Rapidly growing patient lists can heavily skew prevalence calculations. In 2009/10, some practices recorded 'extraordinarily high prevalence', a GPC document reveals.
Some recorded prevalence of up to 200 per cent for disease domains. GP leaders say Darzi centres are largely to blame.
All practices in the bottom 5 per cent of the range of prevalence nationally for each domain receive the same QOF pay weighting.
Because the outliers were so extreme, many practices with prevalence well above national average will have received the same weighting on points in the worst-hit domains as those with zero prevalence.
Vascular domains and the hypothyroid domain are understood to have been heavily hit.
GPC negotiator Dr David Bailey said the problem had caused 'significant variations' in QOF pay. High prevalence practices may have lost thousands of pounds, while those with low prevalence gained. He said Darzi centres may not have benefited financially but 'are just responsible for the skewing of QOF prevalence figures'.
Expanding practices can record abnormally high prevalence because it is calculated by dividing the number of patients on a disease register on 31 March by the practice's registered population on 1 January.
If a practice list grows between these dates, prevalence for disease areas can be substantially skewed. The GPC document explains: 'A new practice with 10 patients on 1 January but which has grown by 31 March to include 11 patients with thyroid disease, would have a prevalence of 110 per cent.'
Jon Ford, head of the BMA's health policy and economic research unit, said some practices recorded prevalence of 200 per cent in disease domains. 'The prevalence was unfeasibly large, making a nonsense of it. People will have lost out,' he said.
The removal in 2009/10 of the square rooting mechanism that damped QOF pay weighting further increased pay variation.
For 2010/11, the 5 per cent mechanism will also be lost.
The GPC document warns: 'The effect of the variations in 2009/10 may have cushioned the loss of money for low prevalence practices to less than predicted. These practices can anticipate further reductions in QOF income in 2010/11.'