2026 UK Comparison: CO Breath Monitors vs Saliva Cotinine Home Tests — Which Is Best for Tracking Your Switch from Smoking to Vaping?

Introduction

If you're switching from smoking to vaping, tracking progress can help keep motivation high and confirm reduced harm. Two common biomarkers used for verification are exhaled carbon monoxide (CO) and cotinine (a nicotine metabolite) in saliva or urine. Both are widely available to UK consumers, inexpensive and used in clinical trials — but they measure different things, work on different timescales, and suit different goals. This comparison explains how each test works, their strengths and limitations, and which is best depending on what you want to measure.

Feature-by-feature comparison

What each test measures

CO breath monitors measure exhaled carbon monoxide, a gas produced by combustion. A raised CO reading is a direct indicator of recent smoking (ie, inhaling smoke from lit tobacco) and does not rise with nicotine from vaping or nicotine replacement therapy.

Saliva cotinine tests measure cotinine, the main metabolite of nicotine. Cotinine reflects nicotine exposure from any source — smoking, vaping, or NRT — and therefore indicates recent nicotine use rather than whether combustion occurred.

Time window and speed

• CO breath: near-instant readings (around 15 seconds) and most indicative of very recent smoking — minutes to a few hours. Common research cutoffs use ≥3–5 ppm to flag recent smoking.
• Saliva cotinine: detects nicotine exposure over a longer window: saliva typically up to ~48–96 hours (urine ~4 days, blood up to ~10 days). Tests generally take longer to process (reports of ~30 minutes) and require a saliva or urine sample.

Accuracy and clinical performance

Both biomarkers perform strongly in clinical settings. Exhaled CO has reported AUCs (area under the ROC curve) around 0.90–0.99 for detecting recent smoking, indicating excellent discrimination for very recent smoking. Salivary cotinine shows AUCs roughly 0.96–0.98, making it extremely sensitive and specific for nicotine exposure when appropriate cutoffs are used.

Source specificity

• CO: specific to combustion — a positive CO points to recent smoked tobacco.
• Cotinine: cannot distinguish source — a positive result flags nicotine from smoking, vaping, or NRT.

Practicality, cost and availability

Consumer CO monitors and at‑home cotinine kits are readily available across the UK. At-home cotinine kits commonly retail around £6–£8, making self-monitoring accessible. CO testing is fast and noninvasive — ideal for immediate behavioural feedback — while saliva cotinine requires producing a saliva or urine sample and waiting longer for a result.

Interpretation and cutoffs

Correct cutoff selection is essential to avoid misclassification. Typical thresholds are:

• CO: ~3–5 ppm to indicate recent smoking.
• Saliva cotinine: ~10–20 ng/ml (urine: 50–200 ng/ml) depending on the context and desired sensitivity.

Using the wrong threshold can mistakenly classify light vaping or recent abstinence as ongoing smoking or vice versa.

Pros and cons

CO breath monitors

• Pros: Rapid (≈15 seconds), noninvasive, immediate feedback; specific to combustion so excellent for confirming recent smoking lapses; portable and easy to repeat throughout the day.
• Cons: Short detection window (minutes–hours) — may miss smoking episodes that occurred more than a few hours earlier; readings can be influenced by environmental CO (eg, traffic fumes) though modern devices reduce this risk; doesn’t measure nicotine exposure from vaping or NRT.

Saliva cotinine tests

• Pros: Detects nicotine exposure over days, highly sensitive and specific for nicotine at suitable cutoffs (AUCs ~0.96–0.98); useful to confirm nicotine abstinence (or ongoing nicotine use) over a longer window.
• Cons: Slower (about ~30 minutes to process), requires a biological sample (saliva/urine) which some find awkward, and cannot tell whether nicotine came from smoking, vaping, or NRT.

How researchers and quit‑services use them together

Because the two biomarkers are complementary, many clinical trials and cessation studies use both: CO for immediate verification of smoked tobacco abstinence and cotinine to confirm recent nicotine exposure over days. Combining tests reduces the chance of false negatives or misinterpretation and gives a fuller picture of behaviour during a quit attempt or switch to vaping.

Which test is best for your goals?

There isn’t a single “best” test for everyone — choose depending on what you want to monitor:

• If your goal is to confirm you’ve stopped smoking combustible cigarettes: a CO breath monitor is ideal. It shows near‑instant changes after even one cigarette and directly reflects combustion exposure. Use repeated daily checks or after times you feel tempted to smoke for immediate feedback.
• If your goal is to track overall nicotine exposure during the switch: saliva cotinine is better because it captures nicotine intake over several days. This is helpful if you’re tapering nicotine strength in e‑liquids or using NRT alongside vaping and you want to know total nicotine exposure.
• If you want robust verification (eg, for research or accountability): use both — CO to detect recent smoked lapses and cotinine to measure cumulative nicotine use.

Practical recommendation and product suggestions

For most people switching from smoking to vaping, a pragmatic approach is:

• Use a consumer CO monitor for quick, immediate checks after cravings or social situations where smoking might occur.
• If you’re adjusting nicotine strength or worried about total nicotine intake, test saliva cotinine every few days to confirm exposure trends.

If you’re using e‑liquids as part of your switch, consider nicotine salt longfills or single‑serving nicotine options to fine‑tune nicotine delivery. For example, the 0mg Crystalize Bar Salts 120ml Longfill (with nicotine shots) and the 0mg Crystalize Bar Salts 60ml Longfill (with nicotine shots) make it easy to mix in nicotine shots to match the strength you need, while the 0.5mg Tick Tock Nicotine Candy 12 Drops can help if you prefer an oral nicotine option for short cravings.

Conclusion

CO breath monitors and saliva cotinine tests each have distinct strengths. CO is the best rapid, specific indicator of very recent smoked tobacco use and gives immediate behavioural feedback; saliva cotinine provides a longer look at nicotine exposure but cannot tell whether nicotine came from cigarettes, vape devices or NRT. For most people making the switch to vaping, using CO for daily immediate checks and cotinine occasionally for confirmation of nicotine intake offers the most informative, practical strategy. Choose the test — or combination of tests — that matches what you want to measure, and interpret results with appropriate cutoffs to avoid misclassification.