How Accurate Are Breathalyzer Tests in San Antonio?

The Intoxilyzer 9000 — San Antonio’s Breath Testing Instrument

The San Antonio Police Department uses the Intoxilyzer 9000 for breath testing in DWI cases. The instrument does not directly measure blood alcohol concentration. It measures alcohol vapor in deep lung air and converts that measurement to an estimated BAC using a fixed 2,100:1 partition ratio — the assumption that 2,100 mL of alveolar breath contains the same alcohol as 1 mL of blood. That assumption is a population average. Research shows individual partition ratios range from approximately 1,700:1 to 2,900:1. For individuals whose actual ratio is higher than 2,100:1, the machine reports a BAC higher than they actually have.

Sources of Intoxilyzer 9000 Error

Partition ratio variation. The fixed 2,100:1 assumption can systematically overestimate BAC for individuals with higher-than-average partition ratios. This is not a malfunction — it is a built-in limitation of breath testing technology.

Mouth alcohol contamination. Alcohol from belching, regurgitation, recent mouthwash use, or dental work can artificially inflate the reading. The required 15-minute observation period before the test is specifically designed to prevent this. Any gap in the observation period creates a challenge to the result’s reliability.

Radio frequency interference. The Intoxilyzer 9000 contains an RFI detector, but interference from police radios and electronic equipment in the SAPD testing environment can still affect accuracy.

Medical conditions. Diabetes, acid reflux, GERD, and low-carbohydrate diets can produce compounds the instrument may read as ethanol, causing false readings.

Elevated body temperature. A fever increases the breath-to-blood ratio, causing the instrument to overestimate BAC.

The Maintenance and Calibration Record

Every Intoxilyzer 9000 has a specific maintenance history obtainable through Texas open records requests. We request the complete maintenance file for the specific instrument used in your test. Common findings that support BAC challenge include missed calibration checks, solutions outside acceptable range, logged instrument errors near the date of your test, and lapsed operator certification for the testing SAPD officer.

Blood Tests and the Bexar County Crime Lab

Blood draws analyzed at the Bexar County Crime Laboratory are also subject to challenge. Improper preservative ratios, inadequate refrigeration, and chain of custody documentation gaps all affect reliability. In-vitro fermentation from an improperly preserved sample can produce additional ethanol not present at the time of the draw. See our full San Antonio BAC defense page.