The short answer: screening is not the same as confirmation
An EtG immunoassay uses antibody-based recognition to estimate whether urinary ethyl glucuronide is above a selected threshold. It is fast, scalable, and practical when a laboratory processes many specimens. LC-MS/MS separates compounds by liquid chromatography and then identifies EtG through characteristic mass transitions. That added analytical selectivity makes LC-MS/MS useful for confirmation and quantitative reporting. Neither method, by itself, reveals an exact drinking time or drink count.
The distinction matters most when a result affects treatment, employment, professional monitoring, supervision, or litigation. A screening result can be appropriate for an initial decision pathway, but a consequential interpretation should follow the laboratory's confirmation policy. Readers should ask whether the reported number came from an immunoassay, LC-MS/MS, or a screen followed by confirmation rather than treating every EtG result as analytically identical.
Related: How an EtG urine test works · How to read an EtG report
How an EtG immunoassay works
An immunoassay combines a urine specimen with reagents designed to respond to ethyl glucuronide. The instrument compares the signal with calibrators and applies a decision cutoff, commonly expressed in nanograms per millilitre. The output may be a qualitative positive or negative classification, a semi-quantitative value, or a numerical result used within the assay's validated range. Performance depends on calibration, reagent lot, quality controls, instrument settings, and the cutoff selected by the laboratory.
Antibody recognition is efficient but not perfectly equivalent to chemically separating the analyte. Structurally related compounds, matrix effects, medication metabolites, or assay-specific cross-reactivity can sometimes affect a screen. That does not make immunoassay unreliable; it explains why screening and confirmation have distinct roles. A validated assay can perform well in routine use, yet an unexpected or disputed positive may deserve a more specific method rather than a confident conclusion based only on the screening label.
How LC-MS/MS identifies and measures EtG
Liquid chromatography–tandem mass spectrometry begins by separating EtG from other components in the prepared specimen. The mass spectrometer then monitors selected precursor and product ions associated with EtG, usually alongside an internal standard. Laboratories validate retention time, calibration range, precision, accuracy, recovery, interference, limit of detection, and limit of quantification. This multi-part identification process provides greater analytical specificity than antibody recognition alone.
LC-MS/MS can measure EtG and ethyl sulfate in the same workflow, depending on the method. A quantitative result still requires interpretation: 600 ng/mL at one collection time is not a universal signature for a particular beverage amount. Urine volume, creatinine, elapsed time, repeated drinking, renal handling, and the laboratory's reporting policy can all influence the meaning. Analytical precision should not be confused with biological certainty.
Related: EtG and EtS compared · Quantitative EtG levels
What comparative studies show
Böttcher and colleagues compared a urinary EtG enzyme immunoassay with an established liquid chromatography–mass spectrometry method across routine specimens. The methods showed strong overall agreement under the study conditions. That finding supports immunoassay as a useful screening approach, while the authors also discussed the importance of an appropriate cutoff where unintentional ethanol exposure is a concern.
A later comparison in a pain-management population examined immunoassay-positive samples using LC-MS/MS at several cutoffs and also measured EtS. A portion of positive screens did not confirm under the compared conditions. The result is not a universal failure rate for every assay or laboratory; it demonstrates why population, cutoff, confirmation criteria, and method-specific performance must stay attached to any percentage quoted from a study.
Cutoffs can create disagreement without an instrument error
A sample can contain measurable EtG and still be classified differently at 100, 200, or 500 ng/mL. If an immunoassay screen uses 500 ng/mL while an LC-MS/MS report lists a lower concentration, the apparent disagreement may reflect reporting rules rather than analytical contradiction. Conversely, a screen near its cutoff may be confirmed below a program's required threshold. Always compare like with like: analyte, unit, cutoff, and collection.
Lower cutoffs generally increase sensitivity to lighter or older exposure, while higher cutoffs increase specificity and reduce some low-level incidental-exposure concerns. This tradeoff is one reason the main EtG calculator displays multiple cutoff ranges instead of a single countdown. A method name without its cutoff is incomplete information, just as a cutoff without the specimen and collection time is incomplete.
