This study found that in the 2006 sample of NSP Survey respondents, older heroin users were not preferentially recruited relative to younger methamphetamine users. NSP clients who reported their last drug injected as methamphetamine were, in fact, more likely to complete the Survey at first NSP visit during Survey week than were heroin injectors. NSP clients who last injected methadone/buprenorphine/buprenorphine-nalaxone combination were most likely to participate in the Survey at first visit, followed by those who last injected morphine. Cocaine injectors were the least likely of all NSP clients to participate. Although these results cannot preclude the possibility that heroin users were preferentially recruited to the Survey in previous years, these data clearly indicate that this was not the case in 2006.
Although some NSP sites may have increased their efforts to recruit younger nonopiate users in 2006, this possibility is not consistent with findings that IDUs aged 35 years or older were more likely than their younger counterparts to participate. It is, of course, important that older IDUs are not overrepresented in surveillance research, because duration of injecting, which is highly correlated with chronologic age,10,27,28 is strongly associated with HCV seropositivity.29-32 Thus, HCV prevalence could be overestimated if older IDUs are preferentially recruited. Higher background prevalence may be associated with a “ceiling” effect, leading to difficulty in detecting trends over time in prevalence estimates. Conversely, a range of research, including prospective cohort studies of HCV incidence, suggest that new initiates to injecting may be particularly vulnerable to HCV infection and that the risk of seroconversion declines with age and duration of injecting.33-38
Compared with men, women were more likely to participate in the 2006 survey. This pattern was documented in the original NSP Survey implementation research8 and is likely to have been the case throughout the existence of the Survey. Research in other areas has also indicated that women are more likely to comply with requests to participate in health-related research,39 a pattern that may be explained by the generally more relational, expressive, and communal nature of women than men40 and the demands of conventional gender roles, in which women are perceived as more compliant and responsive to the needs of others than are men (compare articles by Crawford and Unger41 and Deutsch42). The potential that women may be preferentially recruited into surveillance studies suggests that recruitment strategies to target men, and stratification of main outcomes by gender in future analyses, may usefully be considered.
This study failed to support the hypothesis that the Australian NSP Survey continually accesses an ageing cohort of repeat participants who unduly influence the results. The proportion of repeat participants has remained relatively low and stable during the past 5 years, at approximately 20% of all Survey respondents. Although the median age of Australian NSP Survey participants has increased annually since 2002, the findings presented here indicate that this has occurred across all Australian NSP Survey participants rather than being confined to repeat participants, suggesting that the average age of NSP clients may have also increased over this period. This is consistent with other research, which has identified an increase in the median age of regular heroin users and a possible reduction in initiation to heroin injecting.43 It has been suggested that younger less entrenched heroin users may have dropped out of the heroin market altogether after Australia's 2001 heroin shortage.26 Indeed, since the heroin shortage, the size of the overall population of IDUs in Australia is estimated to have decreased.44,45
Increased use of drugs by means of noninjecting routes of administration (eg, smoking of crystalline methamphetamine46) may further explain this increase in the average age of Australian IDUs, which has also been detected by the Illicit Drug Reporting System (IDRS), Australia's strategic early warning system.47,48 This annual survey has also observed an increase in the average age of samples of IDUs recruited from a range of sites across Australia, from 30.1 years in 2002 to 34.1 years in 2005.49 Indeed, recent comparisons demonstrate striking similarities between the 2006 NSP Survey (N = 1961) and IDRS (N = 914) samples,50 with equivalent mean ages (34.8 vs. 34.5 years); proportions of men (65% vs. 64%), heterosexuals (82% vs. 86%), Indigenous Australians (10% vs. 13%), and participants with a history of incarceration (48% vs. 51%); daily or more frequent injection (47% vs. 46%); and an injecting career of 3 or more years (91% vs. 98%). Similar to the present study, Judd et al6 observed increasing median age among participants recruited for their serial cross-sectional point prevalence HIV surveys from community-based and treatment settings in Greater London but argued this was unlikely to be attributable to an ageing cohort because they observed a similar trend among opiate injectors presenting to treatment services in North London during the same period.
A limitation of this study, and the NSP Survey itself, is the lack of information that is collected from NSP clients who decline to participate, particularly in terms of their risk behaviors. Although the Survey is designed to place as few demands as possible on respondents, who are not reimbursed for their participation, it does involve providing a capillary blood sample and answering a series of questions relating to illicit and socially stigmatized activities; only approximately one third to one half of clients who attend NSPs during Survey week agree to participate.11 Nonetheless, the addition to Response Sheets (which collect information on all clients during Survey week, including nonrespondents) in 2006 of “drug last injected” was a significant improvement on the procedure in previous years, allowing a better delineation of population parameters and, consequently, a greater capacity to address issues around sample representativeness.
