1. Time of day that the test is performed

The best time to do a test is usually the afternoon, as CO levels fall over-night and morning readings can sometimes give misleadingly low results. High readings in the morning, on the other hand, would be strong evidence of heavy inhalation and high nicotine dependence.

2. Type of tobacco used

Different tobacco types yield differing amounts of carbon monoxide and this can lead to higher or lower test results in extreme cases. Cigar and pipe smokers can be classified as ‘Primary’ (those who have never smoked cigarettes) or ‘Secondary’ (those who have switched from cigarettes to pipes and/or cigars). Secondary cigar and pipe smokers may have higher test results because they have continued the inhalation pattern learnt while smoking cigarettes.

3. Daily consumption

Differences in daily consumption account for the major differences between the results of different smokers. As a general rule we estimate that 20 a day smokers are ‘heavier’ smokers than those on 10 a day but the CO test can reveal significant differences between smokers who smoke the same quantity of cigarettes per day. For example: a 15 per day smoker may achieve a test reading far in excess of that obtained by a 30 a day smoker, simply because they inhale more deeply and extract more from each cigarette smoked. Attempts to cut down often cause smokers to get more out of the cigarettes available to them. They will regulate their own nicotine intake (often quite unconsciously) by taking bigger puffs, puffing more frequently and smoking further down the butt.

4. Number smoked so far on test day

Depending on the time of day at which the test is being conducted, the smoker may have smoked a lot or a little. This will be influenced by their normal daily routine, restrictions at work etc. It is a good idea to ask the smoker how representative the test day is of their normal smoking pattern.

5. Time since last smoke

The half-life of carboxyhaemoglobin during sleep is about 5 hours. Physical exercise speeds up the elimination process. If the smoker has not smoked for several hours prior to the test then it may help to give him/her a reading before smoking a cigarette and then another 10 minutes afterwards.

6. Occupation

Some workers are exposed to CO in the course of their working day and this may affect their readings. It is as well to check what they do for a living so that workplace CO levels can be taken into account if relevant. Workers exposed to large numbers of vehicle emissions are particularly at risk of having abnormally high CO levels, irrespective of their smoking habits.²

7. Passive smoking

Whilst passive smoking is an important health issue, passive exposure would need to be extremely heavy to alter a person’s CO reading by more than two or three parts per million. Thus it is not a highly significant factor when interpreting results unless the exposure has been excessive. In most cases it will not be possible to attribute slight variations between non-smokers to passive smoking.3 Air pollution can affect readings slightly, so it is as well to check the ambient (atmospheric) CO level before testing individuals so that unusually high background levels can be allowed for if necessary.

8. Results & smoking status

The standard cut-off point for determining a person’s smoking status on a given day is 10ppm. Readings lower than 10 indicate a non-smoking status and readings of 10 or above indicate that smoking has probably taken place in the preceding 12 to 24 hours. In some cases the individual being tested may get a reading of 10 or higher and yet be adamant that they have not smoked that day. If further investigation cannot reveal a possible reason for this then it is as well not to challenge the individual but to repeat the test on another occasion. Most people will be honest and open about any lapses but if a client cannot bring him or herself to be totally frank, then it serves no useful purpose to force the issue. Consistently high readings above 10pp in a self-reported non-smoker may indicate high levels in the home, which should be investigated.

The Psychological Benefits of CO Testing

The results of an initial test may sound alarming to many smokers but within just a couple of days of stopping CO levels drop right down to normal and it’s very encouraging for them to see the difference after working their way through the first few days without cigarettes. CO monitoring is therefore a powerful motivational tool. Taking the test before patients stop smoking will help you to gauge their level of nicotine dependence, and taking it after they’ve stopped will give them real proof that what they’re doing is working.

Nicola Willis, smoking cessation specialist and author of 'The Three Step Quit Smoking System' published by Bedfont.