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Abstract

Credible estimates of the number of dependent heroin users are needed to plan appropriate public policy responses to the consequences of dependent heroin use.² These consequences, which have negative effects on the community and public health,³ include bloodborne virus transmission,⁴ premature death from overdose,⁵ and crime.⁶ The need for credible estimates is especially pertinent now, because an increased rate of fatal overdoses over the past five years⁴ has prompted claims in the popular media that there has been a substantial increase in heroin use in Australia in recent years.

Household surveys of drug use are not well suited to estimating the prevalence of dependent heroin use, for several reasons. First, heroin use is likely to be under-reported in household surveys because it is an illegal and stigmatised behaviour.⁷ Second, dependent heroin users' lifestyles also make them less likely to live in conventional households and less likely to participate in household surveys, because of either their unavailability at the time the interviewer calls or their reluctance to be interviewed.⁸ Third, household surveys may underestimate the number of dependent heroin users in the population, because heroin users are heavily concentrated in a small number of geographic areas where heroin is most readily available.⁸ When these areas are represented in national samples in proportion to the contribution to the total population, the estimated population rates of heroin use are usually underestimated in national surveys.⁹

There is no widely accepted "gold standard" method for estimating the size of the "hidden population" of dependent heroin users. The most widely used approach has been to look for a convergence of estimates derived by applying a number of different indirect methods of estimation to data on the consequences of dependent heroin use, such as overdose death and persons seeking treatment.¹⁰ We used this strategy to estimate the number of dependent heroin users in Australia.

Our aims were:

• to estimate the number of dependent heroin users in Australia; and

• to compare the estimated population prevalence in Australia with that in comparable European societies.

Data sources

• national data on the number of fatal opioid overdoses per year between 1964 and 1997 compiled by the Australian Bureau of Statistics;

• the NSW Health Department's methadone client database between 1987 and 1998, provided by the Pharmaceutical Services Branch (PSB); and

• data on arrests for heroin offences in the period 1997-1999 from the NSW Police Service.

Each of these datasets is likely to be incomplete to different degrees and in different ways. The number of opioid overdose deaths may be underestimated if the cause of death is not recognised as opioid-related. Data on methadone entrants are more likely to be complete because methadone can not be prescribed in NSW until a registration form is completed and identity checks are undertaken to prevent a person registering more than once. Data on arrests for heroin-related offences do not include heroin-dependent people who are arrested for non-drug-related offences, such as property offences. The aim of the study was to look for convergence of estimates derived from "multiple measures of independent imperfection".¹¹

Methods of estimation

Multiplier methods used the number of national opioid overdose fatalities and NSW methadone maintenance therapy (MMT) clients. For mortality, we used both the conventional multiplier of 100 (which assumes an annual overdose mortality rate of 1%),¹² and a multiplier of 125, derived from a meta-analysis of cohort studies of treated heroin users (which suggests an annual mortality rate of 0.8%).⁹ The multiplier of 3 for MMT was derived from a study of demand for this type of treatment in Australia.¹³

Capture-recapture estimates were based on the numbers of individuals entering MMT (1995-1998) and arrested for heroin offences (1997-1998), using methods described elsewhere.⁹ The capture-recapture estimates were across periods of one year, rather than capture-recapture across different data sources in the same time period.¹⁴

Back-projection methods were applied to data on national fatal opioid overdose and first-time entrants to MMT in NSW to estimate the incidence and prevalence of heroin dependence. The form of back-projection used was that suggested by Becker and colleagues^15,16 (see ref. 17 for details). The current study examined the degree of convergence between the back-projection estimates derived from the two different data sources.

A national estimate was derived directly from national opioid overdose deaths. NSW estimates were derived from NSW MMT and arrest data. The NSW estimates were doubled to produce national estimates. This was done because NSW has consistently accounted for just under half of all opioid overdose deaths in Australia since 1979,⁵ and for just over half of the number of people enrolled in MMT in Australia.⁶

National estimates

The degree of consistency between the best estimates derived from different methods from different data sources was reassuring, especially the agreement between the trends in the two back-projection methods over the period 1970-1997 (Box 2). This consistency provides some reassurance that the true estimate of the number of dependent heroin users in Australia is within our range of estimates.

In the absence of any consensus on differential weighting of the estimates, we have taken the median national estimate of 74 000 as the best estimate. This represents a population prevalence of dependent heroin use in Australia of 6.9 per 1000 adults aged 15-54 years, with a range of 5.8-8.6 per 1000.

Crude estimates of the number of heroin-dependent people in each of the Australian States and Territories can be derived by allocating the estimated number of heroin users in Australia as a whole (74 000) to each jurisdiction in proportion to the contribution that each jurisdiction made to national opioid overdose deaths in the period 1994-1998. On this basis, NSW is estimated to have 35 400 dependent heroin users, just under half of the national estimate (48%). The estimated number of 19 600 heroin-dependent persons in Victoria accounted for 27% of the national estimate. NSW and Victoria therefore account for 75% of all the estimated number of heroin-dependent persons in Australia. The smaller States accounted for the remaining 25%. The estimated numbers of dependent heroin users in the smaller States, particularly Queensland and Tasmania, appear to be underestimates in that they imply that an implausibly high proportion of dependent heroin users are currently receiving treatment. Because better local estimates need to be derived by applying our methods to data in each of the smaller States, we have not reported these estimates here.

