Counting intravenous drug users

Here's a classic of its type. Counting the number of Iv drug users in an area is, for some obvious reasons, difficult.

Capture-recapture methods come from ecology. If you want to count butterflies in a wood, capture ten and mark them with a spot. Come back a week later and capture another ten. How many of this recapture sample are already marked? Two - i.e. 20%? Then your first effort must have captured 20% of the entire population; there are 50 butterflies in the wood altogether.

The Bristol team use a variant of this. Rather than capture- recapture at different points in time, they look for overlap (patients already 'marked,' as it were) in different data sources. For this to work properly the sources must be independent - getting arrested should make no difference to whether or nor you are attending a treatment agency. Then they add a sophistication I don't fully understand - the 'covariate' method.

All in all they found lower death rates than expected (though still appallingly high); a lot of hepatitis B and a lot of undiganosed hepatitis C. The result of all this?

"As a result Bristol PCT increased investment in needle and syringe provision, HCV diagnostic testing, and renewed its efforts to reduce injecting risk and BBV transmission locally. Without the local information showing the scale of the risk the commitment and a sense of urgency by providers and policymakers may not have emerged as quickly. In contrast, the public health message from the mortality study was that the high coverage of O[piate] S[ubstitution] T[herapy] was improving the health of the population, and led to a local commitment to maintain current levels of provision and increase the uptake of OST."

Confirms the opinion expressed 20 years ago in an obscure (but brilliant!) doctoral thesis that the purpose of local epidemiology is not to generate new knowledge but to prompt action; and hence the requirement is for info that is local and immediate. It doesn't have to be accurate to the last decimal point. (But it does have to be rigorous enough to be honest.)