenges are often easier to identify because they involve observable procedural failures.

Blood tests are vulnerable to post-collection errors: fermentation, storage problems, chain of custody gaps, and lab processing mistakes. These challenges often require deeper investigation, including subpoenaing lab records, hiring forensic toxicology consultants, and examining the physical evidence.

In our experience, breath tests tend to have more readily available attack vectors because the margin for error is built into the technology itself. The partition ratio assumption, the sensitivity to mouth alcohol, and the reliance on proper calibration all create openings that a thorough defense can exploit. But blood tests, when compromised by fermentation or chain of custody issues, can produce results that are even more dramatically inaccurate.

The strongest defense strategy depends on the specific facts of your case. That is why we conduct a comprehensive review of all testing evidence, protocols, and records before developing our approach.

Why the Type of Test Matters for Your Defense Strategy

Knowing which test was administered is just the starting point. What matters is what happened before, during, and after the test. Was the observation period properly conducted? Was the machine in calibration? Was the blood sample stored at the correct temperature? Was the chain of custody maintained? These are the questions that determine whether a test result stands or falls.

We have handled DUI cases involving both types of tests across San Bernardino County, and we approach each one with the same level of scrutiny. Prosecutors expect defendants to accept chemical test results as unchallengeable facts. We know from experience that these results are often anything but.

Patrick Silva, Attorneys at Law Will Tear Apart the Evidence Against You

The prosecution’s case is only as strong as its weakest piece of evidence, and chemical test results are frequently that weak link. At Patrick Silva, Attorneys at Law, we have the education, courtroom experience, and technical knowledge to challenge blood draws, breath tests, and every procedure in between. Our track record of winning DUI cases in San Bernardino speaks for itself, and our clients’ reviews confirm what we deliver.

Do not plead guilty based on a number that may be wrong. Call us at 909-500-4819 and let us show you what a real DUI defense looks like.