The square root formula for adjusting prevalences finished a year ago but we are still left with the 5% cut off for the year just gone. This was less well known than the square root formula but its effects could be rather larger. It seems to have become more of an issue for many practices recently - perhaps we are all looking at our budgets just that little bit more closely.
The basic rule is this. You find the practice with the highest prevalence for any given condition and then calculate 5% of that prevalence. Any practices below that 5% value have their prevalence moved up to that level. Simple? No, not really.
The problem is that there are a small number of practices out there that are quite exceptional in their prevalence. You can see the spread of prevalences in the boxplot below (2008-9 data). For those unfamiliar with a boxplot the middle 50% of practices are within the box. The whiskers spread out from this upto 1.5 times the size of the box. The really outlying practices are plotted separately.
As you can see there are high outliers in every area, some more than others. The 5% rule really starts to kick in when the highest outlier is more than 20 times the mean. When this happens more than half of the practices will be bunched together at the 5% level. Prevalence adjustsment can simply stop. Last year, for instance, only three practices in England had a dementia prevalence more than 5% of the maximum. This meaned every other practice received the same prevalence factor.
To illustrate this effect the results for one of these extreme practices below. I am not giving their name as who they are is not really the point. They provide services to a group of patients with significant needs and there is no reason at all to doubt their figures.
Under the current rules this one practice can significantly change the QOF payments to thousands of others. But how many are affected? Well we can use the database to see.
As you can see it is not only in dementia that the 5% rule affects the vast majority of the 8229 practices in England. Learning disabilities, stroke and mental health area all hugely affected and over half of practices are affected in the area of CKD. As a side note you may notice that learning disabilities, heart failure and epilepsy all have the same maximum. This is all down to a single, and highly unusual practice, although this time down to very small numbers of patients. The same practice is also responsible for the highest stroke prevalence. Another "special" practice has the highest rate of mental health problems, although fewer than 100 patient overall.
There is no blame to attach to these practices. They are providing services to often very difficult populations and there is no doubt that they are recording accurately. The problem is with the operation of the rule, now thankfully in its final year. Expect big changes in these areas next year.