Biochemical measurement of cotinine was performed at Bevital A/S, Bergen, Norway. Cotinine levels were measured in serum by liquid chromatography-tandem mass spectrometry(23). For the Swedish cohort cotinine levels were measured in plasma. The laboratory coefficients of variations were 2.0 – 3.0% for repeated analyses within the same day, and were approximatelly 6% between-days. Cases and controls were analyzed in random order and laboratory personnel were blinded to case-control status.
