Fat maps? Fat chance.

It comes to quite something when the best source that I can find for information about QOF analysis comes from GMTV. The big story is the "Fat Map" of the UK apparently produced by Dr Foster and sponsored by Roche. I say apparently but the actual map and report don't seem to feature on the web sites of either.

The data they appear to be using is the QOF obesity register size at PCT level for April 2007 which has been available on this site for ten months now. When you come down to the business rules level this is a measure of the number of patients over sixteen years old who have had a BMI measured (or technically weight measured and BMI calculated) between January 2006 and April 2007 and that BMI was greater than or equal to 30.

A BMI of 30 is not that high these days. For those of you who don't deal with BMIs on a daily basis (basically front line clinicians) Flickr hosts a rather wonderful range of illustrated BMI catagories.

The prevalence has then been calculated by dividing this number by the total registered patient population.

There are thus quite a number of confounding factors.

Firstly and probably most significantly is the enthusiasm of the GP practice for weighing lots of people. If people were not weighed they did not count. For instance a huge patient would not be counted as obese if they did not have a BMI recorded. Getting a high prevalence involved weighing everyone who came through the door who looked like they may have a BMI over 30. There was no incentive to weigh patients with a BMI of less than 30 so it was just not done much - GPs have a pretty good eye for rough BMIs. For this reason even if we could know how many BMIs were measured it would be a bad measure of the obesity prevalence due to the skewed population at the measurement level.

Secondly we have the dodgy denominator. Remember the definition above? It applied only to patients of 16 or over - which is fair enough. BMIs don't really work with children. However to get the prevalence it was divided by the whole population. So if you have a lot of under 16s then your obesity prevalence will tend to be diluted. Similarly if you have a generally aging population then your obesity levels would appear artificially high.

Finally we have areas such as coding which are probably pretty minor.

Wales in general seems to stick out on the map, or at least the bits I could see on news.sky.co.uk Now I don't know a lot about Wales other than what I see on Torchwood but it seems rather odd that the whole of Wales is high (from North to South) and that obesity starts right on the border. Was there a LES or other country specific reason for practices to be incentivised to check BMIs a lot?

So this is a pretty dubious set of statistics on a map. Could it be better? Well perhaps a little. I mentioned the problem of the dodgy denominator above. Is there a better figure that we could use? Certainly there is. Records 22 (recording of smoking status) applies to all patients over 15 and uses that population as its denominator. We could at least correct that error although practice rates of measurement will still be a significant factor. I will try to put the figures together and if Roche or anyone else want to sponsor it they are very welcome!

1 comment:

Anonymous said...

Nicely done, thanks for this.