Nicotine and Cotinine

• When nicotine is absorbed into the body, it breaks down into cotinine. A blood test for nicotine will usually include a measurement of cotinine levels.
  
  

Half-Life

• Nicotine has a half-life of roughly two hours, meaning that half the nicotine in the body converts to cotinine every two hours. Cotinine's half-life is roughly twenty hours.
  
  

Time Frame

• Because nicotine is metabolized so quickly, a blood test will only detect trace amounts after 24 to 72 hours. Cotinine may take up to 10 days to drop to levels undetectable by blood test.
  
  

False Negatives

• Blood test results may fail to detect nicotine if the blood sample more than 24 hours after the person last smoked or chewed tobacco.
  
  

False Positives

• People with certain types of liver defect may show nicotine and cotinine in their blood weeks or months after quitting.