THE idea that a private patient might be admitted into an NHS bed, undergo surgery in an NHS theatre, or attend consultations in an NHS hospital might come as a surprise, but it is not new - and not necessarily a bad thing.

How widespread the practice is in Scotland is unclear - there are no published, national statistics - but it is likely to be extremely low.

Read more: NHS Ayrshire earning £100k a year from private cataracts ops

Even in England, where hospital trusts are entitled to make 49% of their annual income from private activity (compared to 2% previously) as a result of the controversial 2012 Health and Social Care Act, only 150 NHS beds are occupied by private patients at any one time.

The Act does not apply in Scotland and some health boards - for example, NHS Fife - do not treat private patients at all.

From the perspective of the NHS, providing care to a small number of private patients offers it an extra revenue source in cash-strapped times. The £100,000 a year NHS Ayrshire is making from cataracts patients would pay a consultant's salary.

Read more: NHS Ayrshire earning £100k a year from private cataracts ops

So long as private patient numbers are too low to impact on waiting times or theatre space, it should be a net benefit for the health service.

However, NHS Ayrshire's provision of cataracts surgery to private patients coupled with the roll-out of various restrictions to its NHS referrals raises a quandary.

Waiting times for routine operations - cataracts, hip and knee surgeries, for example - are on the rise, not only in Scotland but all over the UK.

Read more: NHS Ayrshire earning £100k a year from private cataracts ops

If health boards tighten the eligibility criteria in such a way that it drives people to the private sector, and some of these patients are subsequently treated in NHS hospitals - while paying for the privilege - it does seem to undermine the free-at-the-point-of-use cornerstone.

NHS Ayrshire denies their restriction have contributed to increased private activity; a hospital sources suggests otherwise.

It will take time for the effect to show up in figures on hospital income and patient activity, but Professor Bell is right to call for clarity. That should apply not only in Ayrshire, but across NHS Scotland.