Could a low‑level ethyl sulfate positive result with a negative ethyl glucuronide in a patient who denies alcohol consumption represent a false positive?

Low-Level Ethyl Sulfate Positivity with Negative Ethyl Glucuronide: Likely a False Positive

Yes, this pattern strongly suggests a false positive from non-beverage alcohol exposure rather than intentional drinking, and the patient's denial should be taken seriously. The isolated low-level EtS positivity without detectable EtG is highly atypical for actual alcohol consumption and warrants investigation of alternative explanations before concluding the patient has relapsed.

Why This Pattern Suggests False Positivity

The Discordant EtG/EtS Pattern is Suspicious

When someone drinks alcohol, both EtG and EtS are typically produced together and appear in urine simultaneously. The presence of EtS without EtG is biochemically unusual for beverage alcohol consumption and should raise immediate suspicion for:

• Ingestion of products containing pre-formed EtS 1
• Cross-reactivity with other alcohol glucuronides from occupational or environmental exposure 2
• Immunoassay interference generating false signals 3

A study specifically examining non-alcoholic wine ingestion found that subjects showed positive EtS but negative EtG in urine, demonstrating superior bioavailability of orally ingested EtS compared to EtG 1. This proves that EtS can appear without EtG from certain exposures.

Common Sources of False-Positive EtS

The AAP guidelines explicitly warn that hand sanitizer, mouthwash, and cough syrup can result in low levels without actual "alcohol use" 4. Additional sources include:

• Hand sanitizers (ubiquitous in healthcare and public settings)
• Alcohol-containing mouthwashes
• Over-the-counter medications with alcohol
• Non-alcoholic beverages containing trace ethanol
• Occupational exposure to solvents 2

Recommended Approach to Resolve This Case

Step 1: Obtain Detailed Exposure History

Question the patient specifically about:

• Hand sanitizer use in the 24-48 hours before testing
• Mouthwash or oral hygiene products
• Any medications, including cough syrups
• Consumption of "non-alcoholic" beverages
• Occupational or household exposure to cleaning products or solvents

Step 2: Consider Confirmatory Testing with PEth

Phosphatidylethanol (PEth) testing is the gold standard for differentiating innocent exposure from actual drinking 5. A study of 18 subjects with positive EtG/EtS who denied drinking found that:

• 5 of 7 subjects had negative PEth results supporting their claim of abstinence
• Only 2 had positive PEth confirming actual drinking
• Negative PEth following low positive EtG/EtS helps elucidate findings and support the patient's claim 5

PEth requires significant alcohol consumption to become positive, making it ideal for this scenario.

Step 3: Repeat Testing Under Controlled Conditions

If PEth is unavailable or you need additional data:

• Repeat urine testing after documented avoidance of all potential sources
• Use first-morning specimens to ensure adequate concentration 4
• Consider testing both EtG and EtS by LC-MS/MS rather than immunoassay to avoid cross-reactivity 2, 3

Step 4: Check for Specimen Validity Issues

Ensure the specimen integrity was adequate:

• Creatinine should be 2-20 mg/mL (dilute specimens can miss substances) 4
• Temperature should have been 90-100°F within 4 minutes of collection 4
• Review for any signs of adulteration

Critical Pitfalls to Avoid

Do not automatically assume the patient is lying based solely on this discordant pattern. The AAP guidelines explicitly state that "laboratory testing is not perfect, and spurious results are possible" 4.

Avoid punitive action without confirmatory evidence. One case report documented a nurse with persistent low EtS without EtG who was ultimately found to have anatomical factors (gastric bypass surgery) contributing to the results, despite prolonged monitoring proving no intentional alcohol use 6.

Do not rely on immunoassay alone for consequential decisions. Recent data show immunoassays can have false-positive rates of 0.15-1.35% and interference rates up to 2.86% 3. Cross-reactivity with other alcohol glucuronides (methyl, propyl, butyl, hexyl) from environmental exposures can trigger positive screens 2.

When to Suspect Actual Drinking

You should reconsider the false-positive hypothesis if:

• Both EtG and EtS are positive (typical pattern for beverage alcohol)
• EtG levels exceed 1,000 ng/mL (high probability of actual consumption) 5
• PEth testing is positive (confirms significant recent drinking) 5
• Repeat testing under controlled conditions remains positive
• Patient has behavioral signs inconsistent with abstinence

Bottom Line

In this specific scenario—low-level EtS positive with negative EtG in a patient denying use—the most likely explanation is exposure to non-beverage alcohol sources rather than intentional drinking 4, 1. Order PEth testing to definitively resolve the question 5, and investigate potential exposure sources before concluding the patient has relapsed. The discordant pattern itself is evidence supporting the patient's denial.