Notwithstanding some (inevitable) bias, large and broad cross-sectional samples of IDUs are recruited each year through this system. Our results thus indicate that, at least in the context of relatively widespread, legal, publicly funded NSPs, such as exist in the United Kingdom, Europe, and, more recently, Canada, such sites can facilitate indispensable surveillance. As demonstrated by our data and elsewhere,6 ageing cohorts need not be interpreted as artifacts of specific methodologies; serial cross-sectional studies can, in fact, detect such phenomena. This study also illustrates the need to consider the issue of gender closely in surveillance with IDUs. Although women are understood to be oversampled in health studies in general, the disproportion of women among IDUs adds to the complexity of assessing the representation of women in research with this population.
These results demonstrate that the concern regarding the Australian NSP Survey preferentially recruiting heroin injectors relative to methamphetamine injectors was not warranted, at least in 2006, when, in fact, methamphetamine injectors were more likely than heroin injectors to complete the Survey. The results also indicate that most participants in the Survey are new to the survey each year and that repeat participants are unlikely to exert an undue influence on the Survey's results. The annual increase in age of NSP Survey participants is not attributable to the increasing age of repeat participants but likely reflects the increasing age of the broader NSP population and, when considered in the light of data drawn from other illicit drug surveillance systems, probably the population of IDUs as a whole. Our findings suggest that inferences derived from the results of these Surveys can reasonably be applied to the population of NSP clients, although because older female pharmaceutic injectors may be overrepresented among NSP Survey participants, recruitment strategies to target specific subpopulations (younger men) and stratification of main outcomes by age and gender in future analyses may usefully be considered. Nonetheless, although the extent to which NSP Survey results can be generalized to Australia's population of IDUs as a whole cannot be ascertained, given the “partially hidden” nature of illicit drug injectors,3 we conceive of the NSP Survey samples in the same manner in which Hope et al4 described their samples of IDUs recruited from drug treatment and nontreatment community settings: a “sample (that) is likely to be as representative a sample of injectors as it is practical to obtain.”
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Collaboration of Australian Needle and Syringe Programs: Directions Australian Capital Territory (ACT); AIDS Council of NSW (Sydney and Hunter); Albury Base Hospital and Albury Community Health Center, Albury; First Step Program, Port Kembla; Health ConneXions, Harm Reduction Program, Liverpool; Hunter NSP Services, Royal Newcastle Hospital, Newcastle; Indo-Chinese Outreach Network (ICON), Bankstown, Cabramatta, and Liverpool; Kirketon Road Center and K2, Kings Cross; NSW Users and AIDS Association (NUAA), Surry Hills; Northern Rivers Area Health Service, Ballina, Byron Bay, Lismore, Murwillumbah, Nimbin, and Tweed Heads; Resource and Education Program for IDUs, Redfern and Canterbury; Responsive User Services in Health (RUSH), Manly, Ryde, and St. Leonard's; St. George NSP, Kogarah; Southcourt Primary Care NSP, Nepean; Sydney Sexual Health Center, Sydney; Sydney West Area Health Service HIV/Hepatitis C Prevention Service, Auburn, Blacktown, Merrylands, Mt. Druitt, and Parramatta; Northern Territory AIDS Council, Alice Springs, Darwin, and Palmerston; Biala Community Alcohol and Drug Services, Brisbane; Cairns Base Hospital NSP, Cairns; Cairns Youthlink, Cairns; Logan Youth Heath Services, Logan; Mackay Sexual Health Services, Mackay; Queensland Injectors Health Network (QUIHN), Brisbane, Gold Coast, and Sunshine Coast; Kobi House, Toowoomba; West Moreton Sexual Health Service, Ipswich; Drug and Alcohol Services South Australia, Adelaide; Hindmarsh Center, Hindmarsh; Nunkuwarrin Yunti Community Health Center, Adelaide; South Australia Voice for Intravenous Education (SAVIVE); AIDS Council South Australia, Norwood; Parks Community Health Service, Adelaide; Port Adelaide Community Health Service, Port Adelaide; Noarlunga Community Health Service, Adelaide; Northern Metropolitan Community Health Service NSP and Shopfront, Salisbury; Devonport Community Health Center, Devonport; Launceston Sexual Health, Launceston; Tasmanian Council on AIDS, Hepatitis and Related Diseases (TasCAHRD), Hobart and Glenorchy; The Link Youth Health Service, Hobart; Health Works, Footscray; Melbourne Inner Needle Exchange, Collingwood; South East Alcohol and Drug Service, Dandenong; St. Kilda NSP; Southern Hepatitis/HIV/AIDS Resource and Prevention Service (SHARPS), Melbourne; WA AIDS Council Mobile Exchange, Perth; Western Australia Substance Users Association (WASUA), Northbridge and Bunbury.