Firstly, there is reassuring convergence between estimates produced by different methods. The two back-projection estimation methods, applied to two very different datasets (new entrants to methadone treatment and opioid overdose deaths), produced estimates that were in close agreement. The ranges of estimates derived by plausibly varying the assumptions also substantially overlapped.¹⁷ The estimates produced by the mortality and treatment multiplier methods and the capture-recapture estimates were similar to those produced by the back-projection methods.

Secondly, the estimated national prevalence rate is in the middle of the range of estimates of problem drug use derived recently by similar methods in Europe (3-8 per 1000 adults aged 15-54 years).¹⁸ The Australian prevalence is the same as the estimated prevalence of heroin dependence in the United Kingdom (7 per 1000 adults aged 15-54 years).¹⁸

Several factors explain the discrepancy between our estimates and media estimates of 200 000 Australian heroin users. Our estimate is of dependent heroin users, who are highly visible in a small number of geographic areas in Sydney and Melbourne. This means that they often attract media attention out of all proportion to their numbers. Within these areas, intoxicated dependent heroin users are often highly visible, dealing and using drugs, and leaving discarded injecting equipment in public places. Dependent heroin users who are involved in criminal activities engage in high rates of breaking and entering houses.¹⁹

The national estimate for 1997 (74 000) represents a doubling of the 34 000 estimated in 1984-1987²⁰ and a 25% increase on the estimate of 59 000 in the period 1988-1993.¹³ During this time, there was a corresponding increase in the rate of heroin dependence from 3.7 per 1000 in 1984-1987 to 5.9 per 1000 in 1988-1993 and to 6.9 per 1000 in 1997. The back-projection estimates produce trends in numbers that are consistent with these earlier estimates (Box 2).

There are three plausible reasons why the number of dependent heroin users has increased in Australia over the past decade. First, there has been a substantial increase in the availability of very pure and very cheap heroin in Australia in the past two decades. The very limited historical data indicate that the purity of street heroin in Sydney has increased from 10% in 1979²¹ to a mean of 60% in 1993-1995.²² The nominal price has remained stable at $30 for a street "cap", but the effective price per ounce of pure heroin has declined from about $16 000 in 1979 dollars to $5000 in 1999 dollars; the difference is even greater when account is taken of inflation.⁹

Second, the high purity and low price have meant that it has been easier for new users to initiate heroin use by non-injecting routes, such as smoking or inhaling the fumes of heroin heated on aluminium foil. This may have permitted more naive users to begin heroin use before making a transition to injecting.¹⁹

Third, the rise in heroin use in birth cohorts born in the latter half of the 20th century is paralleled by similar trends in a range of psychosocial disorders among young adults,²³ including depression, suicide, antisocial behaviour and eating disorders. These trends and the reasons for them have been extensively reviewed by Rutter and Smith,²³ who conclude that the increases are real.

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2. Stimson GV, Judd A. Estimating the scale and nature of drug problems: the relationship between science, policy and drug strategy. In: European Monitoring Centre for Drugs and Drug Addiction. Estimating the prevalence of problem drug use in Europe. EMCDDA Scientific Monograph No. 1. Luxembourg: Office for Official Publications of the European Communities, 1997; 19-26.
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8. Darke S, Ross J, Zador D, Sunjic S. Heroin-related deaths in New South Wales, Australia, 1992-1996. Drug Alcohol Depend 2000; 60: 141-150.
9. Hall W, Ross J, Lynskey M, et al. How many dependent opioid users are there in Australia? NDARC Monograph No. 44. Sydney: UNSW, 2000.
10. Hartnoll R. Cross-validating at local level. In: European Monitoring Centre for Drugs and Drug Addiction. Estimating the prevalence of problem drug use in Europe. EMCDDA Scientific Monograph No. 1. Luxembourg: Office for Official Publications of the European Communities, 1997; 247-161.
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14. Hook EB, Regal RR. Capture-recapture methods in epidemiology: methods and limitations. Epidem Rev 1995; 17: 243-264.
15. Becker NG, Watson LF, Carlin JB. A method of non-parametric back-projection and its application to AIDS incidence data. Stat Med 1991; 10: 1527-1542.
16. Becker NG, Watson LF, Marschner IC, et al. Assessing the extent of the Australian HIV epidemic from AIDS surveillance data. Aust J Pub Health 1993; 17: 226-231.
17. Law M, Ross J, Lynskey M, Hall W. Back projection estimates of the number of dependent heroin users in Australia. Addiction 2000; in press.
18. European Monitoring Centre for Drugs and Drug Addiction. Extended annual report on the state of the drugs problem in the European Union 1999. Luxembourg: Office for Official Publications of the European Communities, 1999.
19. Maher L, Dixon D, Lynskey M, Hall W. Running the risks. Heroin, health and harm in South Western Sydney. Sydney: UNSW, 1998.
20. National Drug Abuse Data System NDADS. How many heroin users are there in Australia? Statistical Update No. 5. March 1988.
21. Williams ES. Australian Royal Commission of Inquiry into Drugs. Canberra: AGPS, 1980.
22. Weatherburn D, Lind B. Drug law enforcement policy and its impact on the heroin market. Sydney: NSW Bureau of Crime Statistics and Research, 1995.
23. Rutter M, Smith DJ, editors. Psychosocial disorders in young people: time trends and their causes. Chichester: John Wiley and Sons, 1995.

Authors' details

Reprints: Professor W D Hall, National Drug and Alcohol Research Centre, University of New South Wales, Sydney, NSW 2052.
w.hallATunsw.edu.au

©MJA 2000
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