Misclassification of smoking habits: An updated review of the literature

Table 1 Distribution of study characteristics among the 294 detailed results

Factor	Level	No. of results analyseda
Body fluid	Urine	78a
	Saliva	90
	Blood	126
Assay method	Chromatography	93
	Spectrometry	72
	Immunoassay	108
	Other	21a
Age groupb	Young	103
	Not young	35
	All ages	108a
	Not stated	48
Study type	General population	189a
	Pregnancy	57
	Diseased or case-control	48
Awareness of validation by cotinine	Yes	22a
	No	47
	Not specified	225
Time of publication	Studies considered in the 1995 review	31
	Studies reported before 2003	109
	Studies reported later	154a
Study quality	Good	36
	Not good	258a
Pregnancy (women only)	Not pregnant	61
	Pregnant	57
Tobacco products considered	Cigarettes	108
	Any smoking	160
	Any tobacco	26a
Sex	Females	118
	Males	48
	Combined	128a
Location	Canada/United States	115a
	Europe	128
	Asia	25
	Other	26

^aFactor levels with this superscript are the levels applicable to the very large study by Palmier J, Lanzrath B, Dixon A and Idowu O[26] considered separately in our analyses.

^bStudies varied in how they reported the age range studied, sometimes giving a specific range of ages, sometimes a mean age and sometimes no information.

The categories were based on the available age information as follows: Young: Upper age limit < 50 or mean age < 30 or a pregnancy study; Not young: Lower age limit 30+ or mean age 60+, thus excluding young people; All ages: Lower age limit < 30 and upper age limit 50+ or lower age limit < 30 and mean age 30+; Not stated: All other combinations.