May 8, 2025
How Scientific Errors in Blood Testing Could Help Beat Your DWI in Houston
Facing a DWI charge in Houston can be overwhelming. Many people believe that if a blood test shows a high blood alcohol concentration (BAC), their case is hopeless. Fortunately, that’s not true. Blood tests — even those using the so-called “gold standard” method of gas chromatography — are not perfect. In fact, scientific mistakes happen more often than you think, and they can make a huge difference in defending your case.
At The Napier Law Firm, we specialize in identifying and exposing errors in DWI blood testing to protect your future. Here’s what you need to know if you’re fighting a DWI charge in Houston.
How Blood Alcohol Tests Work: Gas Chromatography
When police take a blood sample in a DWI case, the lab often uses gas chromatography to measure the BAC.
But gas chromatography is not as simple or foolproof as prosecutors might suggest.
- A gas chromatograph is a “dumb” machine. It doesn’t know what substance it’s detecting until it’s manually “taught” every time it’s used.
- The machine separates substances based on how fast they travel through a column.
- After the substance passes through, it is burned, and the energy produced is measured to estimate how much alcohol is present.
✅ Problem: If the machine is improperly calibrated, incorrectly “taught,” or if there’s contamination, the results can be wrong.
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Common Problems That Can Skew Blood Test Results
Even if gas chromatography is used, mistakes can lead to inflated BAC results. Common issues include:
1. Improper Calibration
The gas chromatograph must be properly calibrated using a linear curve based on known alcohol concentrations.
If the calibration is invalid — for example, non-linear or based on too few samples — the test result can be scientifically unreliable.
2. Carryover and Contamination
If the machine isn’t cleaned properly between tests, leftover alcohol from a previous sample can carry over into the next test, falsely elevating the new sample’s BAC.
- Proper “blanks” must be run to confirm no carryover.
- If contamination occurs, the entire test may be invalid.
3. Preservative Issues and Fermentation
Blood is typically preserved using sodium fluoride to prevent alcohol from forming after collection.
However:
- Too little preservative can allow fermentation, producing new alcohol and inflating results.
- Too much preservative can artificially increase vapor pressure, again inflating the BAC.
This delicate balance means even minor mistakes can make blood evidence unreliable.
4. Storage and Transportation Errors
Blood samples must be kept cold during storage and transport.
If a sample is exposed to heat or left unrefrigerated, fermentation can occur, again raising the BAC without the driver ever drinking more alcohol.
✅ In short: Just because a BAC number exists doesn’t mean it’s accurate.
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What Is Reverse Extrapolation — and Why It’s Dangerous
Reverse extrapolation is when prosecutors try to guess your BAC at the time of driving by adding .015% for every hour that passed between your arrest and the blood draw.
They assume:
- You were already at your highest BAC level (peak absorption).
- Your BAC was declining at a predictable rate.
✅ Problem: Absorption rates vary wildly based on many factors:
- Whether you had eaten (a full stomach delays alcohol absorption)
- Your individual metabolism
- How much you drank and over what period
If you were still absorbing alcohol at the time of the stop, reverse extrapolation could drastically overstate your true BAC at the time you were actually driving.
How The Napier Law Firm Challenges DWI Blood Tests
At The Napier Law Firm, we know how to challenge unreliable blood test results:
- Analyzing Calibration Records: Was the gas chromatograph properly taught and tested?
- Checking for Carryover: Were blanks used to detect contamination?
- Investigating Preservative Levels: Was the blood preserved properly?
- Questioning Storage Conditions: Was the blood transported and refrigerated correctly?
- Attacking Reverse Extrapolation: Were assumptions made that aren’t supported by science?
We build strong, scientific challenges that can expose reasonable doubt — and in DWI cases, reasonable doubt can be everything.
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Arrested for DWI in Houston? Call The Napier Law Firm Today
Don’t let a BAC number decide your future without a fight.
Blood tests can be challenged — and when they are, your DWI case may look very different.
At The Napier Law Firm, we fight aggressively for individuals facing DWI charges, especially first-time offenders who want to avoid a life-changing conviction.
📞 Call us today for a free consultation: 713-470-4097
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Let us use our scientific knowledge, courtroom experience, and passion for defending people like you to protect your rights and your future.