National data show that 39% of the population is aged 45 years or over and that the prevalence of ischaemic heart disease in this group is 8.7%.[w1] [w2] The number of paper records that would have to be examined to confirm the diagnosis of ischaemic heart disease in a typical practice or primary care group population for a given sensitivity can be calculated from the formula: list size´ 0.39´ 0.087´ search sensitivity´ (1/positive predictive value).

The search sensitivity and specificity are derived from the Battersea population, where the prevalence of ischaemic heart disease was found to be 5.5%. To produce the figure we estimated the positive predictive value when the disease prevalence increases using a standard formula.[w3] For example, a primary care group with a national average for population age range and disease prevalence would need to review 100 000´ x0.39´ 0.087´ 0.96´ 100/0.45=7166 notes (where the positive predictive value has increased from 33% to 45% as a result of the increased prevalence of disease found nationally[w1] [w2]) If Read coded cases are not re-examined at this stage, this number falls to about 5400.
 

1. Moser K, Majeed A. Prevalence of treated chronic diseases in general practice in England and Wales; trends over time and variations by the ONS area classification. Health Stat Q 1999;2:25-32.
2. Calman K. On the state of the public health 1997. London: Stationery Office, 1998
3. Altman DG. Practical statistics for medical research. London: Chapman and Hall, 1991:416.