A validation and calibration process on self-reported tobacco with participants: Cotinine levels in building blocks

BACKGROUND: Building Blocks was a pragmatic randomised controlled trial assessing the effectiveness of giving the Family Nurse Partnership (FNP) home-visiting programme to teenage first-time mothers on infant and maternal outcomes up to 24 months after birth (Robling et al., 2016). One of the primary outcomes was to investigate the effectiveness of the intervention in reducing smoking during pregnancy. At baseline and late pregnancy, we collected a large amount of selfreported data on smoking habits from each participant during a face-to-face and telephone interview respectively. It is well known that self-reported smoking can be inaccurate and therefore some participants are likely to report smoking fewer or more cigarettes than they actually do. In the Building Blocks study, we collected urine samples at the same time as the baseline interview and at follow-up in late pregnancy. The cotinine levels within the urine sample were used to supplement the participants’ self-reported behaviour and then further calibrate their number of cigarettes smoked per day (Dukic et al., 2007). However, this calibration approach requires complete and well-synchronized collection of self-reports and urine samples. The main challenge of our study lies in the collection of urine samples, particularly at follow-up stage. Some urine samples were collected at different time points from their interview and some were missing for a variety of reasons, which cause incompleteness in participant’s data and potentially lead to bias in the results. METHODS: We tackled these issues using a validation and calibration process. Firstly, participants were divided into three categories according to their completeness of these outcomes. Secondly, time gaps between the urine sample and self-report dates were assessed over different thresholds. Thirdly, we examined the feasibility of inferring participants’ reporting behaviours at follow-up stage by their baseline outcomes. RESULTS: 870 participants with different levels of non-contemporary outcomes collection at follow-up stage were sub-grouped and investigated over their consistency in reporting behaviours. We further validated 222 participants with incomplete data at follow-up stage and calibrated their self-reported tobacco accordingly, which strengthened the power for the main analysis. DISCUSSION: It is not rare that difficulties arises when collecting data at follow-up stages, especially in populations that may be vulnerable and often mobile as in this study. Rather than losing those participants for key analyses, this proposed process could further validate and calibrate self-reported tobacco of participants for public health studies with similar settings. Because of the costings and challenges in urine sample collections, investigating the participants’ reporting behaviours by some associated factors, such as social and demographic factors, has become one of our follow-up research topics.