Uses and abuses of cotinine as a marker of tobacco smoke exposure

Continine, as a biomarker of exposure to tobacco smoke, is typically measured in blood, saliva, or urine. Cotinine levels in plasma and saliva are similar but urinary levels are higher, generally by a factor of 5 to 6. Cotinine is also detected in a wide range of other fluids, including amniotic fluid and breast milk, placental tissue, umbilical cord blood, semen, cerebrospinal fluid, cervical fluid, and even serum from a testicular biopsy. Cotinine is also detectable in hair at low levels, and here nicotine is used as the marker of exposure. Cotinine is a marker of tobacco smoke exposure, which is very useful in some contexts, but far less useful in others. Care should therefore be taken to take its limitations into account.