Last week's Inside Health on BBC Radio 4 was the first of two editions discussing over diagnosis. The suggestion is that we are creating diseases which are, at best, risk factors. Chronic Kidney Disease (CKD), which has been part of the QOF for a few years now, was given as an example.
It is well worth downloading and listening to. Particular focus was placed on the MDRD formula for calculating the estimated glomerular filtration rate. This is the basis for entry to the register.
The formula (taken from Wikipedia) is
There are only two variables for any given person (race and gender are pretty constant). I see a lot of low eGFRs in older patients and wondered how the formula works. I downloaded Gnuplot and, after a bit of reading the manual, plugged the formula in. I have plotted for non black men but the chart would simply be moved a bit to the left or right in other patients.
For those that don't see that many of these things this is a not untypical range for creatinine. 125 is about the top of the normal range in my lab. You can see how the threshold for CKD (and eGFR of 60 or below) drops from a creatinine of 125ish in a 30 year old to 100ish at 90.
This is actually a smaller effect than I thought it would be but it does give some idea of the shape of the formula. What a low eGFR actually means for individuals remains under some debate.