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Breath and Blood Tests in California DUI Cases — What the Numbers Don't Tell You

A BAC reading looks definitive. It's a number, and numbers feel like facts. But chemical testing in DUI cases — whether by breath or blood — involves equipment, procedures, human handling, and timing variables that can produce results that don't accurately reflect what was in your bloodstream while you were driving. At the Law Office of Nic Cocis, we examine the chemical testing evidence in every DUI case we handle in Murrieta and Southwest Riverside County, because that's where many DUI defenses are actually built.

How Breath and Blood Testing Works — and Where It Fails

Breath Testing: The Assumptions Built Into Every Reading

California's approved evidential breath testing devices — the Draeger Alcotest 9510 is the current primary device — measure deep lung air and apply a conversion factor to calculate a corresponding blood alcohol concentration. The conversion factor — 2100:1, meaning 2,100 milliliters of breath corresponds to 1 milliliter of blood — is a population average. Individual partition ratios vary from approximately 1700:1 to 2400:1. A person with a lower-than-average ratio will produce a breath result that overstates their actual BAC.

The machine doesn't know your partition ratio. It assumes the population average. That assumption can produce a result that's measurably higher than reality.

Beyond the partition ratio, several other factors affect breath test accuracy:

Mouth alcohol.

Residual alcohol in the mouth from recent drinking, burping, or acid reflux can contaminate the sample. Most DUI protocols require a 15-minute observation period before testing to address this — but if that period wasn't observed, the result is suspect.

Medical conditions.

Gastroesophageal reflux disease (GERD) and other conditions that cause stomach contents to enter the esophagus can introduce alcohol into breath samples independent of systemic BAC.

Device calibration and maintenance.

Approved breath test devices require regular calibration and documented maintenance. A device outside its calibration window, or with a maintenance history showing irregular service, produces unreliable results.

Radio frequency interference.

Some older devices were susceptible to interference from radio transmissions — a recognized issue in patrol vehicle environments.

Blood Testing: Chain of Custody and Preservative Concerns

Blood tests are generally considered more accurate than breath tests, but they introduce a different set of challenges — primarily around handling, storage, and the chemical additives in the collection tube.

Collection tubes for blood draws in DUI cases contain two additives: an anticoagulant to prevent clotting and a preservative (sodium fluoride) to prevent fermentation of blood sugars into alcohol after collection. If the preservative was absent, depleted, or improperly mixed, the blood sample can continue to ferment in the tube — producing alcohol that wasn't present in the original sample. A blood sample that was left unrefrigerated for an extended period, or that was drawn into a tube with inadequate preservative, may show a higher BAC than existed at the time of the draw.

Chain of custody documentation tracks who drew the sample, how it was packaged, where it was stored, how it was transported to the lab, and who handled it at the lab. Any gap in that chain creates an argument that the sample tested may not be the sample drawn — or that its integrity was compromised between the draw and the analysis.

How We Challenge Chemical Testing Evidence

We examine the breath or blood test evidence in every DUI case. The process isn't fishing for technicalities — it's applying scientific scrutiny to evidence that the prosecution will present as conclusive.

01

Requesting calibration and maintenance records for the specific breath test device

02

Examining the 15-minute observation period documentation

03

Reviewing the blood draw procedures, collection tube lot numbers, and preservative documentation

04

Analyzing chain of custody records from draw to lab to report

05

Retaining independent toxicology experts where the analysis warrants it

06

Applying rising blood alcohol analysis to the timeline of consumption and testing

What to Expect When You Work with Us

01

Discovery Request

We request every document associated with the chemical test: the device's maintenance log, the calibration certificates, the officer's observation period notes, and for blood tests, the full chain of custody and lab documentation.

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02

Expert Analysis

Where the documents raise questions — an inconsistent calibration record, a missing observation period, a blood sample with inadequate preservative documentation — we retain a qualified forensic toxicologist to analyze the evidence and provide an independent opinion.

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03

Motion or Negotiation

If the chemical testing evidence is unreliable, we challenge its admissibility or challenge the result at trial. If the evidence is solid but other defenses exist, we develop the full defense picture.

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Why Choose the Law Office of Nic Cocis?

Scientific Evidence Focus

Examines chemical testing records in every DUI case — not just the number

Expert Network

Works with forensic toxicologists where the analysis requires independent review

25+ Years of DUI Defense

Knows what to look for in breath and blood test documentation

Multilingual Services

English, Romanian, and Spanish available

Frequently Asked Questions

Blood tests are harder to challenge on accuracy grounds than breath tests, but chain of custody and preservation arguments still apply. The absence of a breath test also removes some defenses — rising blood alcohol is harder to argue when there's only one data point — but blood test results can still be attacked on collection, handling, and lab procedure grounds. We examine the blood draw documentation in every case regardless of whether a breath test was also taken.

Alcohol absorbs into the bloodstream over time. If you consumed alcohol shortly before driving, your BAC may have been below the legal limit while you were driving but continued rising during the period between the stop and the chemical test. The test result reflects your BAC at the time of testing — not at the time of driving. Depending on the timing of consumption, the stop, and the test, the difference between driving-time BAC and testing-time BAC can be legally significant. This defense requires analysis of the consumption timeline and the absorption rate, and it's most viable when the tested BAC is close to the legal limit.

Yes. California Vehicle Code § 23158 gives defendants the right to have their blood sample tested by a laboratory of their choice, at their expense. We routinely request the retained portion of blood samples for independent analysis when the prosecution's result is close to the legal limit or when the collection and handling documentation raises questions. An independent test that produces a different result — or that reveals preservative deficiency in the original sample — is powerful evidence.

Facing DUI Chemical Test Evidence in Murrieta?

A number on a breathalyzer printout isn't the end of the analysis. Contact the Law Office of Nic Cocis for a consultation. We serve clients in Murrieta, Temecula, Menifee, Lake Elsinore, Wildomar, Winchester, Canyon Lake, French Valley, and throughout Southwest Riverside County.

  • Murrieta

  • Menifee

  • Wildomar

  • Canyon Lake

  • Temecula

  • Lake Elsinore

  • Winchester

  • French Valley

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