The hour before the draw

Leave the cigarette. Nicotine has a metabolic effect of its own, and a fasting period is meant to exclude anything that stirs the metabolism, not only food. Vaping falls under the same rule for the same reason: the nicotine is the point, not the smoke.

This is the small part of the answer, and it is easy to follow. The larger part is about what your report will show regardless.

What regular smoking does to a blood count

This is the part that surprises people when they read their own results. Comparing smokers with non-smokers, studies of healthy adults find measurably higher white cell counts, higher haemoglobin, higher haematocrit and a larger mean corpuscular volume among smokers.

The effect scales with how much someone smokes. In a study grouping people by smoking intensity, heavy smokers showed higher red cell and white cell counts, with a specific rise in neutrophils, alongside a lipid profile shifted in the unfavourable direction: higher total cholesterol, triglycerides and LDL, with lower HDL.

So a smoker can arrive with a white cell count flagged high and no infection anywhere. The flag is accurate, the alarm is not. Your doctor reads that line knowing you smoke, which is one of several reasons the laboratory should know too.

What to tell the laboratory, and your doctor

Say that you smoke, and roughly how much. It is not a confession, it is context that changes how a line is read. A white cell count of 11 means something different in a twenty-a-day smoker than in someone who has never smoked.

The same applies to a report you are reading yourself. A flag is a comparison against a range built mostly from people who were not doing what you are doing.

And if you are stopping

Several of these values drift back over months, not days, because the cells themselves are replaced slowly. If a panel is following your progress, the useful comparison is your own previous report rather than the population range printed beside it.

That also means there is nothing to gain from skipping cigarettes for two days to make a result look better. The hour before the draw is worth respecting; the two days are not a strategy.