What a self-recorded BAC log can—and cannot—show

A self-recorded BAC log can show what reading you entered, when the record was preserved, what source photo you attached, and whether the saved record later changed. By itself it cannot prove who supplied the breath sample, whether the device was accurate, whether testing conditions were controlled, or whether a person was medically or legally unimpaired.

1. What the log can preserve

A personal BAC entry can preserve the displayed reading, the date and time, the device you identified, a photograph of the display, and your notes about the context.

When the record is fingerprinted at save time, a later reviewer can also check whether the saved entry has changed. That is evidence about the record’s history—not proof that every underlying fact is true.

• The reading you entered
• The time the entry was preserved
• A source photo or attachment
• The device information you recorded
• Visible gaps and later corrections

2. What the log cannot establish by itself

Without observation or controlled identity checks, the log cannot independently establish who provided the sample. It cannot verify that the device was calibrated, used correctly, or free from interference.

A reading also does not make a medical or legal conclusion. Alcohol can affect driving ability at levels below common legal thresholds, and laws and testing requirements vary.

• Identity of the person tested
• Device accuracy or calibration
• Controlled test conditions
• Medical fitness or clinical sobriety
• Compliance with a specific court or program requirement

3. Why device context matters

NHTSA distinguishes among preliminary screening devices, evidential devices, ignition interlocks, and other alcohol-measurement equipment. Formal programs may control the device, maintenance, calibration, scheduling, identity, and chain of custody.

A consumer breathalyzer used at home does not become an evidential or monitored system because the result is photographed. Record the device model and follow its manufacturer instructions, but describe the result honestly as a self-recorded reading.

4. Why gaps should stay visible

A missing day is not a failed test unless a qualified program defines it that way. It is a gap in a self-recorded sequence.

Do not fill the gap later with a remembered estimate or hide it from a report. A timeline with visible gaps is easier to understand than a polished chart that implies continuous testing.

5. Use language that matches the evidence

Prefer “self-recorded BAC reading,” “display photo,” “entry preserved at,” and “no entry recorded” over “verified sober,” “passed,” or “proved sobriety.”

If someone needs monitored, evidential, clinical, or court-ordered testing, follow that program. A personal log can be supplemental context, but it should not claim equivalence.

The honest claim is narrower and stronger: this is what was recorded, this is when it was preserved, and this is whether the saved record later changed.

Sources and further reading

These primary resources support the general boundaries and terminology in this guide. They do not turn the article into individualized legal, medical, or treatment advice.

• NHTSA: Alcohol Measurement Devices
• NHTSA: Drunk Driving—BAC and driving effects
• NIAAA: Alcohol Metabolism

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