Menza et al. BMC Public Health 2010, 10:774 http://www.biomedcentral.com/1471-2458/10/774 Page 5 of 13 222 assessed for eligibility 127 underwent baseline assessment 127 randomized 95 excluded 81 did not meet behavioral eligibility 3 planned to move within 6 months 11 eligible but refused to participate 70 assigned to contingency management 57 assigned to control Follow-up visits: 6 weeks: 48/70 (69%) 12 weeks: 56/70 (80%) 18 weeks: 53/70 (76%) 24 weeks: 60/70 (86%) Follow-up visits: 6 weeks: 44/57 (77%) 12 weeks: 46/57 (81%) 18 weeks: 46/57 (81%) 24 weeks: 47/57 (82%) 70 included in analysis 57 included in analysis Figure 1 Participant flow through the study. methamphetamine use for <10 years (81% v. 94%, P = 0.05). The largest difference in follow-up between the CM and control group was for the first visit after baseline (69% v. 77%). Of the 22 CM participants loss-to-follow- up at this visit, one was incarcerated, one was in a drug rehabilitation program, three withdrew from the study, and 17 could not be located for study visit attendance. Among the 13 control group participants who did not complete the first follow-up visit, one had died, two withdrew from the study, and 10 could not be located for study visit attendance. Table 1 presents characteristics of the study popula- tion. The only statistically significant difference between the study populations at baseline was that between the distributions of race/ethnicity (P = 0.03). CM intervention adherence Table 2 provides metrics of intervention adherence among CM participants. Participants who earned < $15,000 per year attended more visits than participants who earned ≥$15,000 per year (41% v. 20%, P = 0.04). Participants who reported using methamphetamine weekly or daily at baseline attended fewer visits than participants who reported less frequent use (31% v. 51%, P = 0.02) and participants whose urine contained methamphetamine at baseline attended fewer urine test- ing visits than participants whose urine did not contain methamphetamine (26% v. 43%, P = 0.02). Since the mean detection time of the methamphetamine assay is shorter than the interval between urine testing visits, the proportion of metabolite-free urine samples provided by CM participants during the intervention visits may be over-estimated. Sexual risk At baseline, 6-weeks, 12-weeks, 18-weeks, and 24-weeks, the adjusted probabilities of reporting non-concordant UAI in the prior six weeks were 31.0% (95%CI: 27.6%, 34.3%), 19.7% (95%CI: 16.3%, 23.1%), 12.1% (95%CI: 9.9%, 14.4%), 9.1% (95%CI: 7.2%, 10.9%) and 10.2% (95% CI: 8.3%, 12.1%), respectively. Figure 2 presents the adjusted proportion of men reporting non-concordant UAI at each study visit by study arm. Comparing partici- pants at study visits six weeks apart, participants at the later study visit were less likely to report non-concordant UAI in both the CM (aRR = 0.80; 95%CI: 0.64, 0.99) and control (aRR = 0.84; 95%CI: 0.71, 1.00) arms than partici- pants at the earlier study visit. CM and control participants were similarly likely to report non-concordant UAI during the 12-week inter- vention period. CM participants were less likely than control participants to report non-concordant UAI dur- ing the subsequent 12-week follow-up period, and also reported fewer non-concordant UAI partners than con- trol participants during both the intervention and fol- low-up periods. None of these differences were statistically significant (Table 3). One man in CM arm and one man in the control arm acquired pharyngeal gonoccocal infection (3.4 v. 4.5 per 100 person-years; P = 0.43). There were no incident syphilis, rectal gonococcal, or rectal chlamydial infec- tions. Among the HIV-negative participants, one man in the CM arm and no men in the control arm acquired HIV (8.9 v. 0 per 100 person-years; P = 0.27). Stimulant use During the 24 weeks of follow-up at the 6-week study visits attended by both CM and control participants, 28 (49%) control participants and 44 (63%) CM participants submitted ≥1 sample containing methamphetamine metabolites; 26 (46%) control participants and 23 (33%) CM participants submitted ≥1 sample containing cocaine metabolites; and, 43 (75%) control participants and 55 (79%) CM participants submitted ≥1 sample containing methamphetamine or cocaine metabolites. CM and control participants were comparably likely to submit urine samples positive for methamphetamine at study visits during the intervention period (Table 4); however, during the follow-up period, CM participants were somewhat more likely to submit a urine sample containing methamphetamine (P = 0.11).