The recently published clinical indicators assume clinical coding is reasonably consistent in all trusts (news focus, 24 June). This is some way from reality.

My trust has a proven record of good standards in clinical coding using original case-notes rather than discharge summaries, which often do not include many of the 'investigative' or other procedures which may not be significant for consultant-to-GP correspondence. For example, we code 'external resuscitation' following cardiac arrest and all 'procedures' such as blood transfusions and catheterisation. A significant number of trusts do not include such detail in their clinical coding of episodes, although national standards say they should.

While these 'procedures' - and many more like them - are not surgical interventions in the sense of 'operations', the OPCS codes for them are not included in the list of 'excluded' procedures in the clinical indicator for 'death within 30 days of operation'. The non-surgical procedures are very likely to be applied to the sicker patients, and trusts which (wrongly) do not include such codes may appear to have a much lower death rate than those which carry out clinical coding to a higher standard - a clear example of a perverse incentive to code poorly.

All 1,500 non-surgical OPCS codes should have been excluded rather than the 200 which actually were for this particular indicator. That would have created a more even 'playing field'. This trust made the point about the problem with coding of cardiac resuscitation when the previous indicators were circulated for consultation. We were disappointed to find our comments apparently ignored.

We have no problem with the principle of clinical indicators, but it cannot be helpful to compare flawed data and then assume trusts are comparable. Since this and so much else is being based on clinical coding, it is time for mandatory external audit of clinical coding to common standards to ensure true comparability. Only then will the clinical indicators be a helpful tool.

Sarah Harley Head of information services Pilgrim Health trust Boston