Related: 100, 200, and 500 ng/mL cutoffs · 100 vs 500 cutoff evidence
Cross-reactivity, interference, and unexpected results
An unexpected positive can have several explanations: genuine beverage exposure, ethanol from a non-beverage source, immunoassay cross-reactivity, sample handling, or an interpretation that ignores the numerical level and context. The phrase false positive is therefore too broad unless the analytical and exposure questions are separated. Confirmation helps answer whether EtG itself is present; it does not automatically determine where the ethanol came from.
When a result is unexpected, document alcohol-containing medicines, mouthwash, sanitizer exposure, occupational contact, fermented products, and collection timing without assuming that one source explains the result. Request the numerical EtG value, the screening cutoff, the confirmation method, EtS if available, and urine creatinine. A specific record is more useful than speculation and allows the responsible professional to compare the report with the governing policy.
Creatinine and sample validity still matter
Both immunoassay and LC-MS/MS report concentration per volume of urine. Hydration and time since the last void can therefore change the measured ng/mL value even when the total amount excreted is similar. Laboratories often measure creatinine or other validity indicators to identify unusually dilute specimens. Some programs recollect a dilute negative; others apply a defined policy or consider creatinine-normalized EtG.
Water does not transform an immunoassay into a negative test or accelerate metabolism in a predictable way. It can lower urine concentration and may create a report that is dilute or unsuitable. A complete interpretation should place the EtG result beside creatinine, specimen-validity findings, and the collection schedule. This remains true even when LC-MS/MS has measured EtG with excellent analytical accuracy.
Related: EtG, creatinine, and urine dilution
A practical EtG laboratory-report checklist
Start with the analyte: confirm that the report concerns EtG, EtS, or both. Next identify the specimen, units, numerical concentration, and cutoff. Look for language such as screen, presumptive, confirmed, quantitative, LC-MS/MS, or mass spectrometry. Review creatinine and sample-validity information. Finally, calculate the collection interval from the end of drinking rather than relying on a vague calendar-day estimate.
If the result has meaningful consequences, ask whether confirmation is required by policy and whether the laboratory can provide method documentation. Do not use an online EtG calculator to overrule an actual report. The calculator can organize dose, cutoff, and elapsed-time concepts, but laboratory interpretation belongs to the qualified parties responsible for the test and the decision.
- Analyte and specimen
- Screening or confirmatory method
- Numerical value, unit, and cutoff
- Creatinine and specimen validity
- EtS result when measured
- Collection time and governing policy
Related: Calculate exact elapsed hours · EtG testing FAQ
Frequently asked questions
Is LC-MS/MS always required after an EtG immunoassay?+
Not in every setting. Requirements depend on the laboratory and program. Confirmation is especially important when a presumptive result is unexpected, disputed, near a decision threshold, or likely to produce significant consequences.
Can an EtG immunoassay give a numerical result?+
Some systems produce a numerical or semi-quantitative value, but the result remains method-specific. A number from an immunoassay should not automatically be treated as equivalent to a confirmed LC-MS/MS concentration.
Why can immunoassay and LC-MS/MS results disagree?+
Differences can arise from cutoff rules, calibration, analytical specificity, cross-reactivity, reporting limits, specimen preparation, or timing. Compare the methods and thresholds before concluding that either result is wrong.
Does LC-MS/MS prove beverage alcohol use?+
LC-MS/MS can provide strong analytical evidence that EtG is present. It does not by itself establish the source of ethanol, exact dose, exact time, or impairment.
What should I request after an unexpected screen?+
Request the numerical value, screen cutoff, confirmation result and cutoff, EtS if measured, creatinine, specimen-validity findings, collection time, and the written policy used to interpret the result.
Evidence and limitations
This guide summarizes primary research and public-health guidance. Group-level findings cannot predict one person's result.