A blood alcohol result is supposed to measure the alcohol that was in your blood, and nothing else. Two problems can break that promise. Carryover happens inside the instrument, when a trace of a previous high sample lingers and adds to yours. Contamination happens outside it, when alcohol from some other source finds its way into the sample. Either one can push a number up, and neither shows on the face of the report.
The good news is that careful labs run safeguards designed to catch both, and those safeguards leave a record. The run order, the blanks between samples, and the collection details tell you whether your result was clean or whether something else got counted.
A blank that runs clean between samples proves the system was clear. A skipped or dirty blank leaves the door open to carryover.
What Carryover Is
Carryover is contamination from within the instrument itself. When a sample with a high alcohol level is analyzed, a tiny residue can remain in the injection system, and if the next sample follows too closely without the system being cleared, that residue can add to the next measurement. Laboratories control for this by running blank samples between real ones and by washing the system. A blank that comes back clean shows the instrument was clear before your sample ran. The safeguard works only if it is used and checked.
The Run Order Matters
Because carryover travels from one sample to the next, the sequence of the run is evidence. We look at what ran immediately before your sample. If it was a high-alcohol sample and the blank in between was skipped, or if the blank itself showed alcohol, that is a concrete reason to question your result. This information sits in the batch records and the run log, which is part of what we request when reviewing the gas chromatography data. A result is not just a number in isolation. It is one entry in a sequence, and the entries around it matter.
The blank run is what catches it
Here is the specific thing to look for, because carryover is not invisible if the lab was watching for it. Between samples, a properly run batch includes a blank, a clean solvent or negative sample that should read as nothing. That blank is the check. If a blank run right after a high-reading sample comes back showing a signal, the instrument was carrying material from one sample into the next, and any result measured near that point is suspect, including yours. So two things from the batch record tell the story: what sat in the run order immediately before your sample, and whether the blanks around it came back clean. When the blanks are missing from the record, or one of them is not clean, the lab cannot show your number was measured on an instrument that was not still holding someone else’s alcohol.
Contamination From Outside
Contamination comes from sources outside the instrument. The most familiar is at the draw, where an alcohol-based swab used to clean the skin can introduce alcohol at the puncture site, which is why Florida Administrative Code Rule 11D-8.012 calls for a non-alcohol antiseptic, as covered on the how blood is drawn page. Other sources include contaminated or expired collection tubes, alcohol vapors in the collection or lab environment, and the simplest problem of all, a sample that was mislabeled or swapped so that the result belongs to someone else. That last possibility is the subject of the chain of custody page.
Contamination has ordinary sources, and most of them leave a trace in the collection records, the tube lot, or the chain of custody.
What We Request
We ask for the full batch and run sequence so we can see exactly what ran before and after your sample, the results of the blanks, and the system wash and maintenance records. We pair that with the collection records, the tube lot and expiration, and the chain of custody, so we can test both kinds of problem at once. The goal is to confirm that the alcohol the instrument measured was yours, drawn cleanly, and not a leftover from another sample or an outside source.
Why This Matters
A number that looks precise can still be wrong if it counted alcohol that did not belong to you. Carryover and contamination are recognized ways that happens, and the safeguards against them produce records that either close the question or open it. Reading those records is part of making sure the result reflects the blood, and only the blood.
I do not take a blood-alcohol number in isolation, because it was measured inside a batch of other people’s samples, and what happened around it matters. I pull the full run order and the blanks, and I look at what was measured just before your sample and whether the checks that catch carryover came back clean. When a high sample sat right ahead of yours and the blanks are missing or dirty, that number is not standing on solid ground, and I make the State account for it instead of presenting it as if your sample were the only one in the room.
I started out as an Assistant Public Defender in Florida’s Thirteenth Judicial Circuit, in Tampa, and today I am one of six ACS-CHAL Forensic Lawyer-Scientists in Florida. I work both the science and the procedure in your case the way the State’s own analysts and officers are trained to, and I show a jury the exact point where the evidence does not hold up. Learn more about my background.
Questions About Carryover and Contamination
What is carryover?
It is contamination from within the instrument. A trace of a previous high-alcohol sample can linger in the injection system and add to the next sample if the system was not cleared. Labs run blanks between samples to catch and prevent it.
How would carryover affect my result?
If a high sample ran right before yours and the blank between them was skipped or showed alcohol, residue could have added to your measurement and pushed it up. The run order is where we look for this.
What is a blank?
A clean sample run through the instrument to confirm the system is clear. A blank that comes back clean shows there was no carryover before your sample. A skipped or contaminated blank removes that assurance.
How does outside contamination happen?
Through an alcohol swab at the draw, a contaminated or expired tube, alcohol in the collection or lab environment, or a mislabeled or swapped sample. Florida’s rules call for a non-alcohol antiseptic precisely to avoid the swab problem.
Could my sample have been mixed up with someone else’s?
It is one of the contamination possibilities, and it is why labeling, seals, and the chain of custody matter. We review that documentation to confirm the result belongs to you.
What records show carryover or contamination?
The full batch and run sequence, the blank results, the wash and maintenance records, the collection details, the tube lot and expiration, and the chain of custody. Together they show whether the measured alcohol was really yours.
Related: gas chromatography, calibration, how blood is drawn, chain of custody, and how we challenge a blood test.
Could another sample have affected my blood alcohol result?
It is possible, and it is checkable. Carryover is when a bit of one sample lingers in the instrument and shows up in the next, so a high-reading sample run just before yours can raise your result. Labs guard against it with blank runs that should read clean. If a blank after a high sample is not clean, nearby results, including yours, are in question.
This page is general information, not legal advice, and it does not create an attorney-client relationship. Blood testing in Florida is governed by Fla. Stat. 316.1932 and 316.1933 and the Florida Administrative Code chapter 11D-8, including Rule 11D-8.012 on sample collection. Procedures and rules change, and every case turns on its own facts. Past results do not guarantee a similar outcome.
